L A. A TEXT-BOOK OF VOLUMETRIC ANALYSIS WITH SPECIAL REFERENCE TO THE VOLUMETRIC PROCESSES OF THE PHARMACOPCEIA OF THE UNITED STATES. DESIGNED FOR THE USE OF PHARMACISTS AND PHARMACEUTICAL STUDENTS. BY HENRY W. SCHIMPF, Pn.G. Professor of Inorganic Chemistry in the Brooklyn College of Pharmacy ; Food Inspector of the Department of Health of the City of Brooklyn ; Member of the American Association for the Advancement of Science ; of the American Pharmaceutical Association ; of the Kings County Pharmaceutical Society ; of the Brooklyn Institute ; of the German Apothecaries Society of the City of Neiu York; Honorary Member of the Alumni Association of the Brooklyn College of Pharmacy, etc., etc. SECOND EDITION. FIRST THOUSAND. InriTiRsirr] & NEW YORK : >HN WILEY & SONS. LONDON: CHAPMAN & HALL, LIMITED. 1895. Copyright, 1894, HY HENRY W. SCHIMPK. ROBERT DRUMMOND, ELECTROTYPER ANP PRINTER, NEW YORK. triU7BRSIT7 PREFACE. THIS book is designed for the use of pharmacists, and especially as a text-book for students in pharmacy. In the first portion of the book the author has at- tempted, in explaining the principles of volumetric analysis, to combine thoroughness with simplicity of expression. The United States Pharmacopoeia has been taken as the basis of the work, and the volumetric processes therein given are followed throughout, each step being carefully explained, and chemical equations inserted, wherever deemed necessary. The author has also added descriptions of processes not given in the Pharmacopoeia, but which are worthy of consideration. In teaching volumetric analysis to students in phar- macy the author discovered the necessity for a work especially designed for this class of students. Moreover, the requirements of the new edition of the United States Pharmacopoeia, in which many volumet- ric processes are given, necessitate on the part of the careful pharmacist a knowledge of this branch of ana- lytical chemistry; and no work that has as yet fallen into the hands of the author has seemed to be exactly suited to the needs of the practical pharmacist. Con- sequently the necessity for a book based upon the Pharmacopoeia and free from technicality is apparent. iii IV PREFACE. The latter portion of the book is devoted to descrip- tions of such special analytical processes as the phar- macist may be called upon to use, and such as are taught in the pharmaceutical colleges. The author has selected such processes as can be easily and quickly executed, and has given the gravi- metric only where volumetric processes cannot be employed. In the subject-matter of the book little originality is claimed, but the author has used his own judgment in its selection and arrangement. He has endeavored in the text to give credit wher- ever it was due, and especially acknowledges his indebt- edness to the United States Pharmacopoeia ; Button's Volumetric Analysis ; Hartley's, Simon's, and Attfield's text-books ; Blythe's Food Analysis ; Prescott's Organic Analysis ; Muter's Analytical Chemistry (American edition} ; Lefmann and Beam's Milk and Water Analy- sis ; and Witthaus' and Holland's Urine Analysis. He wishes to express his thanks to Dr. J. F. Gold- ing for the valued assistance he has rendered during the preparation of the book. He is also indebted to Richards & Co., of 41 Barclay Street, N. Y. City, manufacturers of chemical apparatus, from whom sev- eral of the cuts were borrowed. The author submits this work to the consideration of pharmacists, trusting its reception will be at least commensurate with the labor expended in its prepa- ration. HENRY W. SCHIMPF. 365 FRANKLIN AVE., BROOKLYN, N. Y. TABLE OF CONTENTS. PAGE TABLE OF THE ELEMENTS AND THEIR ATOMIC WEIGHTS . . xvii ABBREVIATIONS AND SIGNS . xviii PART I, CHAPTER I. Quantitative Analysis . I The Gravimetric Method I The Volumetric Method e . I CHAPTER II. Standard and Normal Solutions .<,..., 4 Normal Solutions 4 Standard Solutions 4 "Standardized," "Set," or "Titrated "Solutions ... 4 Decinormal Solutions , . . . 8 Centinormal '* . .... ... 8 Semi-normal " . ' * r 9 Double-normal " 9 Empirical 9 To "Titrate" . 9 Residual Titration . .9 CHAPTER III. Indicator defined 10 Litmus Tincture .10 Phenolphthalein T. S. , 10 V vi TABLE OF CONTENTS. PAGE Methyl-Orange T. S n Rosolic Acid T. S , n Turmeric T. S n Cochineal T. S n Eosin T. S u Brazil-wood T. S n Fluorescein T. S. . n Potassium Chromate T. S. . . , . . . .11 Potassium Ferricyanide T. S. . . . . . . .11 CHAPTER IV. GENERAL PRINCIPLES . . . .12 CHAPTER V. WEIGHTS AND MEASURES USED IN VOLUMETRIC ANALYSIS. 15 Graduation of Instruments 16 Table showing Expansion and Contraction of Liquids at Different Temperatures 16 CHAPTER VI APPARATUS USED IN VOLUMETRIC ANALYSES . .17 The Burette . . .17 Mohr's Burette 17 Glass-cock Burette 17 Oblique-cock Burette 18 Mohr's Foot Burette with Rubber Ball 19 GayLussac's Burette . . . . . . .19 Bink's Burette . . .19 Bead Stop ........... 2O Burette Stand 25 Measuring-flask : .... 20 Test-mixer ... 21 Pipettes 21 Single-volume Pipettes .00 21 Graduated Pipettes . . 21 Bead Pipette 22 Nipple Pipette 22 Burette attached to Reservoir ..... 24 TABLE OF CONTENTS. Vll CHAPTER VIL PAGE USE OF APPARATUS . . . .27 Cleaning the Instruments . b 27 Filling the Burette 27 Reading the Instruments ....... 28 Half-blackened Card . 29 Erdman's Float . 30 CHAPTER VIII. CALCULATING RESULTS . . , .31 Rules for finding Percentage 31 Factors or Coefficients . . . . . . . -33 Table of Approximate Normal Factors for Alkalies and Acids . 35 CHAPTER IX. ANALYSES BY NEUTRALIZATION . '36 Alkalimetry . ., . ... . . . " . .36 Preparation of Standard Oxalic-acid Solutions . . . .39 Preparation of Standard Sulphuric and Hydrochloric Acid Solutions .. ... . . . . . 40, 41 Estimation of Alkaline Hydroxides * . . . .43 Potassa j ... .. ^ ."., ... . * 43 Liquor Potassa 44 Soda 45 Liquor Soda 45 Aqua Ammonia . . . 46 Fortior .46 Spirit of Ammonia r .. " . ... . . .47 Estimation of Alkaline Carbonates , * . . . . . 47 Potassium Carbonate . ' *. ... .' . . . .48 Potassium Bicarbonate % 49 Sodium Carbonate ... . ;... . > : ^ *.';! .T . . . 49 Sodium Bicarbonate . . . . . . .50 Ammonium Carbonate . * . . . . .51 Lithium Carbonate ... . ,. . . . . .51 Borax ....... .... 54 Estimation of Organic Salts of the Alkalies , . . . 54 Potassium Tartrate . . . . 55 Potassium and Sodium Tartrate . v . . .57 viii TABLE OF CONTENTS. PAGE Potassium Bitartrate ...,... 58 Lithium Citrate .... o .... 59 Potassium Citrate .... .... 60 Potassium Acetate . . . . . . .61 Sodium Acetate 62 Lithium Benzoate , ........ 63 Sodium Benzoate ...... ... 64 Lithium Salicylate . . 66 Sodium Salicylate 67 Table showing Normal Factors for the Organic Salts of the Alkalies 68 Acidimetry ........... 68 Special Vessels for Preserving Alkali Solutions . . . .69 Preparation of Normal Alkali Solutions . . . . .69 Acetic Acid 73 " Diluted 74 " Glacial 76 Vinegar ' 74 Free Mineral Acids in Vinegar . 75 Citric Acid 76 Lime and Lemon Juice . 77 Hydrobromic Acid ... 77 Hydrochloric Acid ... 78 Hypophosphorous Acid . 79 Lactic Acid o . . . . 80 Nitric Acid . 81 " Diluted . . . . ' 82 Phosphoric Acid . 82 " Diluted . c o 82 " " (Stolba's Method) ...... 82 Sulphuric Acid . 86 " " Aromatic 87 " " Diluted 87 Tartaric Acid 87 Table showing Normal Factors, etc., for the Acids . . .88 Estimation of Alkaline Earths .88 Preparation of Normal Sodium Carbonate V. S, . . . .89 Liquor Calcis .90 Calcium Carbonate 9 1 " Bromide ..,.<>.... 92 TABLE OF CONTENTS. IX PAGE Calcium Chloride ......... 93 Barium Chloride 93 " Nitrate 93 Strontium Lactate .. ......94 CHAPTER X. ANALYSIS BY PRECIPITATION . ,96 Estimation of Haloid Sails ........ 97 Preparation of Decinormal Silver Nitrate V. S. . . . 97 Ammonium Bromide . 99 Lithium Bromide 101 Potassium Bromide . , 101 Sodium Bromide 102 Strontium Bromide 103 Calcium Bromide 103 Zinc Bromide 104 Potassium Iodide . 105 " " Personnel Method 106 Sodium Iodide 107 Strontium Iodide ...;.. . 108 Zinc Iodide 108 Ammonium Chloride , 109 Potassium Chloride 109 Sodium Chloride .no Zinc Chloride . . . . . . . . . .no Syrup of Hydriodic Acid in 4. F errous Iodide . .112 Ferrous Bromide 117 Saccharated Ferrous Iodide . * * * . . . . 116 Preparation and Use of Standard Potassium Sulphocyanate V. S. (Volhard's Solution) .113 Hydrocyanic Acid 117 Potassium Cyanide . . . . . . . . 120 Silver Nitrate 121 " Fused 123 " " Diluted . 123 " Oxide 124 Liquor Plumbi Subacetatis .... e ... 124 T$ble showing Factors of Substances estimated by Precipitation. 125 X TABLE OF CONTENTS. CHAPTER XL PAGE OXIDIMETRY 127 Estimation of Ferrous Salts . . . . . . . .126 Preparation of Standard Solution of iKMnO^ and K-^Cr^Oi . 129 Estimation of Ferrous Salts by IC i Cr^O^ 133 Saccharated Ferrous Carbonate . . , . . .138 Ferrous Sulphate , 140 Estimation of Ferrous Salts by zICMnO* . . . . .141 Ferrum Reductum 143 Ferrous Sulphate , . 145 Estimation of other Oxidizable Substances . . . . .145 Hypophosphorous Acid 146 Calcium Hypophosphite 148 Ferric Hypophosphite . . . 149 Potassium Hypophosphite 150 Sodium Hypophosphite ........ 151 Hydrogen Peroxide . 152 Barium Dioxide . .157 Oxalic Acid .158 Table of Substances which may be Estimated by Oxidation . 160 CHAPTER XII. ANALYSIS BY INDIRECT OXIDATION , . 161 Preparation of Standard Solution of Iodine . . -;..', .162 Arsenous Acid . . . . . . :/. *63 Liquor Acidi Arsenosi, U. S. P. . . .; . f . . . 164 Liquor Potassa Arsenitis, U. S. P 165 Sulphurous Acid ...... ... 165 Sodium Sulphite . 166 Potassium Sulphite . ... 167 Sodium Bisulphite ... . . 168 Sodium Thiosulphate ... 168 Antimony and Potassium Tartrate 169 Table of Substances which may be Estimated by Iodine . .171 CHAPTER XIII. ESTIMATION OF SUBSTANCES READILY REDUCED . 172 Preparation of Standard Solution of Sodium Thiosulphate . .173 Estimation of Free Iodine 175 TABLE OF CONTENTS. xi PAGE Liquor lodi Compositus 176 Tincture of Iodine 177 Aqua Chlori . . . , 177 Calx Chlorata ... 178 The Arsenous Acid Process 180 Preparation of A rsenous-acid Solution . . . . . 1 8 1 J 10 Liquor Sodae Chloratae 181 Estimation of Ferric Salts .183 Ferric Chloride . 184 Liquor and Tinctura Ferri Chloridi . . ... 185 Ferric Citrate 186 Liq. Ferri Citratis 187 Ferri et Ammonii Citras 188 " " Potassii Tartras 188 " " Ammonii Tartras . 188 Ferri Phosphas . . . . . . . . " . .188 Ferri et Quininse Citras 189 Ferri et Strychninae Citras 191 Ferri et Ammonii Sulphas 192 Ferri Pyrophosphas ....... 194 Ferri Valerianas '. 195 Liq. Ferri Acetatis 196 " " Nitratis 197 " " Subsulphatis 9 198 " " Tersulphatis ........ 199 Hydrogen Peroxide, Estimation of, by Kingzett's Method . . 200 N Table of Substances Estimated by Sodium Thiosulphate V. S. 201 PART II. CHAPTER XIV. SANITARY ANALYSIS OF WATER . . .202 Collection of Sample 202 Color 203 Odor ....* 203 Reaction 204 Suspended Matter . . . . . . . . 204 Xli TABLE OF CONTENTS. PAGE Total Solids .... 204 Organic and Volatile Matter or Loss on Ignition . . . 205 Chlorine 206 Ammonia .... ...... 207 Nessler's Solution ...... .. 207 Albuminoid Ammonia 6 210 Nitrates 211 Nitrites 214 Oxygen-consuming Power 216 Phosphates e . . . . 21? Hardness, Temporary and Permanent 219 Interpretation of Results 224 CHAPTER XV. ESTIMATION OF CO 2 IN THE ATMOSPHERE . . 233 Table showing Volume of .001 gm. of CO 8 at various Temper- atures 237 CHAPTER XVI. ESTIMATION OF ALCOHOL IN TINCTURES AND BEVERAGES . 238 Table for Ascertaining the Percentages of Alcohol in Spirit from the Specific Gravity , . 240 CHAPTER XVII. ESTIMATION OF TANNIN . . . . 242 G. Fleury's Method . . . ; . 242 Lowenthal's Method . . . . . ' . . . 243 CHAPTER XVIII. ESTIMATION OF OLEIC ACID . . . 246 CHAPTER XIX. ANALYSIS OF SOAP .... 249 CHAPTER XX. DETERMINATION OF THE MELTING-POINT OF FATS . .251 CHAPTER XXI. ESTIMATION OF OIL OR FAT IN EMULSIONS AND OINTMENTS. 252 Soxhlet Apparatus ... 253 TABLE OF CONTENTS. Xlll CHAPTER XXII. PAGE ESTIMATION OF STARCH IN CEREALS, ETC. . . 255 CHAPTER XXIII. ESTIMATION OF SUGARS . . . .259 CHAPTER XXIV. ESTIMATION OF GLYCERIN . . .26 CHAPTER XXV. ESTIMATION OF PHENOL . 266 Preparation of Standard Bromine Solution 266 By Koppeschaar's Method ....... 268 Dr. Waller's Method .272 Assay of Crude Carbolic Acid 273 CHAPTER XXVI. PEPSIN 275 Valuation of Pepsin, U. S. P. Method . 277 Bartley's Method 278 CHAPTER XXVII. DETERMINATION OF THE DIASTASIC VALUE OF MALT AND PANCREATIC EXTRACTS . . . .281 Robert's Method 281 Park, Davis & Co.'s Method .283 CHAPTER XXVIII. VOLUMETRIC ESTIMATION OF ALKALOIDS . . 285 Table showing the Behavior of Some of the Alkaloids with Indicators 289 Table showing Factor for Various Alkaloids when Titrating with Acid V. S. . .... 290 20 Estimation by Mayer's Reagent . ...... 290 Alkaloidal Assay by Immiscible Solvents 292 CHAPTER XXIX. ESTIMATION OF ALKALOIDAL STRENGTH OF SCALE SALTS. 295 General Method for the Estimation of the Alkaloidal Strength of Extracts 295 xiv TABLE OF CONTENTS. Assay of Extract of Nux Vomica, U. S. P t 296 " " Extract of Opium ... ... 298 " " Tincture of " 300 " " Gum Opium , <= . 301 " " Cinchona, U. S. P. - . . . . . . .302 " " Fl. Extr. of Ipecac . ... 304 " " Ipecac Root 306 Estimation of the Strength of Resinous Drugs .... 306 CHAPTER XXX. ESTIMATION OF GLUCOSIDES. . . , 308 CHAPTER XXXI. Milk . . . . .309 Average Composition . . ...... 309 Colostrum 310 Reaction *... 310 Specific Gravity 310 Lactometer 311 Table for Correcting the Sp. Gr. of Milk according to Tem- perature . . . . . . . . .312 Adulterations of Milk 313 Total Solids and Water 313 Fat, Adam's Method 314 Werner-Schmidt Method 315 Calculation Method ...... . 316 Calculation of Per Cent of Added Water . . . . .317 Total Proteids . 318 Milk Sugar 318 CHAPTER XXXII. BUTTER 319 General Composition .......<,. 319 Reichert's Process for the Detection of Foreign Fats . . . 319 Rapid Method for the Detection of Oleomargarine . . . 320 CHAPTER XXXIII. URINE 322 Reaction 322 TABLE OF CONTENTS. XV PAGE Composition ... 322 Specific Gravity 3 2 4 Total Solids 325 Chlorides . . . " 326 Phosphates 3 2 7 Sulphates 327 Total Acidity 328 Urea 329 Uric Acid 329 Abnormal Constituents 33 Albumen 33O Blood 333 Pus 333 Sugar 034 Bile 338 Examination of Urinary Deposits . . . . . 339 Analysis of Urinary Calculi 340 PART III. GASOMETRIC ANALYSIS .... 342 CHAPTER XXXIV. THE NITROMETER .... 342 Charles' and Boyle's Law 344 CHAPTER XXXV. ASSAY OF SPIRIT OF NITROUS ETHER . . 346 Assay of Amyl Nitrite 349 " " Sodium Nitrite 350 Estimation of Nitric Acid in Nitrates 350 CHAPTER XXXVI. ESTIMATION OF SOLUBLE CARBONATES . . 352 CHAPTER XXXVII. ESTIMATION OF UREA IN URINE . . . 353 I. By Doremus' Ureometer. II. By the Gas-tube Method. III. By Squibb's Urea Apparatus 353 XVI TABLE OF CONTENTS. CHAPTER XXXVIII. PAGE HYDROGEN DIOXIDE* .... 357 Its Assay by the Nitrometer, and bySquibb's Urea Apparatus 357 APPENDIX. INDICATORS 360 REAGENTS AND TEST SOLUTIONS 369 A LIST OF ELEMENTS OCCURRING IN VOLUMETRIC METHODS, THEIR SYMBOLS, AND ATOMIC WEIGHTS. Name. Exact Atomic Weights according to Meyer and Seubert, adopted by the U. S. P. Approximate Atomic Weights. Aluminium-. ... . Al Sb 27.04 IIQ 6 27.0 1 2O O Arsenic As 74 Q 7C o Barium Ba 1*^6 O 1^6 Q Bismuth . . Bi 208 o 208 o Boron B JO Q no Bromine Br lu.y 7O 76 80 o Cadmium . Cd III H 1 1 T <> Calcium . Ca NaOH o 040 Na 2 CO 3 1 06 OOC'I NaHCO 3 8/1 o 084 KOH *6 o 056 Potassium carbonate > KoCO, 1 08 o 060 Potassium bicarbonate KHCO 3 IOO O IOO Ammonia (gas) . . . NH 3 17 o 017 Ammonium carbonate, normal . ... Ammonium carbonate, commercial.. Linr* . .... . (NH 4 ) 2 C0 3 N s HnC a 6 CaO 9 6 157 c 0.048 0.052i o 028 Calcium hydroxide . Ca(OH) 2 7J. OO17 CaCO 3 IOO o 050 HNO 3 A-j Hydrochloric acid . HC1 U J 76 A Sulphuric acid -. . H 2 SO 4 08 Oxalic acid crystallized H 2 C 2 O 4 2H 2 O y 126 o 063 Acetic acid HC 2 H 3 O 2 60 o 060 * This is the coefficient by which the number of cc. of normal solu- tion used is to be multiplied in order to obtain the quantity of pure substance present in the material examined. 36 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. CHAPTER IX. ANALYSIS BY NEUTRALIZATION. THIS is based upon the fact that acids are neutralized by alkalies and alkalies by acids. The strength of an acid is estimated by the quantity of alkali that is required to neutralize it. This process is called acidimetry. The strength of an alkali is found by the quantity of an acid that is required to neutralize it. This process is called alkalimetry. The stronger the acid, the more alkali is required, and vice versa. A substance is said to be alkaline when it turns red litmus blue; phenolphthalein, red; turmeric, brown; etc. Acid, when it turns blue litmus red; red phenol- phthalein, colorless, etc. The principal alkaline substances are the hydroxides and carbonates of sodium, potassium, and ammonium, and the hydroxides and oxides of calcium, barium, and strontium, and the alkaloids. When an acid is brought in contact with an alkali combination takes place, and a neutral salt is formed. This combination takes place in definite and invariable proportions ; thus : If 1 12 parts of potassium hydroxide are mixed with 98 parts of absolute sulphuric acid the alkali as well as the acid will be neutralized. If only 80 parts of the acid have been added the mixture would still be alkaline, for it requires 98 parts of the A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 37 acid to neutralize it. If more than 98 of the acid have been added the mixture would consist of potassium sulphate and free sulphuric acid. The reaction is thus illustrated : 2KOH + H,SO 4 = K 2 S0 4 + 2H 2 0. O- H,= 2 2 = 3 2 S =3 H, =*J 4 = 64 112 98 Sodium hydroxide will unite with oxalic, and form a neutral compound in the proportion of 80 parts by weight of the former and 126 parts by weight of the latter, as the equation shows : 2NaOH + H 2 C 2 4 . 2H 2 O = Na 2 C 2 O 4 + 4H 2 O. 2Na = 46 6H = 6 2O =32 2C = 24 2H = 2 6O = 96 80 126 NH 4 OH + HC1 = NH 4 C1 + H 2 O. N = 14 H = i. 5 H= 5 01 = 35-4 35 3^.4 Ammonium Hydrochloric hydroxide. acid. Na 2 CO 3 + 2HC1 = 2NaCl + H,O + CO 2 106 72.8 Sodium carbonate. 38 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Upon a careful perusal of the foregoing equations it will be seen that since definite weights of acids neu- tralize definite weights of alkalies the quantity of a certain alkali in solution can be easily determined by the quantity of an acid solution of known strength re- quired to neutralize it, and vice versa. If we make a solution of oxalic acid of such strength that IOOO cc. of it contains 63 gms. of the crystallized acid, i cc. of it will neutralize .056 gm. of KOH, .040 gm. of NaOH, or .035 gm. of NH 4 OH. Thus if 10 gms. of solution of KOH be treated with this oxalic-acid solution and it is found that 25 cc. of it are required to neutralize the alkali, the alkali solution contains 25 x .056 = 1.4 gms. of pure KOH. Since the acid and alkali as well as the neutral salt which is formed are colorless, and no visible change takes place during the reaction, it is necessary to add some substance which by change of color will show when the neutralization is complete. Such a substance is known as an indicator. A number of these are spoken of on page 10. Neutralization is sometimes called saturation. ALKALIMETRY. Preparation of Acid Volumetric Solutions. It is possible to carry out the titration of most alkalies with only one standard acid solution, but the standard acids are frequently required in other processes besides mere saturation, and it is therefore advisable to have a variety. The standard oxalic acid is preferred by some be- cause of the ease with which it may be prepared, pro- A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 39 vided a pure acid can be had. It does not, however, keep very long, and when used for titrating carbonates with methyl orange as an indicator the end reaction is not very distinct. Oxalic acid cannot very well be used for the titration of alkaline earths, since it forms insoluble compounds with these metals. Sulphuric acid V. S. is preferred by others. A pure acid can be gotten without difficulty, and the standard solution made with it is totally unaffected by boiling, which cannot be said of either nitric or hydrochloric acid. Sulphuric acid, however, forms with alkaline earths insoluble compounds. For this reason standard solution of hydrochloric acid must frequently be em- ployed. Normal Oxalic Acid V. S., U. S. P. H a C 2 O 4 + 2H,O = 125.7. *' gms. in i litre. Dissolve 62.85 g ms - (*63 gms.) of pure oxalic acid (see below) in enough water to make, at or near 15 C., exactly 1000 cc. Pure oxalic acid, crystallized, is in the form of colorless, transparent, clinorhombic crystals, which should leave no residue when ignited upon platinum foil. It is completely soluble in 14 parts of water at 15 C. If the acid leaves a residue on ignition it should be purified by recrystallization, as directed by the U. S. P. N I cc. of oxalic acid V. S. is the equivalent of NaOH .................... 0.03996 gm. KOH .................... 0.05599 " NH, ..................... 0.01701 " 40 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Decinormal Oxalic Acid V. S., U. S. P. H 3 C 2 O 4 + 2H 2 = 125.7. ' 5 S ms - in * litre - Dissolve 6.285 g ms - (*&3 gms.) of pure oxalic acid in enough water to make, at or near 15 C., exactly 1000 cc. N I cc. of oxalic acid V. S. is the equivalent of 10 NH 3 0.001701 gm. KOH 0.005599 " NaOH 0.003996 " Normal Hydrochloric Acid V. S., U. S. P. HC1 = 36.37. 3|.37 | gms> in j litre> Mix 130 cc. of hydrochloric acid of sp. gr. 1.163, with enough water to measure, at or near 15 C., 1000 cc. Of this liquid (which is still too concentrated) meas- ure carefully into a flask 10 cc., add a few drops of phenolphthalein T. S., and gradually add from a burette N - potassium hydroxide V. S. until a permanent pale N pink tint is produced. Note the number of cc. of - potassium-hydroxide solution consumed, and then dilute the acid so that equal volumes of this and the N - KOH V. S. neutralize each other. i Example. Assuming that the 10 cc. of the acid N solution required 12 cc. of the KOH, each 10 cc. of A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 4! the acid must be diluted to 12 cc., or the whole of the remaining acid in the same proportion. After the dilution a new trial should be made. 10 cc. of the acid V. S. should require exactly 10 cc. of the alkali. This solution is exactly equivalent in neutralizing N power to oxalic acid V.S. Normal Sulphuric Acid V. S., U. S. P. H 2 SO 4 = 97.82. S.9 1 gms> in Mix carefully 30 cc. of pure concentrated sulphuric acid (sp. gr. 1.835) with enough water to make about 1050 cc., and allow the liquid to cool to about i 5 C. Titrate 10 cc. of this liquid in the manner described N under hydrochloric acid, and dilute it so that equal volumes of the acid and the alkali will neutralize each other. Note. It is recommended in the U. S. P. that when a normal acid solution is required the normal sul- N phuric acid should be employed in place of oxalic. The oxalic-acid solution has a tendency to crystallize on the point of the burette. Decinormal Sulphuric Acid V. S., U. S. P. H 2 S0 4 = 97.82. . gms . in a litre . Dilute 10 cc. of the normal sulphuric-acid solution with enough water to make 100 cc. The standardization of normal acid solutions may 42 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. also be effected by the use of pure anhydrous sodium carbonate. Pure anhydrous sodium carbonate may be obtained by heating to dull redness a few grammes of pure sodium bicarbonate for about 15 minutes. The result- ing carbonate is practically free from impurity. The sodium bicarbonate loses on ignition one half of its carbonic acid gas: 2NaHCO 3 + Heat = Na a CO 3 + CO a + H a O. The bicarbonate should, however, be tested before igniting, and if more than traces of chloride, sulphate, or thiosulphate are found, these may be removed by washing a few hundred grammes, first with a saturated solution of sodium bicarbonate, and afterward with distilled water. 0.53 gm. of the pure anhydrous sodium carbonate is accurately weighed and dissolved in about 20 cc. of water in a flask and a few drops of methyl orange T. S. added as indicator. The acid to be " set " or " stan- dardized " is then run into the sodium-carbonate solu- tion until a permanent light-red color is produced. It N should require exactly IO cc. of the y acid solution. If 8 cc. of the acid solution are consumed to bring about the required result, then every 8 cc. must be diluted to 10 cc., or the whole of the remaining solu- tion must be diluted in this proportion : Na,C0 3 + H 2 S0 4 = Na 2 S0 4 + H 2 O + CO a . 2)106 2)98 N 53 gms. 49 = to 1000 cc. --V. S. ; 0-53 g m - = to I0 cc l A TEXT- BOOK OF VOLUMETRIC ANALYSIS. 43 Instead of methyl orange, litmus tincture may be used. The carbonic-acid gas which is liberated in this reaction turns litmus red; the contents of the flask should therefore be boiled for a few minutes to drive off the CO 2 , when the blue color will return. More acid is then run in until the mixture after boiling remains of a neutral color ; indicating that just enough acid has been added to complete the reaction expressed in the foregoing equation. ESTIMATION OF ALKALINE HYDROXIDES. A definite quantity of the substance is taken (gen- erally weighed), and diluted with or dissolved in a little water in a flask or beaker. A few drops of a suitable indicator are now added, and the standard acid solution allowed to flow in until the last drop added just causes the color to change, the flask being agitated after each addition of the acid solution. Potassa. KOH * |^9 u. S. P. Weigh carefully i gm. of potassa, dissolve it in a small quantity of water, add a drop of phenolphtalein solution as indi- N cator, and titrate with sulphuric acid V. S. until the red color just disappears. Each cc. of the normal acid solution used represents .056 gm. of pure potassa. To find percentage, multiply the factor (.056) by the num- N ber of cc. of V. S. used, and then multiply the prod- uct by 100. Potassium hydroxide having great affinity for carbonic-acid gas, which it absorbs out of the air, generally contains small quantities of carbonate. There- 44 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. fore in titrating as above described it should be boiled once or twice toward the end of the reaction in order to drive off any CO 2 which may be present. This gas, which has an acid reaction with phenolphtalein, would otherwise cause an incorrect estimation. This precau- tion should be taken with the other alkaline hydroxides. The U. S. P. requirement is that 0.56 gm. of potassa be neutralized by not less than 9 cc. of the normal acid solution, each cc. corresponding to 10 per cent of pure potassium hydroxide. The equation is 2KOH + H 2 S0 4 = K 2 S0 4 + H 2 0. 2)112 2)98 N 56 gms. 49 gms. in 1000 cc. of V. S. This shows that 56 gms. of KOH are neutralized by 1000 cc. of V. S. I Each cc. of this solution will therefore neutralize 0.056 gm. of KOH. Liquor Potassa, U. S. P. This is an aqueous solu- tion of potassium hydroxide (KOH) containing about 5 per cent of the hydroxide. It is estimated volumetrically in the same manner as potassa, 10 gms. of the solution of potassa being taken, N each cc. of the V. S. representing 0.056 gm. of KOH. By multiplying the factor by the number of cc. of N - V. S. used, the quantity of absolute KOH in the 10 gms. of liquor taken is obtained. The percentage is then found by multiplying the quantity so obtained by 100 and dividing by the num- ber of grammes of the liquor taken. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 45 N Thus if 9 cc. of the V. S. were used, the 10 gms. taken contained 9 X 0.056 = 0.504 gm. Then 10 gms. : 0.504 :: 100 : x. x = 5.04^. 28 gms. of the U. S. P. liquor potassa should require N about 25 cc. of the acid V. S., each cc. representing .2f c of KOH. Soda, (NaOH j 39*9 6 y s p.)._! gm . of soda is carefully weighed, dissolved in a small quantity of water, a few drops of phenolphthalein added, and then titrated with normal sulphuric acid V. S. until the red color of the indicator is just discharged. This equa- tion shows the reaction : 2NaOH + H,SO 4 = Na 2 SO 4 + H 2 O. 2)80 2)98 40 gms. = 49 gms. or 1000 cc. of V. S. Thus each cc. represents 0.040 gm. of NaOH. I gm. N should require 22.5 cc. of acid V. S., which indi- cates ^ .040 X 22.5 = .900 .900 X TOO - = 90$ Liquor Soda, U. S. P. This is an aqueous solution, containing about 5$ of the hydroxide (NaOH). 10 grammes of liquor soda are taken mixed with a little water, a few drops of phenolphthalein are added, and N then from a burette the y sulphuric acid V. S. in the 46 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. manner described above. Each cc. required represents 0.040 gm. of NaOH. If 12.5 cc. were required, then 0.040 X 12.5 = .500. .500 X loo Aqua Ammonise, U. S. P. An aqueous solution of ammonia (NH 3 = 17.01) containing \Q% by weight of the gas. Three grammes of ammonia water are diluted with a little water, a few drops of rosolic acid T. S. are added, N and then sulphuric acid V. S. slowly from a burette until the yellow color indicates that all the alkali is neutralized. Phenolphthalein is not suitable as an indi- cator for ammonia. Litmus may be used, but it is not as delicate an indicator as rosolic acid. N Each cc. of acid V. S. used represents 0.017 gm. NH 3 or *o.O35 gm. NH 4 OH, as shown by the equation 2NH 3 4 H,0 } + H * S < = ( NH <)' SO < + 2H '' 2NH 4 OH 2NH 3 . H 2 Q 2)98 ^ ^T 49 = to 1000 cc. 3 acid V. S. 35 17 i N If the 3 gms. required 17.8 cc. ~ acid V. S., then it contained 17.8 X .017 gm. = 0.3026 gm. According to the U. S. P., 3.4 gms. should require 20 cc. of normal acid V. S. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 47 Aqua Ammonias Fortior, U. S. P. (Stronger Am- monia Water). An aqueous solution of ammonia (NH S ) containing about 28$ by weight of the gas. This is estimated in the same manner as aqua ammonia, two grammes of the stronger ammonia water being taken instead of three. Spiritus Ammoniac (Spirit of Ammonia). This is an alcoholic solution of NH 3 , containing 10$ by weight of the gas. 3.4 grammes (or 4.2 cc.) of the spirit are diluted with N water and treated with sulphuric V. S. Each cc. of N the acid solution used represents .017 gm. of NH 3 or 0.5$. 20 cc. should be required. Rosolic acid is the indicator. ESTIMATION OF ALKALINE CARBONATES. When carbonates are treated with acids carbonic- acid gas is liberated. This gas shows an acid reaction with most indicators, and the reaction will seem to be completed before the alkali is entirely neutralized. To avoid this the process is conducted at a boiling temperature in order to drive off the CO a . The stand- ard acid being added until two minutes' boiling fails to restore the color indicating alkalinity. If the titra- tion is conducted at a boiling temperature it is advisa- ble to attach to the lower end of the burette a long rubber tube with a pinch-cock fixed about midway on the tube. The boiling can then be done at a little distance 48 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. from the burette and the expansion of the standard solution therein thus prevented. Another and better method is to use methyl orange as an indicator, and conduct the process by simple titration without the use of heat. Methyl orange is not affected by CO 2 . When methyl orange is used as an indicator, standard sul- phuric acid, and not oxalic acid, should be employed. The reaction of the latter with this indicator is not very sharp. Potassium Carbonate, K 2 CO 3 = j * I I ^' 91 - Weigh carefully one gramme of the salt, dissolve in a small quantity of water in a beaker or flask, add a few drops of methyl orange T. S., and titrate with normal sul- phuric acid until a faint orange-red color appears. K 2 C0 3 + H 2 S0 4 = K 2 S0 4 + H,0 + CO 2 . 2)138 2)g8_ N 69 49 = grammes in 1000 cc. V. S. N Each cc. of H 2 SO 4 , therefore, represents 0.069 gramme (more accurately 0.068955 gramme) of pure dry potassium carbonate. Thus if 14.4 cc. of the normal acid were required, the salt contained 14.4 X .069 = .9936 grammes of pure K 2 CO 3 , or 99.36 per cent. The U. S. P. requirement is that 0.69 grammes of the salt be neutralized by not less than 9.5 cc. of normal acid, corresponding to 95$ of the pure salt.* When methyl orange is used the end reaction is not very well defined, and practice is required to obtain good results. If it is desired to use litmus or phenol- A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 49 phthalein, it will be necessary to boil the solution as de- scribed above. *.o6 9 X9-5 =0.6555 0.6555 X ioo Potassium Bicarbonate, KHCO 3 = j # ' ^ i oo The process is exactly the same as that for the car- bonate. 2KHC0 3 + H 2 S0 4 - K 2 S0 4 + 2H 2 O + 2CO 2 . 2)200 2)98 N ioo 49 = to grammes in 1000 cc. of acid. N Each cc. of acid represents o. I gramme (more ex- actly 0.09988 gramme) of pure KHCO 3 . The U. S. P. requirement is that I gramme of the salt be neutralized by not less than 10 cc. of normal acid (corresponding to ioo per cent of the pure salt). Sodium Carbonate, Na 1 CO,+ ioH,O= j #^5^- Dissolve two grammes of sodium carbonate in suffi- cient water, add a few drops of methyl orange, and titrate as described under potassium carbonate. Na 2 C0 3 + ioH 2 + H 2 S0 4 = Na 2 SO 4 + 1 1 H 2 O + CO 2 . 2)286 2)98 N 143 49 = grammes in 1000 cc. acid V. S. N Each cc. of acid V. S. represents 0.143 gramme of crystallized sodium carbonate. 50 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. The U. S. P. directs that the salt be deprived of its water of crystallization by heat immediately before being weighed, and that i gramme of the anhydrous carbonate should neutralize not less than 18.7 cc. of N sulphuric acid, corresponding to 98.9$. Sodium Carbonate (exsiccated). Operate upon i gramme of the salt as described. Na 3 C0 3 + H 2 S0 4 = Na 2 S0 4 + H 2 O + CO 9 . 2)106 2)98 N 53 49 = grammes in 1000 cc. acid. N Each cc. of the acid represents .053 gramme of anhydrous sodium carbonate. The U. S. P. require- ment is that not less than 13.8 cc. of normal sulphuric acid should neutralize i gramme of the salt, corre- sponding to about 73 per cent of anhydrous sodium carbonate. .053 X 13-8 = 7314 or 73.14^ Sodium Bicarbonate, NaHCO 3 = j ^ 3 ' 85 . Oper- ate upon i gramme of the salt, and proceed in the usual way. 2NaHCO s + H 2 SO 4 = Na 2 SO 4 + 2H 2 O + 2CO,. 2)168 2)98 N 84 49 = to grammes in looocc. - acid. N Thus each cc. of acid represents .084 gramme of pure sodium carbonate. According to the U. S. P., 0.85 gramme of sodium A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 51 bicarbonate should require not less than 10 cc. of nor- mal sulphuric acid, which corresponds to at least 98.6$ of the pure salt. .084 X 10 .84 Lithium Carbonate, Li 2 CO 3 = j J^* 7 . H 2 S0 4 = Li,S0 4 + H S + CO, 2)74 2)98 N 37 49 = grammes in 1000 cc. acid. N Each cc. of -~ acid represents 0.037 gramme of lithium carbonate (more accurately .03693). 0.5 gm. of dry lithium carbonate are mixed with about 20 cc. of water in a beaker, a few drops of methyl orange T. S. added, and titration proceeded with until a faint orange-red color of the solution indicates the complete neutralization of the lithium carbonate. To comply with the U. S. P. test, 0.5 gm. should require for complete neutralization not less than 13.4 cc. of normal sulphuric acid, corresponding to at least 98.98 per cent of the pure salt. 0.03693 X 134 = 0.494862 gm. 0.494862 X IPO = 98.98$ Ammonium Carbonate, N 3 H 11 C 2 O 6 = j ^ 6 ' 77 . Normal ammonium carbonate has the formula 52 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. but the normal salt loses upon exposure NH 3 and H 2 O. The commercial salt, therefore, gen- erally is a mixture of carbamate and bicarbonate. (NH 4 ) 2 CO 3 - NH,= NH 4 HCO 3 ; (NH 4 ) 2 CO 3 - H 2 O = NH 4 NH 2 CO 2 . The commercial carbonate is therefore generally expressed thus: NH 4 HCO,.NH 4 NH 1 CO, or N 3 H n C 3 O 5 - For estimating the ammonium carbonate the U. S. P. recommends the following procedure: Dis- solve 7.84 gms. of unaltered ammonium carbonate in water to the volume of 90 cc. Take 30 cc. of this solution (which contains 2.613 gms. of the salt), add a few drops of rosolic acid T. S., and titrate with N - H 2 SO 4 V. S. until the violet-red color is replaced by N yellow. 50 cc. of the H 2 SO 4 should be required before this change takes place, corresponding to loofo of pure salt. 2N,H U CA + 3H 2 S0 4 = 3 (NH 4 ) 2 S0 4 + 4 CO 2 + 2H 2 O. 6)313.54 6)254 N 52.256 49 = to 1000 cc. acid V. S. Each cc., therefore, represents 0.052256 gm. of am- monium carbonate. 50 cc. = 50 X .052256 = 2.6128 gms. 2.6128 X ioo -- -, --- 2.613 A TEXT-BOOK OF VOLUMETRIC ANALYSIS, 53 Although rosolic acid, on account of its sensitiveness to ammonia, is recommended in the U. S. P. process, yet it must be remembered that this indicator is af- fected by CO 2 , and therefore great care should be exercised in this estimation. It must also be remem- bered that if heat is employed to dispel the CO 2 it is apt to occasion a loss of ammonia. Methyl orange is not affected by CO 2 and might be employed in this case, but it is not as sensitive to ammonia as rosolic acid. The method usually employed by skilled analysts is to add a measured excess of the standard acid solu- tion, and thus convert the ammonium carbonate into the less volatile ammonium sulphate ; then gently boil to get rid of CO 2 , and titrate back with a standard alkaline V. S. (using litmus as an indicator) until the excess of acid is neutralized. The quantity of free acid is thus found, which, when deducted from the amount of acid first added, gives the quantity which was required to neutralize the ammonium carbonate. Thus, 2.613 g m - m solution of ammonium carbo- N nate are treated with 70 cc. of H 2 SO 4 V. S., which is more than sufficient to neutralize it ; the solution is then gently boiled to drive off CO a , a few drops of N litmus tincture added, and then titrated with KOH V. S. until the litmus no longer shows an acid reaction and the solution is neutraL N Let us assume that 20 cc. of the KOH V. S. were N used. By deducting the 20 cc. from the 70 cc. of - 54 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. acid first added we find that 50 cc. of the acid went into combination with the ammonium salt. Thus, 50 X .052256 = 2.6128 (*2.6i3) 2.613 X ioo - = loofo 2.613 Borax,Na 2 B 4 7 .ioH 2 0^| ^o.92^_ Two gms> of borax are dissolved in a small quantity of water, a few drops of tincture of litmus are added, and the solution titrated with normal oxalic acid V. S., or some other acid V. S. i Boric acid is liberated during the operation, which colors the litmus wine-red. This is not regarded, and the titration is continued until the bright red, due to the action of free oxalic acid, makes its appearance. Apply the following equation : Na 2 B 4 7 .ioH a O + H a C 2 O 4 . 2 H 2 O 2)382 2)126 N 191 63 gms. = looo cc. V. S. = Na.C.0. + H.BA + I2H.O. N Thus each cc. of - - oxalic acid V. S. represents 0.191 gm. crystallized borax. ORGANIC SALTS OF THE ALKALIES. The tartrates, citrates, and acetates of the alkali metals are converted by ignition into carbonates, the whole of the base remaining in the form of carbonate. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 55 Each molecular weight of a normal tartrate gives when ignited one molecular weight of carbonate : K.C.H.O. = K.CO.. Every two molecular weights of an acetate or an acid tartrate give one molecular weight of carbonate : 2KC.H.O, - K,CO, ; 2KHC 4 H 4 6 = K,C0 8 . Every two molecular weights of a normal citrate give three molecular weights of carbonate : These reactions are taken advantage of in volumet- ric analysis, and the tartrates, citrates, and acetates of the alkalies . are indirectly estimated by calculating upon the quantity of carbonate formed by burning them, the quantity of carbonate being found by titra- tion in the usual manner. Potassium Tartrate, K 2 C 4 H 4 O 6 .H 2 O = j ^ 2 ^ 6 .- Two gms. of the salt are placed in a platinum or por- celain crucible and heated to redness in contact with the air until completely charred ; that is to say, until nothing is left in the crucible but carbonate and free carbon. The crucible is now cooled, and its contents treated with boiling water, which dissolves the potassium car- bonate, the carbon being separated by filtration. In order to obtain every trace of carbonate it is well to wash the crucible with several small portions of hot 56 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. water, and add the washings to the rest of the filtrate through the filter. If the salt is completely carbonized the filtrate will be colorless, but if the carbonization is not complete the solution will be more or less colored, and should be rejected, and a fresh quantity of the salt subjected to ignition. To the filtrate, which contains potassium carbonate, N add a few drops of methyl-orange, and titrate with - sulphuric acid V. S. until a light orange-red color appears and the carbonate is neutralized. The following equations will explain the reactions : 2(K,C L H.O..H.O) / + 50, = 2K,CO, + 6CO,+ 6H,O ; 488 276 then 2K 2 CO 3 + 2H 2 SO 4 = 2K 2 SO 4 + 2H 2 O + 2CO 2 ; 276 196 therefore 2(K 2 C 4 H 4 6 .H 2 0) = 2K 2 C0 3 = 2H 2 S0 4 , 4)488 4)276 4)196 N 122 gms. = 69 gms. = 49 gms. = 1000 cc. V. S., N and each cc. of H 2 SO 4 represents 0.122 gm. of po- tassium tartrate. Example. Two gms. of potassium tartrate treated as N described above require 16.3 cc. of -- H 2 SO 4 V. S. It therefore contains 0.122 X 16.3 == 1.9886 gms. 1.9886 X 100 -- = 99-43^ A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 57 Potassium and Sodium Tartrate (Rochelle Salt), KNaC 4 H 4 6 .4H,0 = j ^5 I ^TIfe salt is treated in exactly the same way as described for potassium tartrate. When ignited the double tartrate is converted into a double carbonate of potassium and sodium : 50, = 2KNaCO, + 6CO.+ I2H 2 O ; 244 then 2KNaCO 3 + 2H,S0 4 = 2KNaSO 4 + 2CO a + 2H 2 O ; 244 196 therefore 2KNaC 4 H 4 O 6 .4H a O = 2KNaCO 8 = 2H,SO 4 , 4)564 4)244 4)196 N 141 61 49 =iooocc. V. S. N and each cc. of H 2 SO 4 represents 0.141 gm. of KNaC 4 H 4 O 6 .4H,0. The U. S. P. directs t'hat 1.41 gms. of Rochelle salt when completely decomposed by ignition should leave an alkaline residue, which requires not less than locc. N of H 2 SO 4 for complete neutralization, corresponding to loofo of the pure salt. The factor is 0.141 ; 10 cc. = .141 X 10 = 1.41. 1.41 58 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Potassium Bitartrate (Cream of Tartar), KHC 4 H 4 O 6 = 1 *l88 ^ e es ^ mat ^ on f this salt is affected in the same way as the tartrate. The bitartrate having but one atom of potassium in its molecule, it takes two molecules to form one mole- cule of carbonate. 2 KHC 4 H 4 6 + 5 2 = K 2 C0 3 + ;C0 2 376 138 then K 2 C0 8 + H 2 S0 4 = K 2 S0 4 + H 2 -f CO a ; 138 98 therefore 2KHC 4 H 4 O 6 = K 2 C0 3 = H 2 S0 4 , 2)376 2)138 2)98 N 188 69 49 =1000 cc. of V. S. and each cc. of H 2 SO 4 V. S. = 0.188 gm. of KHC 4 H 4 0, Another way of estimating bitartrate is to dissolve a N weighed quantity in hot water and titrate with po- tassium hydrate until neutral, and thus the amount of tartaric acid existing as bitartrate is found. The bitar- trate is always acid in reaction. This latter is the U. S. P. method. In detail it is as follows: 1.88 gms. of the bitartrate are dissolved in 100 cc. of hot water, a few drops of phenolphthalein T. S. added, N and then titrated with KOH V. S. until a faint pink color indicates that all of the acid has been neutralized. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 59 Not less than 9.9 cc. of the normal alkali should be re- quired, corresponding to 99$ of pure salt. The following equation will show the reaction : KHC 4 H 4 6 + KOH = K 2 C 4 H 4 6 + H 2 O. 188 56 = 1000 cc. of *? KOH V. S. i Each cc.of y KOH V. S. represents .188 gm. of KH C 4 H 4 6 . If 9.9 cc. are required for neutralization, then 9.9 X .188= 1. 8612 gms. i. 8612 X ioo Lithium Citrate, Li 3 C 6 H 6 O 7 = | ^oo,57._ T his salt is estimated in the same way as the other organic salts. I gm. of the salt is thoroughly ignited in a porce- lain crucible, and the resulting lithium carbonate mixed N with 20 cc. of water and titrated with H 2 SO 4 V. S. after having added a few drops of methyl-orange T. S. N Each cc. of the V. S. used before neutralization is effected represents ,070 gm. of pure lithium citrate. The U. S. P. salt requires not less than 14.2 cc. of the '-''"' :: The following are the reactions: 2Li,C.H s O, + 90, = 3 Li,CO s + 5 H,0 + 9 CO, ; 420 222 60 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. then 3 Li 2 C0 3 + 3 H,S0 4 - 3 Li 2 S0 4 + 3 H 2 + 3 CO 2 ; 222 294 therefore 2Li 3 C 6 H 6 7 = 3 Li 2 C0 3 = 3 H 2 S0 4 6)420 6)222 6)294 70 gms. 37 gms. 49 gms. =rooo cc. of the sulphuric acid V. S., and thus each cc. of H 3 SO 4 V. S. = .070 gm. of the pure lithium citrate. If 14.2 cc. of the normal acid are required, then I gm. of the salt contains .070 X 14.2 = .994 gm., or 99.4$. If the more accurate factor .069856 is used, the per cent will be 99.2. Potassium Citrate, K,C.H 6 O 7 .H a O j ^323-59. TWO gms. of the salt are placed in a platinum or porcelain crucible and thoroughly ignited at a red heat in con- tact witli air. The potassium citrate is thus converted into potas- sium carbonate, carbon, and gases. When the crucible is cool, hot water is added to its contents, and the solu- tion of potassium carbonate thus obtained is filtered to separate the carbon. To the solution, which must be colorless, add a few drops of methyl-orange T. S., N and titrate with H 2 SO 4 V. S. until the change of color indicates complete neutralization. Each cc. of the N - H a SO 4 required before neutralization is effected represents 0.108 gm. of the pure salt. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 6l 2(K.C.H.O,.H.O) + 90, = 3 K,CO, + 3 CO, + 7 H,O ; 648 414 then 3 K,CO, + 3 H,SO. = 3 K,S0 4 + 3 CO, + 3 H 5 O ; 414 294 therefore 2K 3 C 6 H 6 O 7 -H 2 O = 3 K 2 CO 3 = 3H a SO 4 . 6) 6 48 6)414 6) 2 94 N 108 gms. 69 gms. 49 gms. = 1000 cc. acid. N Thus each cc, of -- acid represents 0.108 gm. of pure potassium citrate. The U. S. P. directs that 1.080 gms. of potassium citrate be thoroughly ignited at a red heat, and that the alkaline residue should require for complete neutra- N lization not less than 10 cc. of H 2 SO 4 V. S. (corre- sponding to 100$ of the pure salt), using methyl-orange as indicator. The factor, as has been shown, is o. 108 for potassium citrate. .108 X 10= 1.08 1.08 X ioo -Tbir -- 100 * Potassium Acetate, KC 2 H 3 O, = j ,97-89 __ In esti . mating potassium acetate the salt is ignited and the residue treated in exactly the same manner as in the estimation of the citrates and tartrates before men- 62 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. tioned. According to the U. S. P., " if I gm. of potas- sium acetate be by thorough ignition converted into carbonate, the residue should require for complete N neutralization not less than 10 cc. of H 2 SO 4 V. S. (corresponding to at least 98 per cent of pure potas- sium acetate), methyl-orange being used as indicator." 2KC.H.O. + 40, = K,CO S + 3 H,0 + 3 CO, : 196 138 then K 2 CO 3 + H 2 S0 4 = K 2 S0 4 + H a O -f- CO, ; 138 98 therefore 2KC 2 H 3 2 = K 2 C0 3 = H 2 S0 4 . 2)196 2)138 2)98 N 98 gms. 69 gms. 49 gms. = 1000 cc. ~' H 2 SO4 . N Each cc. therefore of H 2 SO 4 V. S. corresponds to .098 gm. of potassium acetate. If IO cc. are required to neutralize the residue from I gm. of potassium acetate, the salt contains 10 X -098 = 0.98 gm., or T 9 Q 8 ^ of I gm., which is Sodium Acetate, NaC 2 H 3 O 2 .3H 2 O = { #Jf|' 74 "- This salt is estimated in the same manner as the potas- sium acetate U. S. P. 1.36 gm. of the salt is ignited until completely carbonized, the residue is treated with hot water, the solution thus obtained is filtered, and to the filtrate a few drops of methyl-orange T. S. N are added, and then the sulphuric acid until neutra- A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 63 lization is effected. 10 cc. of the latter should be re- quired. 2(NaC a H 3 0, . 3H a O) + 40, = Na,CO 3 + 9 H a O 272 106 then Na a CO 3 + H a S0 4 = Na a SO 4 + H a O + CO a ; 106 98 therefore 2(NaC 3 H 3 2 . 3 H U 0) = Na a CO 3 =H 2 SO 4 . 2)272 2)106 2)98 136 gms. 53 gms. 49 gms., or 1000 cc. H 2 SO 4 . Each cc. therefore represents 0.136 gm. of sodium acetate. N If 10 cc. of the acid are required to neutralize, multiply the factor 0.136 gm. by 10 = 1.36 gms. 1.36 X ioo Lithium Benzoate, LiC 7 H B O a j *"7' 72 . This salt when ignited chars, emits inflammable vapors having a benzoin-like odor, and finally leaves a residue of lithium carbonate mixed with free carbon. It may therefore be estimated in the same manner as are the citrates, tartrates, and acetates. One gm. of the salt is placed in a porcelain crucible and thoroughly ignited. The resulting residue, consisting of lithium carbonate and free carbon, is then mixed with 64 A TEXT-BOOK OF VOLUiMETRIC ANALYSIS. about 20 cc. of water and a few drops of methyl-orange. N The titration is then begun, and each cc. of the - H 2 SO 4 V. S. used represents about 0.128 gm. of pure lithium benzoate. The U. S. P. requires the salt to be 99.6^. The reactions are expressed as follows : 2LiC ; HA+ I50 2 = Li 2 C0 3 -f 5 H 2 + i 3 C0 2 ; 256 74 then Li 2 C0 3 + H 2 S0 4 = Li 2 S0 4 + H 2 + CO, ; 74 98 therefore 2 LiC 7 H 6 2 = Li 2 C0 3 = H 2 SO 4 . 2)256 2)74 2)98 N 128 gms. 37 gms. 49 gms. or 1000 cc. H 2 SO 4 . N If 7.8 cc. of H 2 SO 4 V. S. are used to neutralize the residue from the ignition of the lithium benzoate", then .128 X 7-8 = .9984 gm. ; then '" 4 y - = 99.84$ Sodium Benzoate, NaC 7 H B O 2 = *,- J g nite 2 gms. of the salt in a porcelain crucible until com- pletely carbonized. Dissolve the residue in about 20 cc of hot water, filter the solution, rinse the crucible with a little water, and add it through the filter to the A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 65 first filtrate. Then add a few drops of methyl-orange N T. S. and titrate with H 2 SO 4 until neutralization is effected, as shown by the indicator. It should require N not less than 13.9 cc. of the H 2 SO 4 V. S., which cor- responds to 99.8^0 of pure salt. The following are the reactions : 2NaC,H s 5 + i S 0. = Na,C0 3 + 5 H,O + I 3 CO,; 288 106 then Na 2 C0 3 + H 2 SO 4 = Na 2 S0 4 + H 2 O + CO, 106 98 therefore 2NaC 7 H B O a = Na 2 CO 3 = H 2 SO 4 . 2)288 2)106 2)98 N 144 gms. 53 gms. 49gms. or looocc. H 2 SO 4 V. S. N Each cc. of H 2 SO 4 V. S. therefore represents 0.144 gm. of sodium benzoate, or more accurately 0.14371. If 13.9 cc. are required, then the 2 gms. contain 0.14371 X 13-9= i -997 569- 1-997569x100 _. nabout The salicylates of the alkalies are estimated in the same way as are the benzoates, tartrates, etc. 66 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Lithium Salicylate, LiC 7 H 5 O 3 = j ^ 6S . Lith- ium salicylate when heated is decomposed, an odor of phenol is emitted, and a residue of lithium carbonate and carbon is left. It may therefore be estimated as are benzoates, tartrates, citrates, etc. The process is as follows : Two gms. of the salt are ignited in a porcelain crucible, so as to convert it into carbonate. This carbonate is mixed with about 20 cc. of hot water, a few drops of methyl-orange T. S. added, and then titrated with N H 2 SO 4 until neutralized. Not less than 13.8 cc. should be required, each cc. representing 0.14368 gm. of the pure salt. The reactions are: 2LiC 7 H 6 3 + i 4 2 = Li 2 C0 3 + 5H 2 0+ i 3 C0 2 ; 287.36 74 then Li 2 CO, + H 2 S0 4 = Li 2 S0 4 + H 2 + CO, ; 74 98 therefore 2LiC 7 H 5 O 3 = UCO 3 = H a SO 4 . 2)287.36 2)74 2)98 N 143.68 gms. 37 gms. 49 gms. or 1000 cc. acid. N Each cc. of H 2 SO 4 therefore represents 0.14368 gm. of lithium salicylate. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 67 N If 13.8 cc. of H 2 SO 4 are required for neutraliza- tion, then .14368 X 13.8 = 1.982784. 1.982 X ioo - = 99.13$. Sodium Salicylate, NaC,H 5 O, = | *j^ 67 . This salt, when heated, is decomposed, inflammable vapors and an odor of phenol being given off, and a residue of sodium carbonate and free carbon being left. No volumetric process is given in the U. S. P. for the estimation of this salt. The foregoing processes, however, may be applied to it, the alkaline carbonate which is left being titrated with sulphuric acid V. S., N each cc. of H 2 SO 4 V. S. representing 0.15967 gm., or approximately o. 160 gm., of the pure salicylate. 2NaC 7 H 5 3 + I 4 3 = Na 2 C0 3 + 5 H 2 O + i 3 CO a ; 319.34 106 then Na.CC>, + H 2 S0 4 = Na 2 S0 4 + H 2 O + CO, 1 06 98 therefore 2NaC 7 H 5 3 = Na,CO, = H,SO 4 . 2)319.34 2)106 2)98 159.67 gms. 53 gms. 49 gms. or IOQO cc. 68 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. TABLE SHOWING THE APPROXIMATE NORMAL FACTORS, ETC., OF THE ORGANIC SALTS OF THE ALKALINE METALS. Substance. Formula. Molecular Weight. Equivalent Weight in Carbonate. Normal Factor.* Lithium benzoate. ... . . . LiC 7 H B O 2 128 07 O 1 28 " citrate Li 3 C 6 H 6 O 7 2IO III O O~O " salicylate .... LiC 7 H 5 O 3 IAA. Sodium acetate " benzoate NaC 2 H 3 O 2 .3H 2 O NaC 7 H 5 O a 136 Idd. o/ 53 e -3 0.136 O 1.1-1 " salicvlate NaC 7 H 5 O 3 1 60 C o 1 60 Potassium acetate KC,H t Oi 08 60 bitartrate " citrate KHCH 4 Oj K 3 CfH B O 7 H 2 O v 188 6 9 0.188 o 1 08 tartrate K 2 C and fi U a burette with a portion of this solution. Dissolve 0.63 gm. of pure oxalic acid in about 10 cc. of water in a beaker or flask, add a few drops of phe- nolphthalein T. S., and then carefully add from the A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 71 burette the potassium-hydroxide solution, agitating frequently and regulating the flow to drops towards the end of the operation until a permanent pale-pink color is obtained. Note the number of cc. of the po- tassa solution consumed, and then dilute the remainder so that exactly 10 cc. of the diluted liquid will be re- quired to neutralize 0.63 gm. of oxalic acid. Instead of weighing off 0.63 gm. of the acid, 10 cc. of its nor- mal solution may be used. Example. Assuming that 8 cc. of the stronger po- tassa solution had been consumed in the trial, then each 8 cc. must be diluted to 10 cc., or the whole of the remaining solution in the same proportion. Thus if 8 cc. must be diluted to 10 cc., 1000 cc. must be di- luted to 1250 cc. 8 : 10 : : 1000 : x x 1250 cc. It is always advisable to make another trial after diluting. 10 cc. should then neutralize 0.63 gm. of pure oxalic acid. Centinormal Potassium Hydroxide V. S., KOH = { ^5-99 contains j 0-5599 gn- in , litre ._ This is made by diluting 10 cc. of the normal solution with enough distilled water to make 1000 cc. Normal Sodium Hydroxide V. S., NaOH { * 96 C ntainS 40 ^ I-: } '" ' litre-Dissolve 54 gms. of sodium hydroxide in enough water to make about 1050 cc. at 15 C. (59 F.), and fill a burette with a portion of this solution. Dissolve 0.63 gm. of pure oxalic acid in about 10 cc. of water in a flask or beaker, add a few drops of phe- nolphthalein T. S., and then carefully add from a burette 72 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. the soda solution, agitating the flask or beaker fre- N quently, as directed under KOH V. S., until a per- manent pale-pink color is produced. Note the number of cc. of soda solution consumed, and then dilute the remainder of the solution so that exactly 10 cc. will be required to neutralize 0.63 gm. of pure oxalic acid. Example. If 8 cc. of the stronger soda solution had been consumed in the trial, then each 8 cc. must be diluted to 10 cc., or the whole of the remaining so- lution in the same proportion. Thus if 980 cc. should be still remaining, this must be diluted with water to make 1225 cc. Now make a new trial with the diluted solution to see whether 10 cc. will be required to neutralize 0.63 N gm. of oxalic acid (or 10 cc. of oxalic acid V.. S.). The neutralizing power of this solution is exactly N the same as that of potassium hydroxide V. S., and may be employed in place of the latter, volume for volume. The following acids may be tested with either 01 these alkaline solutions : I Acidum aceticum. dilutum. " " glaciale. " citricum. " hydrobromicum dilutum. " hydrochloricum. " " dilutum. hypophosphorosum dilutum, " lacticum. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 73 Acidum nitricum. dilutum. " phosphoricum. dilutum. " sulphuricum. " " aromaticum. dilutum. " tartaricum. Acidum Aceticum, HC 2 H 3 O 2 = | JJ&* 6 . -- The U. S. P. acetic acid contains 36$, by weight, of absolute HC,H,O a and 64$ of water. Mix 3 gms. of the acid with a small quantity of water, add a few drops of phenolphthalein T. S., and titrate with normal potassium hydroxide V. S. until a permanent pale-pink color is obtained, and apply the following equation : HC,H,0 4 + KOH = KC 2 H 3 3 + H,O. 60 56 N Thus 56 gms. or looocc.of KOH V. S. will neutral- ize 60 gms. of acetic acid ; therefore each cc. of N Y KOH V. S. represents .060 gm. of acetic acid. If 1 8 cc. are required to neutralize 3 gms. of the acid, it contains 18 X .060 = 1.08 gms. of absolute acetic acid. 1. 08 X 100 According to the U. S. P., 6 gms. of the acid should re- N quire 36 cc. of y KOH V. S. for complete neutraliza- tion. 74 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Acidum Aceticum Dilutum. A solution contain- ing 6$, by weight, of absolute acetic acid. The estimation is conducted exactly as the above. The diluted acetic acid U. S. P. should contain 6$ of absolute acid. 24 gms. should require 24 cc. of * KOH V. S. 24 X .060 = 1.440 1.440 X IPO ^ g 24 Vinegar. Vinegar is impure diluted acetic acid. Its strength may be estimated in the same manner as acetic acid. Phenolphthalein must be used as an indi- cator. Litmus will give only approximate results, be- cause potassium and sodium acetate both have a slightly alkaline reaction with litmus, but show no reaction with phenolphthalein.* The absence of mineral acids must be assured before the volumetric test is applied. The strength of vinegar may also be estimated by distilling 1 10 cc. until 100 cc. come over. The 100 cc. will contain 80$ of the whole acetic acid present in the 1 10 cc., and may be titrated ; or the specific gravity of the distillate may be taken, and, by consulting the table below, the per cent strength of the distillate found. By adding 20$ to this the strength of the original vinegar is obtained. Vinegar usually contains from 3$ to 6f of acetic acid. * Even dark-colored vinegar may be titrated in this way when diluted. If the color, however, is too dark, litmus-paper or phenol- phthalein paper may be used by bringing a drop of the liquid in con- tact with the paper from time to time during the titration. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 75 ACETIC ACID TABLE. Per cent Per cent Per cent of Absolute Specific Gravity of Absolute Specific Gravity af . J 15 C. of Absolute Specific Gravity Acetic at 1 59 F. Acetic j 59 F. Acetic at j 59 p. Acid. Acid. Acid. I I.OOO7 26 .0363 51 1.0623 2 I.OO22 27 0375 52 1.0631 3 1.0037 28 .0388 53 1.0638 4 I.OO52 29 .0400 54 1.0646 5 1.0067 30 .0412 55 1.0653 6 1.0083 31 .0424 56 I. 0660 7 I.OOgS 32 .0436 57 1.0666 8 I.OII3 33 .0447 58 1.0673 9 I.OI27 34 0459 59 1.0679 10 I.OI42 35 .0470 60 1.0685 ii I.OI57 36 .0481 61 1.0691 12 I.OI7I 37 .0492 62 1.0697 13 I.OI85 38 .O5O2 63 1.0702 14 I.O2OO 39 0513 64 1.0707 15 I.O2I4 40 .0523 65 1.0712 16 1.0228 41 0533 66 1.0717 17 1.0242 42 0543 67 I.072I 18 I.O256 43 .0552 68 1.0725 19 I.O27O 44 .0562 69 1.0729 20 1.0284 45 .0571 70 1-0733 21 1.0298 46 .0580 1.0737 22 I.03II 47 .0589 72 1.0740 23 1.0324 48 .0598 73 1.0742 24 1-0337 49 .0607 74 1.0744 25 1.0350 50 .0615 75 1.0746 ESTIMATION OF FREE MINERAL ACIDS IN VINEGAR. Mr. Hehner has devised the method given below, which has the merit of being speedy, scientific, and accurate. The method is based upon the fact that acetates of the alkalies are always present in commercial vinegar, and when vinegar is evaporated to dryness, and the ash ignited, the acetates of the alkalies are converted into carbonates. If the ash has an alkaline reaction no free mineral acid is present. If, however, the ash is 76 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. neutral or acid some free mineral acid must be present. The quantitative process in detail is as follows : 50 cc. N of vinegar are mixed with 25 cc. of soda or potash V. S. The liquid is evaporated to dryness on a water- bath, and the residue carefully incinerated at the low- est possible temperature, to convert the acetates into N carbonates. When cooled, 25 cc. of sulphuric acid 10 V. S. are added, the mixture heated to expel CO, and filtered. The filter is washed with hot water, phenol- phthalein T. S. added, and the filtrate and washings N N carefully titrated with -- alkali. Each. cc. of alkali 10 10 used represents 0.0049 gm. H 2 SO 4 or 0.003637 gm. HC1. Acidum Aceticum Glaciale. Three grns. of glacial acetic acid are mixed with a small quantity of water, a few drops of phenolphthalein T. S. added, and the solu- N tion titrated with potassium hydroxide V. S. until a very pale pink color appears. Each cc. represents .06 gm. of absolute acetic acid. 49.5 cc. are required by 3 gms. of the U. S. P. acid. 49.5 X .06 = 2.970 gms. 2.970 X IPO = 3 Acidum Citricum, H 3 C 6 H 6 O 7 .H 2 O = j ^' S .- 3.5 gms. of citric acid are dissolved in a sufficient quantity of water, a few drops of phenolphthalein added, N and the solution titrated with potassium hydroxide A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 77 V. S. until a very pale pink color appears. Each cc. of N L potassium hydroxide consumed before neutralization is effected represents .070 gm. of the pure acid, and 50 cc. should be required. The reaction is expressed by the following equation: = K 3 C 6 H 5 7 + 4 H 3 0. 3)210 3)168 70 56 Thus 56 gms, of KOH or 1000 cc. of its normal solu- tion represent 70 gms. of pure crystallized acid, and each cc. represents .070 gm. Therefore 50 X .070 =3.5 gms. Lime-juice or Lemon-juice, the chief constituent of which is citric acid, may be estimated by titrating N with potassium hydroxide V. S. in the same manner as other acid solutions. Lime-juice contains on an average 7.84$, rarely as much as io#, and very seldom as little as 7$ of citric acid. Commercial lime-juice frequently contains sulphuric, hydrochloric, or tartaric acid. Therefore before apply- ing this test the absence of notable quantities of these acids must be insured by qualitative tests. Acidum Hydrobromicum Dilutum (Diluted Hydro- bromic Acid), HBr = j ^ a 7 6 . A liquid containing 10 78 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. per cent, of pure hydrobromic acid (HBr) and 90 per cent, of water. 8.1 gms. of the acid are diluted with a small quan- tity of water, a few drops of phenolphthalein T. S. N added, and then potassium hydroxide V. S. added from a burette, until a very faint pink color is pro- N duced. Note the quantity of alkali used, and mul- tiply this by the factor .081 gm. to obtain the weight of HBr in the diluted acid taken. The reaction is expressed by the following equation : HBr + KOH = KBr + H 2 O. N 81 gms. 56 gms. = 1000 cc. of V. S. Each cc. therefore represents .081 gm., or I per cent, of HBr. If this acid is made with tartaric acid and potassium bromide, a white, crystalline precipitate will be pro- N duced upon the addition of the alkali, some of which will be neutralized by the dissolved potassium bitartrate and the excess of tartaric acid, and an incor- rect indication will be given. Acidum Hydrochloricum (Muriatic Acid), HC1 ) *^64 ^ li*! 11 ^ containing 31.9 per cent., by weight, of absolute HC1 and 68.1 per cent, of water. 3 gms. of hydrochloric acid are diluted with a little water, a few drops of phenolphthalein added, and then N - potassium hydroxide V. S. from a burette, until a A TEXT-BOOK OF VOLUMETRIC ANALYSIS. /Q N faint pink color is produced. Note the quantity of - alkali used, and apply the following equation : HC1 + KOH = KC1 + H 3 0. 36.4 gms. 56 gms. = 1000 cc. V. S. N Each cc. of alkali required before the acid is neu- tralized represents .0364 gm. of pure HC1. 3.64 gms. of the U.S. P. acid should require for com- N plete neutralization 31.9 cc. of KOH V. S. Diluted hydrochloric acid, U. S. P., contains 10 per cent, of absolute HC1. 3.64 gms. of the diluted acid N should require for neutralization 10 cc. of KOH V. S. Let us assume that the 3 gms. of hydrochloric acid required 20 cc. of - KOH V. S. Then 20 X .0364 = .7280 gm. of pure HC1 in 3 gms. of the acid. .7280 X ioo -y- - = 24.26^ Acidum Hypophosphorosum Dilutum (Diluted Hypophosphorous Acid). The U. S. P. acid contains 10 per cent, of absolute HPH 8 O 2 = j **j|' 88 . 80 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. This acid is estimated in exactly the same way as the acids previously noticed : * HPH a 3 + KOH = KPH.O. + H 2 O. 66 gms. = 56 gms. = 1000 cc. alkali. N Thus each cc. of alkali represents .066 gm. of HPH.O,- Take 5 gms. of the acid, dilute it with a small quan- tity of water, add a few drops of phenolphthalein T. S., N and titrate with KOH V. S. until a very faint pink N color appears. If 8 cc. of the alkali are used, the 5 gms. contain 8 X .066 = .528 gm. 5 : .528 : : IOO : x. x = 10.56$ 6.6 gms. of the U. S. P. acid should require for neu N tralization 10 cc. of KOH V. S. Acidum Lacticum, HC 3 H 5 O 3 = j /979._ An or- ganic acid containing 75 per cent., by weight, of abso- lute lactic acid and 25 per cent, of water. 5 gms. of lactic acid are slightly diluted with water, a few drops of phenolphthalein T. S. added, and then the N KOH V. S. from a burette, until a pale-pink color is produced. Note the quantity of normal alkali used, A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 8 1 and multiply that number by .090 gm. to get the quan- tity of absolute acid in the 5 gms. taken. HC 3 H 6 3 + KOH = KC 3 H 6 3 + H Q O, go gms. = 56 gms. = 1000 cc. KOH V. S. and i cc. of y KOH = .090 gm. of HC 3 H 5 O 3 - N If 40 cc. of KOH are required for neutralization of the 5 gms. of the lactic acid, then X 40 -09 = 3-6o gms. 5 : 3.6 : : 100 : x. x = 72% Acidum Nitricum (Nitric Acid), HNO 3 = j *^ 2 ' 89 . The U. S. P. acid contains 68 per cent., by weight, of absolute nitric acid and 32 per cent, of water. Take 3 gms. of nitric acid, dilute with a little water, add a few drops of phenolphthalein T. S., and then N pass into the mixture from a burette -- potassium hydroxide V. S. until neutralization is effected, and the liquid acquires a faint pink color. Apply the following equation : HN0 3 + KOH = KN0 3 + H 2 O. 63 gms. 56 gms. = 1000 cc. KOH V. S. N Thus each cc. of KOH V. S. required before neu- tralization is effected represents 0.063 gm. of absolute nitric acid. 82 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. N If 30 cc. of the alkali are required, then the 3 gms. contain .063 X 30 = 1.890 gms. 3 : 1.89 : : 100 : ;r. ^ = 63$ 3.145 gms. of the U. S. P. acid require 34 cc. of N - KOH V. S., which corresponds to 68$ of absolute acid. Acidum Nitricum Dilutum, U. S. P., contains 10$ of absolute nitric acid, and is estimated in the same way as the nitric acid. Acidum Phosphoricum (Phosphoric Acid), H 3 PO 4 97. ^ ,p he ^ ^ p ac}d contains g^ Q aDSO i u t e 95 orthophosphoric acid and 15$ of water. Take I gm. of phosphoric acid, dilute it witk water, add a few drops of phenolphthalein T. S., and titrate N with potassium hydroxide V. S. until neutralization is complete and the liquid has acquired a faint pink color. 2)98 2)112 49 gms. 56 gms. = 1000 cc. KOH V. S. N Thus each cc. of KOH required represents .049 gm. of absolute orthophosphoric acid. If i gm. of the acid requires for neutralization 18 cc. of y KOH V. S., it contains ,049 X 1 8 = .882 gm. or 88.2^ A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 83 0.98 gm. of the U. S. P. acid should require 17 cc. of KOH V. S., which means 85^ of absolute phos- phoric acid. In the estimation of phosphoric acid litmus cannot be used as an indicator, for the disodic or dipotassic hydric phosphate (Na 2 HPO 4 or K 2 HPO 4 ) which is formed when the standard alkaline solution is added to free tribasic phosphoric acid is slightly alkaline to lit- mus, but not to phenolphthalein. It is recommended, therefore, in order to estimate phosphoric acid alkalimetrically, to prevent the forma- tion of soluble phosphate of the alkali, and to bring the acid into a definite compound with an alkaline earth as follows : The free acid in a diluted state is placed in a flask and a known volume of normal alkali in excess added in order to convert the whole of the acid into a basic salt. A few drops of rosolic acid are now added, and sufficient neutral BaCl 2 solution poured in to combine with the phosphoric acid. The mixture is heated to boiling, and while hot the excess of alkali is titrated with _ acid, i The suspended basic phosphate, together with the liquid, possesses a rose-red color until the last drop or two of acid, after continuous heating and agitation, gives a permanent white or slightly yellowish milky appearance, when the process is ended. The volume of normal alkali, less the volume of nor- mal acid, represents the amount of alkali required to convert the phosphoric acid into a normal trisodic or tripotassic phosphate. 84 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. H 3 P0 4 + 3 KOH = K 3 P0 4 + 3H 2 0. 3)98 3)168 N 32.66 gms. 56 gms. = 1000 cc. of KOH V. S. N Thus i cc. of alkali = .03266 gm. of H 3 PO 4 . Diluted phosphoric acid is estimated in the same manner. Phosphoric Acid may also be estimated by Stolba's method, as Ammonio-magnesian Phosphate. O.2 gm. of phosphoric acid is supersaturated with ammonia water, so as to convert all of the acid into ammonium phosphate and leave an excess of the alkali. H 3 PO 4 + 2NH 4 OH = (NH 4 ) 2 HPO 4 + 2H a O. 98 132 An excess of magnesia mixture* is now added in order to precipitate all of the phosphoric acid in the form of ammonio-magnesian phosphate. (NH 4 ),HP0 4 + MgS0 4 = Mg(NH 4 )P0 4 + NH 4 HSO 4 132 137 The precipitate is washed, first with ammonia water, and then the ammonia is entirely removed by washing with alcohol of 50$ or 60$ strength. The precipitate is now dissolved in a measured ex- N cess of hydrochloric acid V. S., a few drops of methyl-orange T. S. added, and the excess of acid * Magnesia Mixture. Dissolve TO gms. of magnesium sulphate and 20 gms. of ammonium chloride in 80 cc. of water, add 42 cc. of ammonia water, set aside for a few days in a well-stoppered and filter. It should never be used freshly made. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 85 N found by titrating back with potassium hydrate. N The difference between the number of cc. of HC1 N added and the quantity of KOH used gives the quan- tity of HC1 which went into combination with the am- monia-magnesian phosphate. Mg(NH 4 )P0 4 + 2HC1 = NH 4 H a P0 4 + MgCl a . 137 72.8 By consulting the equations given, it will be seen that 72.8 gms. of HC1 are equivalent to 137 gms. of Mg(NH 4 )PO 4 , or 132 gms. of (NH 4 ) 2 HPO 4 , or 98 gms. of H 3 PO 4 . This means that 1000 cc. of a decinormal [ ] solu- tion of HC1, containing 3.64 gms. of the acid, repre- sents ^ of each of these quantities ; and one cc. of N - HC1 thus represents 0.0049 m - of phosphoric acid. In this estimation care must be taken that all free ammonia is removed from the precipitate, and that the whole of the ammonia-magnesian phosphate is decom- N posed by the acid before titration with the alkali. 10 This may be insured by using a rather large excess of the acid and warming. Example. To the precipitate of ammonia-magne- sian phosphate obtained from 0.2 gm. of phosphoric N acid, 50 cc. of HC1 are added. In titrating back 15.3 86 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. N cc. of KOH are required. Hence 34.7 cc. of the acid went into combination with the double salt. Then 34.7 X .0049 0.17003 gm., .17003 X 100 and = 85.01$ of absolute phosphoric acid. This method is said to give good results. Acidum Sulphuricum, H 2 SO 4 = j 97 g 82 . U. S. P. sulphuric acid contains 92.5 per cent., by weight, of ab- soluted sulphuric acid and 7.5 per cent, of water. Aromatic Sulphuric Acid U. S. P. contains 18.5$ of absolute sulphuric acid, by weight. Diluted Sulphuric Acid U. S. P. contains 10$ by weight of absolute sulphuric acid. Operate upon I gm. of the strong acid or upon 5 gms. of either dilute or aromatic sulphuric acid. One gm. of sulphuric acid is diluted with about 10 cc. of water. Add a few drops of phenolphthalein T. S. N and titrate with potassa V. S. until the acid is neu- tralized and the solution has acquired a faint pink N color. Each cc. of alkali solution represents 0.049 gm. of absolute sulphuric acid. The reaction is shown by the following equation : H 3 S0 4 + 2KOH = K 2 S0 4 + 2H,0. 2)98 2)112 N 49 gms. 56 gms. = 1000 cc. of KOH V. S. N If 18 cc. of KOH V. S. are required for the com- plete neutralization of the sulphuric acid, then it con- tains 1 8 X .049 gm. = 0.882 gm. I : 0.882 : : 100 : x x = 88.2$ absolute sulphuric acid. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 87 Diluted Sulphuric Acid is estimated in the same way. Operate upon 5 gms. instead of upon I gm. Aromatic Sulphuric Acid contains ethyl sulphuric acid. Therefore in estimating the sulphuric acid in this preparation it must be boiled with water for a few minutes so as to decompose the ethyl sulphuric acid. The mixture is then allowed to cool, and titrated in N the usual manner with KOH V. S., using phenol- phthalein as indicator. The U. S. P. requires that 4.89 gms. when mixed with 15 cc. of water and boiled for several minutes should, after cooling, be neutralized by not less than 18.5 cc. of -KOH. i Acidum Tartaricum (Tartaric Acid), H 2 C 4 H 4 O 6 = # . Dissolve 3.75 gms. of tartaric acid in suffi- cient water to make a solution, add a few drops of phenolphthalein T. S., and then pass into the solution N from a burette potassium hydroxide V. S. until a faint pink tint is acquired by the solution, and apply the equation H,C 4 H 4 6 J_ 2 KOH = K,C 4 H 4 O 6 + 2 H 2 O, 2)150 2)112 N 75 gms. 56 gms. = 1000 cc. KOH V. S. Thus each cc. required for the neutralization of the acid represents 0.075 gm. If 50 cc. are required, then 50 X .075 3.75 gms, or 88 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. TABLE SHOWING THE APPROXIMATE NORMAL FACTORS, ETC., FOR THE ACIDS. Acid. Formula. Molecular Weight. Normal Factors.* Acetic ... HC 2 H 3 O 2 60 060 Citric. H 3 C 6 H 5 7 .H 2 HBr 210 81 .070 08 1 Hydrochloric HC1 q5 A o^6j. Hypophosphorous HPH 2 O 2 66 066 Lactic HC 3 H 5 O 3 QO OQO Nitric HNO 3 6-? 061 H 3 PO 4 08 O4.Q H 2 SO 4 08 .OJ.Q Tartaric. . H 2 C 4 H 4 O 6 I ^O O7C Phosphoric, after conversion into a neutral phosphate and retitrating with acid = 03266 Phosphoric acid, as ammonia-magnesian phosphate with decinormal acid = 0049 * This is the coefficient by which the number of cc. of normal solu- tion used is to be multiplied in order to obtain the quantity of pure acid in the sample analyzed. ESTIMATION OF THE SALTS OF THE ALKALINE EARTHS. Standard solution of hydrochloric or of nitric acid is preferred by many operators for the titration of caustic or carbonated alkaline earths. These acids have the advantage over most other acids in forming soluble salts. The hydroxides may be estimated by any of the indicators, but as they readily absorb CO 2 out of the air, they are generally contaminated with more or less carbonate, and the residual method should be used, i.e., a known excess of standard acid should be added, the mixture boiled to expel any trace of CO 2 , and reti- trated with standard alkali. The carbonates are of course estimated in the same way. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 89 If methyl-orange is used, heat need not be employed, unless it is impossible to dissolve the substance in the cold. A good excess of acid is, however, generally sufficient. Soluble salts of calcium, barium, and strontium, such as chlorides, nitrates, etc., may be readily estimated as follows : A weighed quantity of the salt is dissolved in water, cautiously neutralized if it is acid or alkaline, phenol- phthalein is added, the mixture heated to boiling, and standard solution of sodium carbonate delivered in from time to time, with boiling until the red color is permanent. This process depends upon the fact that sodium carbonate forms with soluble salts of these bases in- soluble and neutral carbonates. CaCl, + Na,CO 3 = CaCO 3 + 2NaCl. Ba(N0 3 ) 2 + Na 2 C0 3 = BaCO 3 + 2NaNO 3 . Magnesium salts cannot be estimated in this way, as magnesium carbonate affects the indicator. The alkaline earth salts may also be estimated by dissolving them in water, precipitating the base as car- bonate, with an excess of ammonium carbonate and some free ammonia. The mixture is heated for a few minutes, and the carbonate separated by filtration, thoroughly washed with hot water till all soluble matters are removed, and then titrated with normal acid V. S. as carbonate. Normal Sodium Carbonate V. S. Na 2 CO 3 = ] * 06 contains #5* [ gms. in I litre. This solu- tion is made by dissolving 53 gms. of pure sodium car- 90 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. bonate (anhydrous) previously ignited and cooled, in distilled water, and diluting to I litre at 15 C. (59 F.). If pure salt is not at hand the solution may be made as follows : About 85 gms. of pure sodium bicarbonate, free from thiosulphate, chloride, etc., are heated to dull redness (not to fusion) for about fifteen minutes to expel one half of the CO 2 ; it is then cooled under a desiccator. When cool, weigh off 53 gms. and dissolve it in distilled water to I litre at 15 C. (59 F.). This solution should N neutralize acid V. S. volume for volume. i Liquor Calcis (Lime-water), Ca(OH) a = j ^ 3 ' 83 .- The U. S. P. directs lime-water to be estimated with deanormal oxalic acid V. S., using phenolphthalein as indicator. Take 50 cc. of lime-water, add a few drops of phenol- N phthalein, and then carefully from a burette oxalic acid V. S. until the red color is just discharged. 20 cc. N of the acid V. S. should be required for the neutra- 10 lization. This corresponds to 0.14 (0.148) per cent, of calcium hydroxide. Ca(OH), + H 2 C 2 4 . 2H 2 = CaC 2 O 4 + 4 H,O. 20)74 20)126 3.7 gms. 6 3 gms. or 1000 cc. V. S. 10 N Each cc. of oxalic acid V. S. represents .0037 gm. of Ca(OH) 2 . U i * B A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 91 Then .0037 X 20 0.074 gm. .074 X 100 = 0.148$ Syrupus Calcis, U. S. P. (Liquor Calcis Saccharatus, Br. P.). This is estimated in exactly the same way as the lime-water, except that the solution is weighed for analysis, not measured, as its specific gravity is much higher than that of water. Operate upon about 25 grammes. Calcium Carbonate, CaCO 3 = ^'. No meth- od is given for the estimation of calcium carbonate in the Pharmacopoeia, but the following process may be used : One gm. of calcium carbonate is mixed with 5 cc. of water. A measured excess of normal sulphuric acid V. S. is now added, and the solution boiled to drive off the CO 2 . Then add a few drops of phenolphthalein N T. S., and titrate with - alkali V. S. until a faint pink color is obtained. N Note the quantity of alkali used, and deduct this N from the quantity of acid first added, and the remain- der will represent the amount of acid which combined with the calcium. N Each cc. of acid V. S. represents .05 gm. of CaC0 3 . CaC0 3 4- H,S0 4 = CaSO, + H 2 O + CO a . 2)100 2)98 N 50 gms- 49 gms. or 1000 cc. acid V. S. 92 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. N Assuming that 30 cc. of H 2 SO 4 V. S. were added to N the i gm. of CaCO 3 , and that 11 cc. of KOH V. S. were required to bring the mixture back to neutrality, N then 19 cc. of H 2 SO 4 were actually required to saturate the CaCO 3 . Therefore .05 X 19 = -95 gm., or 95$. Calcium Bromide, CaBr 2 = j *^g 43 .This salt when dissolved in water may be estimated directly with normal solution of sodium carbonate. One gm. of the salt is dissolved in a small quantity of water. The solution is neutralized, if it is acid or alkaline, heated to boiling, a few drops of phenol- N phthalein T. S. added, and the solution titrated with - sodium carbonate V. S. delivered cautiously, with boil- ing, until the red color is permanent. CaBr 2 + Na 2 CO 3 = CaCO 3 + 2NaBr. 2)200 2)106 TOO gms. 53 gms. or 1000 cc. Na 2 CO 3 V. S. N Each cc. of Na 2 CO 3 V. S. represents o.i gm. of cal- cium bromide. If 9 cc. are used, the salt contains o.i X 9 .9 gm., or 90$, of pure CaBr 2 . Another way is to add an excess of ammonium-car- bonate solution with some free ammonia to the solu- tion of calcium bromide, in order to precipitate all the base in the form of carbonate. The carbonate is then A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 93 separated by filtration, thoroughly washed with hot water to remove all soluble matters, and then titrated as directed for carbonate. CaBr, = CaCO, = H 2 SO 4 . 2)200 2)100 2)98 100 gms. 50 gms. 49 gms. or 1000 cc. V. S. N Each cc. of acid thus represents o.i gm. of CaBr 2 . See U. S. P. method, page 103. Calcium Chloride, CaCl 2 = -L 110 ^ 5 . This salt ( I IO.o may be estimated in exactly the same way as described for the bromide. CaCl 2 + Na 2 C0 3 = CaCO 3 + 2NaCl. 2)110.8 2)106 55.4 gms. 53 gms. or 1000 cc. V. S. N i cc. Na 2 CO 3 .0554 gm. of CaCl,. CaCl, = CaCO 3 = H 2 SO 4 . 2)110.8 2)100 2)98 55-4 50 49 gms. or 1000 cc. V. S. i N i cc. - H 2 SO 4 = .0554 gm. of CaCl 3 . Barium Chloride, BaCl 2 , and Barium Nitrate, Ba(NO 3 ) a . These two salts are estimated in the same way as the soluble salts of calcium noted in the previous chapter. The factor for BaCl 2 is 0.10385 gm., the factor for Ba(NO 3 ) 2 is 0.13045 gm., using normal volumetric solutions. 94 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Strontium Lactate,Sr(C,H.O 1 ),+3H,O= j .^jjj 5 . 1 -33 g ms - f the salt, rendered anhydrous before being weighed, by careful drying at 110 C. (230 F.), is ignited, until most of the carbon has disappeared, and then mixed with 10 cc. of water. A few drops of methyl orange T. S. are now added, and the mixture N titrated with -- H 2 SO 4 V. S. until a faint red color is produced. N 9.9 cc. of the acid should be required, correspond- ing to 98.6$ of the pure salt. The first step in this process is to drive off the water of crystallization. (Sr(C,H A), + 3 H,0) + heat = Sr(C 3 H s O s ), + 3 H,O ; 318.78 264.88 then Sr(C,H B 3 ), + 60, = SrC0 3 + 5 CO 2 + 5 H,O. 264.88 147.15 Thus Sr(C 1 H i O i ).= SrCO i =H 1 S0 4 . 2)264.88 2)147.15 2)98 N 132.44 73.57 49 S ms - or IO cc - ~ acid V. S. N Thus each cc. of H 2 SO 4 represents 0.13244 gm. of pure anhydrous strontium lactate. If 9.9 cc. are required, then 0.13244 X 9-9 = I.3UI56 gms. '3IIIS6XIOQ = ^ 1-33 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 95 In this process, if the ignition is carried too far, the strontium carbonate is decomposed into strontium oxide. Magnesium salts may be estimated by precipitating as ammonia-magnesian phosphate, and titrating this precipitate as directed for phosphoric acid. 06 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. CHAPTER X. ANALYSIS BY PRECIPITATION. THE general principle of this method is that the determination of the quantity of a given substance is effected by the formation of a precipitate, upon the addition of the standard solution to the substance under examination. \+* The end of the reaction is determined in three ways : 1. By adding the standard solution until no further precipitate occurs, as in the estimation of chlorides, etc., by silver nitrate. 2. By the use of an indicator. This may either be contained in the liquid under analysis ; or used exter- nally, by frequently bringing a portion of it in contact with a drop of the liquid during the titration. The titration is continued until the slightest excess of the standard solution is shown by the production of a characteristic reaction with the indicator. 3. By adding the standard solution until a precipi- tate is produced, as in the estimation of cyanogen by standard silver solution. The first of these endings can only be applied with accuracy to silver and chlorine estimations, as the silver chloride which is formed is almost perfectly insoluble and has a tendency to curdle closely by shaking, so as to leave a clear supernatant liquid. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 97 Most of the other precipitates, such as barium sul- phate, calcium oxalate, etc., although heavy and insol- uble, do not readily and perfectly subside, because of their finely divided or powdery nature. They must therefore be excluded from this class. In these cases, therefore, it is necessary to find an indicator which brings them into class 2. The third class comprises only two processes, viz., the determination of cyanogen by silver, and that of chlorine by mercuric nitrate. ESTIMATION OF HALOID SALTS. The estimation of these salts is based upon the powerful affinity existing between the halogens and silver, and the ready precipitation of the resulting chloride, bromide, or iodide. Standard solution of silver nitrate is used for this purpose, and for the sake of exactness and conven- ience is made of decinormal strength, and in many cases it is advisable to use centinormal solutions. /N\ The Decinormal Silver Nitrate V. S. is offi- \io/ cial. AgN0 3 = -j *^ 55 I ^' 955 igms. are contained 1*169.7 16.97 f* in i litre. Dissolve 16.97 gms. of pure silver nitrate in sufficient water to make, at or near 15 C. (59 F.), exactly 1000 cc. I litre of this solution thus contains T V of the molecular weight in grammes of silver nitrate. It is therefore a decinormal solution. If pure crystals of silver nitrate are not readily ob- tainable, and pure sodium chloride is at hand, a solu- tion of the silver nitrate may be made of approximate 98 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. strength, a little stronger than necessary, and then standardized by means of the sodium chloride, as fol- lows : 0.117 gm. of sodium chloride is dissolved in water, and a burette is filled with the solution of silver nitrate to be standardized. The silver solution is now slowly added from the burette to the sodium-chloride solution contained in a beaker until no more precipi- tate of silver chloride is produced. If neutral potassium chromate is used as an indi- cator, the end of the reaction is shown by the appear- ance of yellowish-red silver chromate. This indication is extremely delicate. The silver nitrate does not act upon the chromate until all of the chloride is converted into silver chloride. In the above reaction 20 cc. of silver nitrate should be required. But since the silver-nitrate solution is too strong, less of it will complete the reaction, and the solution must be diluted so that exactly 20 cc. will be required to precipitate the chlorine in 0.117 g m - f NaCl. Thus if 17 cc. are used, each 17 cc. must be diluted to 20 cc., or each 170 cc. to 200 cc., or the entire re- maining solution in the same proportion. After dilution a fresh trial should always be made. Nitrate of silver solution should be kept in dark amber-colored, glass-stoppered bottles, carefully pro- tected from dust. Titration by decinormal silver nitrate V. S. may be managed in various ways, adapted to the special prep- aration to be tested. I. In most cases it is directed by the U. S. P. to be used in the presence of a small quantity of potassium chromate T. S. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 99 2. In some cases it is added until the first appear- ance of a permanent precipitate, as in potassium cya- nide and hydrocyanic acid. 3. It may be used in all cases without an indicator by observing the exact point when no further precipi- tate occurs. But since this consumes too much time in waiting for the precipitate to subside, so as to render the supernatant liquid sufficiently clear to recognize whether a further precipitate is produced by the addi- tion of the silver solution, it is impracticable. It may, however, be practised in the case of ferrous iodide, where the addition of potassium chromate T. S. would be improper, since it reacts with the iron. 4. It may be added in definite amount, known to be in excess- of the quantity required, and the excess measured back by titration with decinormal potassium sulphocyanate V. S., or even with decinormal sodium chloride V. S. (residual titration). N In case an excess of the - - silver nitrate V. S. is 10 added accidentally, it is only necessary to add a defi- N nite volume of a solution of the salt under exami- 10 N nation, and finish the titration with - - silver nitrate, deducting, of course, the same number of cc. of silver solution as has been added of the salt solution. Ammonium Bromide, NH 4 Br = -j #97-77. 3 of the salt are dried at 100 C. (212 F.) and dissolved in water to the measure of 100 cc. 10 cc. of this solu- tion are placed in a beaker, a few drops of potassium chromate T. S. added, and then the decinormal silver 100 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. nitrate V. S. carefully added from a burette, until a permanent red coloration is produced. The red col- oration is due to the formation of red chromate of silver, which takes place after all of the bromine has combined with the silver. Apply the equation : NH 4 Br + AgNO 3 = AgBr + NH 4 NO,. 10)97.77 10)169.7 N 9-777 gms. 16.97 gms. or 1000 cc. AgNO 3 V. S. N Thus each cc. of the V. S. represents .009777 gm. of NH 4 Br. 3 gms. of the U. S. P. salt should require 30.9 cc. of 5 AgNO, V. S. But as a rule this salt contains an impurity (am- monium chloride) which will be precipitated by the silver nitrate as well as the bromide. The presence of this impurity must therefore be taken into account in calculating the percentage of bromide. NH 4 C1 + AgNO, == AgCl 3 + NH 4 NO 3 . io)53.38 _ 5.338 16.97 gms. = 1000 cc. of V. S. The amount of the salt examined equivalent .to N IOOO cc. of silver solution is first calculated by simple proportion : 30.9 cc. : .3 gm. : : 1000 cc. : x. x = 9.708. Then 9777 -9708 =y. y A TEXT-BOOK OF VOLUMETRIC ANALYSIS. IO1 N .069 = the excess of AgNO 3 V. S. used up by the ammonium chloride, reckoned in terms of bromide (NH 4 Br) ; and since 5.338 gms. of NH 4 C1 = 9777 gms. of NH 4 Br, the excess which NH 4 C1 can consume is represented by 9.777 5.338 = 4-439- Therefore, as 4-439 : 5-338 : : .069 : z. z = 0.08297. 0.08297 = the amount of ammonium chloride present in x grammes of the sample taken. Lastly, calculate the percentage by simple propor- tion : 9.708 : .0829 : : 100 : P. P= 0.85$ of NH 4 C1. Lithium Bromide, LiBr = | J? 6 ' 77 . Dissolve 0.3 ( 7 gm. of dry lithium bromide in 10 cc. of water, add 2 drops of potassium chromate T. S., and then titrate with decinormal silver nitrate V. S. until a permanent red color of silver chromate makes its appearance. N 0.3 gm. of the U.S. P. salt requires 35.3 cc. of -- V. S. LiBr + AgNO 3 = AgBr + LiNO 3 . 10)86.77 10)169 -7 N 8.677 gms. 16.97 gms. or 1000 cc. AgNO 3 V. S. Thus each cc. of AgNO 3 V. S. = 0.008677 gm. cf pure lithium bromide. Potassium Bromide, KBr = j *j* 8 ' 79 . Operate upon o.i gm. of the salt dissolved in about 10 cc. of IO2 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. water. Add a few drops of potassium chromate T. S., N and titrate with AgNO 3 V. S. until a permanent red color of silver chromate is produced. According to the U. S. P., 0.5 gm. of the well-dried salt should require 42.85 cc. of ^ AgN0 3 V. S. KBr + AgNO 3 = AgBr+ KNO 3 . io)"8-79 10)169.7 N 11.879 S ms - l6 -97 gms. or 1000 cc. AgNO 3 V. S. Thus each cc. represents .011879 gm. of KBr. Po- tassium chloride is a common impurity; to calculate it proceed as for NH 4 C1, 74.4 of KC1 being equal to 118.79 of KBr - Sodium Bromide, NaBr = j *j 2 ' 76 - This salt is tested in exactly the same manner as the potassium bromide. A convenient quantity to operate upon is O.I gm. The U. S. P. directs that 0.3 gm. of the well-dried salt be dissolved in 10 cc. of water, two drops of potas- sium chromate T. S. added, and the mixture titrated with decinormal silver nitrate V. S. until a permanent red color of silver chromate appears. Note the number of cc. required to produce this effect, and multiply this number by the factor 0.010276 gm. This will give the quantity of NaBr present in the sample taken. According to the U. S. P., not more than 29.8 cc. of the standard silver solution, corresponding to at least 97.29$ of the pure salt, should be required. The chloride which is present as an impurity may A TEXTBOOK OF VOLUMETRIC ANALYSIS. 1 03 be calculated in the same manner as ammonium chlo- ride, 5.837 gms. of the chloride being equal to 10.276 gms. of sodium bromide. Calcium Bromide, CaBr 2 j **99' 43 . This salt may be tested as described on page 92. The U. S. P. method is as follows: 0.25 gm. of the well-dried salt is dissolved in 10 cc. of water; 2 drops of potassium chromate T. S. are then added, and the solution titrated with decinormal silver nitrate V. S. until a permanent red color is produced. 25 cc. of the standard silver-nitrate solution should be required to produce this result, corresponding to 99.7$ of the pure salt, a greater amount of the standard solution indicat- ing the presence of calcium chloride, a smaller amount indicating other impurities. CaBr 2 + 2AgNO 3 = 2AgBr + Ca(NO 3 ) 2 . 2)199.43 2)339.4 10)99.715 10)169.7 N 9.9715 gms. 16.97 gms. or 1000 cc. AgNO 3 V. S- N Thus each cc. of AgNO 3 V. S. represents .0099715 gm. of CaBr 2 . Therefore 25 cc. represent .0099715 X 25 = 0.2492875 gm. .2492875 X IPO 0.25 ^ , Strontium Bromide, SrBr, + 6H 2 O = j This salt is tested volumetrically, according to the U. S. P., in the following manner: 0.3 gm. of strontium bromide, rendered anhydrous by 104 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. thorough drying before being weighed, is dissolved in 10 cc. of water, 3 drops of potassium dkchromate T. S. are added, and then the decinormal silver nitrate V. S. is poured in from a burette until all of the bromide has combined with the silver nitrate and a permanent red coloration is produced. Not more than 24.6 cc. of decinormal silver nitrate V. S. should be required, corresponding to at least 98$ of the pure salt. SrBr 2 + 2AgN0 3 = 2AgBr + Sr(NO 3 ) 2 . 2)246.82 2)339.4 10)123.41 10)169.7 N 12.341 gms. 16.97 gms. or 1000 cc. AgNO 3 V. S. N Thus each cc. of AgNO 3 V. S. represents 0.012341 gm. of strontium bromide. Zinc Bromide, ZnBr 2 = | ^' 62 . This salt is es- timated as follows : 0.3 gm. of the dry salt is dissolved in 10 cc. of water, 2 drops of potassium chromate T. S. are added, and then decinormal silver nitrate V. S. is poured in from a burette until all of the bromide has combined with silver nitrate, and a permanent red color is produced. Note the number of cc. of the standard silver solution used, and multiply this num- ber by the factor shown by the following equation, to obtain the amount of pure zinc bromide in the quan- tity taken : ZnBr 2 + 2AgNO 3 = 2AgBr + Zn(NO 3 ) 2 . 20)224.62 20)339.4 N 11.231 gms. 16.97 gms. or 1000 cc. AgNO 3 V. S. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. IO5 Thus each cc. represents .011231 gm. of pure ZnBr 3 . The U. S. P. salt should require 26.7 cc. of decinormal silver nitrate V. S. to produce the desired reaction, cor- responding to not less than 99.95$ of the pure salt. Thus 0.011231 X 26.7 = 0.2998677 gm. 0.2998677 X IPO Potassium Iodide KI = . This is esti- ide, KI = j mated, according to the U. S. P., in a manner similar to the haloid salts just considered. 0.5 gm. of the well-dried salt is dissolved in 10 cc. of water, 2 drops of neutral potassium chromate T. S. N are added, and then the AgNO 3 V. S. slowly added from a burette until a permanent red color of silver chromate is produced. Not more than 30.25 cc. nor less than 30 cc. of decinormal silver nitrate V. S. should be required. This quantity corresponds to 99.5$ of the pure salt. KI + AgNO 3 10)165.56 10)169.7 N 16.556 gms. 16.97 gms. or 1000 cc. AgNO 3 V. S. N Each cc. of - - AgNO 3 V. S. thus corresponds to 0.016556 gm. of potassium iodide. Thus 0.016556 X 30 = 0.49668 gm. 0.49668 X IQQ = 99 3 ^ 106 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Potassium iodide may also be estimated volumet- N rically by mercuric chloride V. S., the termination of the operation being indicated by the formation'of a red precipitate. 4KI + HgCl a = 2KC1 + HgI 2 .2KI (soluble), (i) 2HgI 2 . . (2) This process originated with M. Personne, and is founded on the fact that if a solution of mercuric chlo- ride be added to one of potassium iodide, in the pro- portion of one equivalent of mercuric chloride to four of potassium iodide, red mercuric iodide is formed, which dissolves at once to a colorless solution. The slightest excess of mercuric chloride will cause a bril- liant red precipitate to make its appearance, HgI 2 . 4KI + HgCl 2 = 2KCl+HgI 3 .2KI (soluble). 20)662.24 20)270.54 33.112 gms. 13.527 gms. or 1000 cc. of standard solution. Thus each cc. of standard solution of the above strength represents 0.033112 gm. of potassium iodide, which means that I cc. is the largest quantity of this standard solution which can be added to 0.033112 gm. of potassium iodide without producing a permanent precipitate. The above solution of mercuric chloride is not N strictly a V. S. Potassium iodide is a univalent salt ; 20 and since four molecules of it are precipitated by one molecule of mercuric chloride, the latter is chemically equivalent to four atoms of hydrogen ; and \ of its A TEXT-BOOK OF VOLUMETRIC ANALYSIS. IO? molecular weight in grammes, dissolved in water to N one litre, is a normal solution, and ^ of this is a V. S. The author of this process states that neither chlo- rides, bromides, nor carbonates interfere with the re- action. Sodium Iodide, Nal = j *j 49 ' JJ 3 . Dissolve 0.5 gm. of the well-dried salt in 10 cc. of water, add 2 drops of potassium chromate T. S., and then pass N into the solution from a burette AgNO 3 V. S. until a permanent red coloration is produced. Note the number of cc. used, and multiply this by the factor. Nal + AgNO 3 = Agl + NaNO 3 . 10)149.53 10)169.7 N I 4-953 S ms - J 6'97 g ms - r 1000 cc. AgNO 3 V. S. N Each cc. of AgNO 3 V. S. represents 0.014953 gm. of Nal. N Assuming that 33.4 cc. of -- AgNO 3 V. S. were re- quired, each representing 0014953 gm. of Nal, then the quantity tested contained 33.4 X 0.014953 gm. or 0.4994302 gm. 0.49943Q2 X IPO 8 v 0.5 The U. S. P. requirement is that the salt contain at least, of pure Nal. 108 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Strontium Iodide, Sri, + 6H 2 O = \ ^1' 12 .~ 0.3 ( 44^-3 gm. of strontium iodide, rendered anhydrous before being weighed, is dissolved in 10 cc. of water, 3 drops of potassium dichromate T. S. are then added, and the N AgNO 3 V. S. run in from a burette until a perma- nent red coloration is produced. Apply the following equation : SrI 2 + 6H 2 + 2 AgN0 3 = 2 Agl + Sr (NO 3 ) 2 + 6H 2 O. 2)448.12 2)339.4 10)224.06 10)169.7 M 22.406 gms. 16.97 gms. or 1000 cc. AgNO 3 V. S. - N This equation shows that each cc. of the AgNO 3 V. S. represents 0.022406 gm. of Srl a . Zinc Iodide, ZnI 2 = j ^ jg' 1 . Dissolve 0.5 gm. of dry zinc iodide in 10 cc. of water, add 2 drops of po- tassium chromate T. S., and then run into the mixture N from a burette AgNO 3 V. S. until a permanent red color is produced, indicating that all of the iodide has been precipitated in the form of silver iodide. Each cc. N of the silver solution used represents 0.015908 gm. of zinc iodide. ZnI 2 + 2AgN0 3 = 2 Agl + Zn(N0 3 ) a . 2)31^.16 2)339.4 10)159.08 10)169.7 N 15.908 gms. 16.97 gms. oriooocc. AgNO 3 V. S. A TEXT- BOOK OF VOLUMETRIC ANALYSIS. 109 The U. S. P. directs that not less than 31 cc. nor N more than 31.4 cc. of AgNO 3 V. S. be required to produce the desired result, 31 cc. corresponding to 98.62$ and 31.4 cc. to 99.9$ of pure zinc iodide. 0.015908 X 31.4 = 0.4995112 gm. of ZnI 3 . Then . : : 0.4995112x100^ Ammonium Chloride, NH 4 C1 = j *H'f- It is estimated in the same manner as the other soluble haloid salts. A weighed quantity of the salt is dissolved in a small quantity of water and the solution titrated with - silver-nitrate solution until no more precipitation takes place, or, if potassium chromate T. S. has been added as indicator, until a red color makes its appear- ance. NH 4 C1 + AgN0 3 = AgCl + NH 4 N0 3 . 10)53.38 10)169.7 N 5.338 gms. 16.97 gms. or 1000 cc. V. S. N Thus each cc. of V. S. used represents 0.005338 gm. of NH 4 C1. Potassium Chloride, KC1 = j ^4-40__ Thisisesti _ mated in the same manner as the. above, applying the following equation : KC1 + AgN0 3 = AgCl + KNO 3 . io)74-4 10)169.7 N 7.44 gms. 16.97 gms. or looocc. AgNO 3 V. S. 1 10 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. N Thus each cc. of V. S. represents 0.00744 gm. of KC1. Sodium Chloride, NaCl = j Jg' 37 . A weighed quantity of the well-dried salt, say 0.2 gm., is dissolved in about 10 cc. of water and the solution mixed with N a few drops of potassium chromate T. S. Then : AgNO, V. S. is run in from a burette until all the chloride is precipitated and a permanent red color of silver chromate is produced. The U. S. P. directs that 0.195 gm. of the salt should N require not less than 33.4 cc. of AgNO 3 V. S. to pro- duce this reaction. The following equation shows the reaction which takes place between the sodium chloride and the silver nitrate : NaCl + AgN0 3 = AgCl + NaNO 3 . 10)58.37 10)169.7 ^ 5.837 gms. 16.97 gms. or looocc. AgNO 3 V. S. Each cc. of the standard solution thus represents 0.005837 gm. of NaCl. 005837 X 33-4 = 0.194958 gm. of NaCl. 0.194958^000 = 0.195 Zinc Chloride, ZnCl. - { *\lffi- - This salt is tested in exactly the same way as the other haloid salts. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Ill Dissolve 0.3 gm. of the dry salt in about 10 cc. of water, add a few drops (2 drops) of potassium chromate N T. S., and then run into the mixture from a burette, AgNO 3 V. S. until a permanent red color is produced. It should require 44.1 cc. of the standard silver solu- tion to produce this result. The reaction is shown by the following equation : ZnCl, + 2AgN0 3 = 2AgCl + Zn(NO 8 ) 2 . 2)135.84 2)339.4 io) 67.92 10)169.7 6.792 gms. 16.97 gms. or 1000 cc. V. S. N Thus it is seen that each cc. of the - AgNO 3 V. S. represents 0.006792 gm. of ZnCl 2 . 0.006792 X 44-i = 0.2995272 gm. 0.2995272 X IPO = 99.84$ The U. S. P. requires 99.! Syrupus Acidi Hydriodici, a syrupy liquid contain- ing about ij> of HI U. S. P. HI = | ^7-53. Oper- ate upon 15 grammes. The reaction which occurs is as follows : HI + AgN0 3 = Agl + HN0 3 . 10)127.5 10)169.7 N 12.75 g ms - J 6.97 gms. or 1000 cc. - AgNO 3 V. S. The end of the reaction is shown by the cessation of the formation of a precipitate. 112 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Since nitric acid is liberated, potassium chromate is not admissible as indicator. The U. S. P. directs that the syrup be neutralized by ammonia water before titration. This prevents the formation of nitric acid, and admits of the use of potassium chromate as indicator. (31.875) *32 gms. of the syrup, neutralized, and mixed with 2 drops of the indicator, should require 25 cc. of decinormal silver-nitrate solution to produce a permanent red tint. Each cc. represents 0.01275 gm. of HI. 0.01275 X 25 =0.31875 gm. 0.31875 X IPO . 31.875" Syrupus Ferri lodidi, a syrup containing about 10$ by weight of ferrous iodide (FeI 2 ) U. S. P. FeI 2 = j # 394. Take 2 gms. of the syrup, mix it with a N small quantity of water, and run in the silver solu- tion. The close of the reaction is shown by the cessa- tion of the formation of a precipitate. Potassium chromate is not admissible as an indicator in this case. FeI 2 + 2AgN0 3 - 2AgI + Fe(NO 3 ),. 2)308.94 2)339.4 10)154.47 10)169.7 N 15.447 gms. 16.97 gms. or 1000 cc. V. S. TO Thus each cc. represents 0.015447 gm. of ferrous iodide. The U. S. P. method originated with Volhard. It has the advantage over the direct method for haloids A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 113 with chromate indicator, in that it may be used in the presence of nitric acid. It thus enables the haloids to be estimated in the presence of a phosphate or other salt which precipitates silver in a neutral, but not in an acid solution. It depends upon entirely precipitating the chloride, in the presence of nitric acid, by a known excess of standard solution of silver nitrate, and then estimating the excess of silver left uncombined, by the aid of a standard solution of potassium sulphocyanate, using ferric alum as an indicator. The sulphocyanate has a greater affinity for silver than it has for iron, and therefore so long as any silver is in solution, the sulphocyanate will combine with it and form a precipitate of silver sulphocyanate. As soon as the silver is all taken up, the sulphocya- nate will combine with the ferric alum and strike a brownish-red color. The sulphocyanate solution is to be made of such strength that it corresponds with the silver solution, volume for volume. The difference between the volume of silver solu- tion originally added, and the volume of sulphocyanate solution used, will give the volume of silver solution equivalent to the haloid salt present. Decinormal Potassium Sulphocyanate V. S. (Vol- hard's Solution), KSCN = \ # 9&99 ' 9^99 1 gms . in ( *97 *9-7 ) * I litre. Dissolve 10 gms. of pure crystallized potas- sium sulphocyanate (thiocyanate) in 1000 cc. of water. This solution, which is too concentrated, must be adjusted so as to correspond in strength exactly with decinormal silver nitrate V. S. For this purpose in- 114 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. N troduce into a flask 10 cc. of AgNO 3 V. S., 0.5 cc. of ammonio-ferric sulphate T. S., and 5 cc. of diluted nitric acid. Run into this mixture from a burette the sulphocya- nate solution. At first a white precipitate of silver sulphocyanate is produced, giving the fluid a milky appearance, and then, as each drop of sulphocyanate falls in, it is sur- rounded by a deep brownish-red cloud of ferric sulpho- cyanate, which quickly disappears on shaking, as long as any of the silver nitrate remains unchanged. When the point of saturation is reached and the silver has all been precipitated, a single drop of the sulphocyanate solution produces a faint brownish-red color, which does not disappear on shaking. Note the number of cc. of the sulphocyanate solu- tion used, and dilute the whole of the remaining solution so that equal volumes of this and of the decinormal silver nitrate V. S. will be required to pro- duce the permanent brownish-red tint. (The same tint of brown or red to which the volumetric solution is adjusted must be attained when the solution is used in volumetric testing.) Assuming that 9.5 cc. of the sulphocyanate solution were required to produce the reaction, then each 9.5 cc. must be diluted to make 10 cc., or the whole of the remaining solution in the same proportion. Always make a new trial after the dilution to see if the solutions correspond. The U. S. P. method for estimating syrup of ferrous iodide is as follows : 1.5447 gms. (*i.55 gms.) of the syrup and 10 cc. of A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 11$ water are introduced into a flask, 1 1 cc. of decinormal silver nitrate V. S. are added, then 5 cc. of diluted nitric acid, and 5 cc. of ferric ammonium sulphate T. S. The decinormal potassium sulphocyanate V. S. is now run into the mixture from a burette until a reddish- brown tint is produced, which does not disappear upon shaking. Not more than I cc. should be required. This corresponds to 10$ of ferrous iodide. The re- actions which take place are shown by the following equations: Pel, + 2AgNO s = 2AgI + Fe(NO 3 ),; . (i) 15.447 gms. 16.97 gms. or 1000 cc. AgNO 3 V. S. 10 AgN0 3 + KSCN = AgSCN + KNO 3 ; . (2) 16.97 gms. 9.699 gms. or 1000 cc. KSCN V. S. Fe,(NH 4 ),(S0 4 ) 4 - = Fe 2 (SCN) 6 + (NH 4 ) 3 S0 4 + 3K 2 SO, (3) The Fe a (SCN) 6 gives the brownish-red color to the solution. The object of the nitric acid is to acidulate the solu- tion, facilitate the precipitation of the silver, and to oxidize the ferrous nitrate. In the above case 1 1 cc. of silver nitrate are originally added. If I cc. of potassium sulphocyanate be re- quired, it shows that I cc. of the silver-nitrate solution was in excess, and that 10 cc. went into combination with the ferrous iodide. The equation shows us that Il6 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. each cc. of silver nitrate V. S. represents 0.015447 gm. of ferrous iodide ; then 10 cc. represent 0.015447 X 10 = 0.15447 gm., and -1544^X200 1-5447 of FeI 3 in the U. S. P. syrup. Saccharated Ferrous Iodide. The process for estimating this compound is exactly the same as that for syrup of ferrous iodide. 1.5447 gms. (*i.55 gms.) of the saccharated ferrous iodide are dissolved in about 20 cc. of water in a small flask, and to this solution is added first 22 cc. of N AgNO 3 V. S., then 5 cc. of diluted nitric acid, and N 5 cc. of ferric ammonium sulphate T. S. The KSCN V. S. is then run in, from a burette, until the reddish-brown color of ferric sulphocyanate is produced. N Not more than 2 cc. of the KSCN V. S. should be required. This corresponds to 20% of pure ferrous iodide. N 22 cc. of silver nitrate 10 N 2 cc. of potassium sulphocyanate N = 20 cc. of silver nitrate, 10 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 1 17 which reacted with the ferrous iodide, then 0.015447 X 20 = 0.30894 gm., 0.30894 X IPO _ 1-5447 Syrup of Ferrous Bromide, U. S. P. 1880, FeBr 2 = *4. This syrup may be tested in the same manner as the syrup of ferrous iodide, either by the direct method, using the cessation of precipitation as the end reaction, or by the residual method with potassium sulpho- cyanate. The factor is 0.01077. Hydrocyanic Acid, HCN = j ^6.9^_ Dilute hy _ drocyani cacid may be estimated by weighing out about 5 gms., and adding to this sufficient soda or potassa solution to convert the acid into sodium or potassium cyanide (NaCN or KCN), and leave the solution strongly alkaline. To this solution is added the decinormal silver- nitrate solution until a permanent turbidity occurs. This turbidity is due to the precipitation of silver cyanide, and affords a delicate proof of the completion of the reaction. The difficulty experienced in this process is in the conversion of the acid into the cyanide. The sodium cyanide has a strong alkaline reaction, turning litmus blue when only a small proportion of the acid has been neutralized. If the titration is conducted before the acid is com- pletely neutralized, that which is free will not be acted Il8 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. upon. Indeed, cyanide of sodium may be estimated in the presence of hydrocyanic acid in this way. According to Senier, the following procedure will answer well : To the dilute hydrocyanic acid add soda solution to a strong alkaline reaction, determined by litmus tinc- N ture. Then titrate with - - silver nitrate V. S., drop 10 by drop, from the burette. If the liquid becomes acid, add a little more soda solution to bring it back to alkalinity, and continue the titration until the turbidity indicates the end of the reaction. The liquid must be kept alkaline throughout the process. It is not well to add too much soda solution at the beginning, as this would use up too much of the silver solution, and make the reading a trifle too high. The following equations, etc., explain the reactions : 2HCN + 2NaOH = 2NaCN + 2H 2 O ; 10)53.96 10)97.96 5.396 gms. 9.796 gms. 2NaCN + AgNO, = AgCN,NaCN + NaNO 3 . 10)97.6 10)169.7 N 9.796 gms. 16.97 gms. or 1000 cc. V. S. It is seen that 5.396 gms. of real HCN are equivalent to 9.796 gms. of sodium cyanide, and represent 16.97 N gms. of silver nitrate, or looocc. of the V. S. That - N is, 1000 cc. of the AgNO 3 V. S. may be added to a solution containing 9.796 gms. of sodium cyanide, and A TEXT-BOOK OF VOLUMETRIC ANALYSIS. I IQ no precipitate be produced ; but if one or two drops more of the standard solution be added, a precipitate is at once formed. N Each cc. of AgNO 3 V. S., which fails to produce a precipitate with a solution of sodium cyanide, repre- sents 0.009796 gm. of NaCN, which is equivalent to .005396 gm. of HCN. With 2 molecular weights of sodium or potassium cyanide, one molecule of silver nitrate forms a double salt, having the composition NaCN,AgCN, and which is soluble. When more silver-nitrate solution is added, this solu- ble double salt is decomposed, and a precipitate of silver cyanide occurs, thus: AgCN,NaCN + AgNO 3 = 2AgCN + NaNO 3 . The U. S. P. method is as follows : A weighed quantity of the acid is mixed with suffi- cient of an aqueous suspension of magnesia to make an opaque and decidedly alkaline mixture. To this a few drops of potassium chromate T. S. are N added, and the - silver solution delivered from a burette until the red color of silver chromate appears. 1.35 gms. of the diluted acid is mixed with enough water and magnesia to make an opaque mixture of about 10 cc. Add to this 2 or 3 drops of potassium chromate T. S., and then from a burette deliver the decinormal silver nitrate V. S. until a red tint is pro- duced which does not again disappear by shaking. 120 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Each cc. of the standard silver solution used, repre- sents 0.002698 gm. of absolute HCN. HCN + AgNO 3 = AgCN + HNO 3 . 10)26.98 10)169.7 2.698 gms. 16.97 gms. or 1000 cc. silver nitrate V. S. 10 Potassium Cyanide, KCN = j *>' 1 . This salt may be estimated in the following manner : I gm. of the salt is dissolved in sufficient water, and into the solution, is delivered in drops the standard silver solution until a precipitate appears which is not redissolved on agitation. If 0.65 gm. of KCN are taken, not less than 45 cc. of N - Q AgNO 3 V. S. should be required. 2KCN + AgNO 3 = AgCN,KCN + KNO 3 . 10)130.02 10)169.7 13.002 gms. 16.97 gms. or 1000 cc. AgNO 3 V. S. Thus each cc. of the standard silver solution repre- sents 0.013 g m - f KCN. 0.013 X 45 = 0-585 gm. .585 X IPO _ .65 Cyanides maybe estimated also by iodine, according to Fordos and Gelis. This process depends upon the fact that potassium cyanide decolorizes iodine, potassium iodide and cyanogen iodide being formed. When iodine solution is added to a solution of po- A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 121 tassium cyanide, the iodine is decolorized as long as there is any undecomposed cyanide present. The following equation expresses the reaction KCN + I 2 = KI + CNI, 2)65.01 2)153.06 10)32.505 10)126.53 N 3.2505 gms. 12.653 g ms - or I00 cc - iodine V. S. Thus each cc. of the volumetric solution represents 0.00325 gm. of KCN. The end of the reaction is known by the yellow color of the iodine solution becoming permanent. Silver Nitrate, (Argenti Nitras) AgNO 3 = j *|^;* 5 . Nitrate of silver and other salts of this metal may be volumetrically estimated by standard solution of sodium chloride. The silver salt is dissolved in sufficient water in a beaker, and a decinormal volumetric solution of sodium chloride run in until a precipitate is no longer pro- duced. The estimation may also be performed by retitration as follows : To the silver solution contained in a beaker add a N measured excess of -- sodium chloride V. S., and then, 10 after adding a few drops of potassium chromate T. S., N titrate the mixture with silver nitrate V. S. until a 10 permanent red color appears. Deduct the number of cc. of silver nitrate V. S. from the quantity of sodium chloride V. S. and the quantity of the latter is ob- tained which actually combined with the silver solution under examination. 122 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. The sulphocyanate method of Volhard may also be employed in the estimation of silver. ^ Sodium Chloride V. S., NaCl = \ *l'M. 10 ( 55.4 5*o- r gms. in I litre. Dissolve 5.837 gms. of pure sodium chloride in enough water to make exactly 1000 cc. at the ordinary temperature of the atmos- phere. Check this solution with decinormal silver nitrate V. S. The two solutions should correspond, volume for volume. Pure Sodium Chloride may be prepared by passing into a saturated aqueous solution of the purest com- mercial chloride of sodium a current of dry hydro- chloric-acid gas. The crystalline precipitate is then separated and dried at a temperature sufficiently high to expel all traces of free acid. The U. S. P. method for silver nitrate is as follows : *O.34gm. (0.3391 gm.) of silver nitrate is dissolved in 10 cc. of distilled water, and the solution carefully N titrated with NaCl V. S. until precipitation ceases. 20 cc. of the standard solution should be required. AgNO, + NaCl = AgCl + NaNO 3 . 10)169.55* 10)58.37 N J 6.955 gms. 5. 837 gms. or looocc. NaCl V. S. Each cc. of the standard solution represents 0.01695 5 gm. of pure AgNO 3 . 0.016955 X 20 = 0.3391 gm. 0.3391 X ioo , ^ - 100$ 339 1 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 123 Argenti Nitras Dilutus (Mitigated Caustic). This may be estimated in the same manner as the above. The U. S. P. method is as follows : I gm. is dissolved in 10 cc. of distilled water, to this N is added 20 cc. of NaCl V. S. and a few drops of N potassium chromate T. S., and the excess of NaCl N V. S. found by titration with AgNO 3 V. S. until a permanent red color is produced. Not more than 0.5 cc. of the latter should be required. This indicates N that 19.5 cc. of NaCl V. S. were actually required to completely precipitate the silver nitrate tested. Therefore 0.016955 x 19.5 = -3306225 gm- Argenti Nitras Fusus (Moulded Silver Nitrate. Lunar Caustic). This is treated in exactly the same manner as the above. 0.34 gm. of the lunar caustic is dissolved in water, and 20 cc. of standard sodium chloride added ; not more than I cc. of this should be in excess, as shown by retitration with silver nitrate V. S., using chromate indicator. This corresponds to about 95$ of pure silver nitrate. Silver Oxide, Ag,O = 231.28. May be converted into nitrate by solution in nitric acid, and then test- ing as above for silver nitrate. There will prob- ably be some free nitric acid present if this is done, 124 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. and therefore the sulphocyanate method is best em- ployed. The Sulphocyanate Method. A weighed quantity of the silver salt is dissolved in water, some diluted nitric acid and ammonium ferric sulphate solution are N added, and the mixture then titrated with potassium 10 sulphocyanate V. S. until a permanent reddish-brown color of feric sulphocyanate is produced. The following equation explains the reactions : AgN0 3 + KSCN = AgSCN + KNO 3 . 10)169.55 10)96.99 16.955 gs. 9.699 gms. or 1000 cc. standard V. S. Thus each cc. of the standard V. S. represents 0.016955 gm. of pure silver nitrate, or 0.010766 gm. of metallic silver. Liquor Plumbi Subacetatis (Goulard's Extract). This is an aqueous solution containing about 25$ of lead subacetate, the formula of which is approximately Pb 3 O(C 3 H 3 O 2 ) 2 = 546.48. This is estimated by precipi- tation with sulphuric acid. (13.6622 gms.)*J3.67 gms. of the solution are diluted with 50 cc. of water, a few drops of methyl-orange added, and the mixture titrated with normal sulphuric acid until the lead is completely precipitated and the mix- ture has assumed a red color. The red color indicates an acid reaction. The reaction is illustrated by the following equation : 4)546.48 4)196 N 136.62 gms. 49 gms. or 1000 cc. - H 2 SO 4 V. S. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 125 N Thus each cc. of H a SO 4 V. S. represents 0.13662 gm. of the subacetate. If 25 cc. of the standard solution are required, then the solution under analysis contains 0.13662 X 25 = 3.4155 gms. 3.4155 Xioo 13.662 The Diluted Solution of Lead Subacetate (Lead Water) may be estimated in the same manner. 126 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. TABLE OF SUBSTANCES ESTIMATED BY PRECIPITATION, GIVING FORMULA, MOLECULAR WEIGHT, STANDARD SOLUTION USED, AND FACTOR. Name. Formula. Molec- ular weight. Standard Solu- tion Used. Factor. Acid hydrobromic HBr 80.76 AgNO 3 0.008076 IO " hydrocyanic HCN 26.98 IA.NO, 0.002698 " hydriodic Ammonium bromide " chloride HI NH 4 Br NH 4 C1 127.53 97-77 53.38 tt 0.01275 0.009777 0.005338 " iodide Calcium bromide NH 4 I CaBr Q M4-54 " 0.014454 0.0099715 " chloride CaCl 2 J 99-43 110.65 0.005532 Ferrous bromide FeBr 2 215.40 " 0.01077 " iodide F I tr& CIA AgNO 3 and 0.015447 e 2 300.94 IO 10 Lead acetate . Pb(C a H,O a VlH a O 378.0 ^ H 2 S0 4 0.189 " subacetate Pb 2 0(C 2 H 3 2 ) a 546.48 " 0.13662 Lithium bromide LiBr 86.77 -AgNO, IO 0.008677 Potassium bromide KBr 118.79 0.011879 chloride cyanide KC1 KCN 74-40 65.01 u 0.00744 0.01300 " iodide KI 165.56 ifc 0.016556 " sulphocyanide KSCN 96.99 4 * 0.009699 Silver (metallic) Ag u 215.32 - NaCl or 0.010766 10 -KSCN IO " nitrate AgNOa I 6O ^ l< fiocc " oxide **& M ^3 Ag 2 231.28 M 0.011564 Sodium bromide NaBr 102.76 AgNO 3 0.010276 " chloride, NaCl 58.37 10 0.005837 " iodide Nal Strontium bromide iodide SrBr 2 .6H 2 O SrI 2 .6H 2 O 1 49-53 354-58 " 0.014953 0.012341 Zinc bromide ZnBr 2 44 .1 14 ^ fci chloride ... ZnClo 22 4- 2 M o.oi 1231 " iodide ZnI 2 I 35-4 318.16 it o.oo 792 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 127 CHAPTER XI. OXIDIMETRY ANALYSIS BY OXIDATION. AN extensive series of analyses are made by this method, with extremely accurate results in fact, the results are generally more accurate than any which can be obtained by weighing. The principle involved in this method is extremely simple. Substances which are capable of absorbing oxygen or are susceptible of an equivalent action are subjected to the action of an oxidizing agent of known power, and the quantity of the latter required for complete oxidation ascertained. The substances which are used as oxidizing agents in volumetric analysis are potassium dichromate, po- tassium permanganate, iodine, etc. The reducing agents, or deoxidizers, are sodium thio- sulphate, oxalic acid, arsenous oxide, stannous chloride, metallic zinc, and magnesium. Thus ferrous oxide (FeO), an oxidizable substance, is ever ready and willing to take up oxygen, while potassium dichromate and permanganate are always ready to give up some of their oxygen. .When po- tassium permanganate gives up its oxygen in this way it loses its color, and in volumetric analysis advantage is taken of this fact. When the permanganate, which is added in drops from a burette, is no longer decolor- 128 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. ized, the iron salt is completely oxidized. The reac- tion is as follows : roFeO + 2KMnO 4 = 5Fe 2 O 3 + 2MnO + K 2 O. Ferrous oxide. Ferric oxide. The oxidation of ferrous oxide by potassium dichro- mate is shown by the following equation : 6FeO + K 2 Cr 2 7 = 3 Fe 2 O 3 + Cr 2 O 3 + K 2 O. An oxidation is always accompanied by a reduction, the oxidizing agent being itself reduced in the opera- tion. As seen in the above equations, the manganic compound is reduced to a manganous compound, and the chromic to a chromous compound. ESTIMATION OF FERROUS SALTS. Ferrous salts are estimated by oxidizing them either with potassium dichromate or potassium perman- ganate. In some respects the dichromate possesses advan- tages over permanganate. 1. It may be obtained in a pure state. 2. Its solution does not deteriorate upon standing as does that of permanganate. 3. It is not decomposed by contact with rubber as the permanganate is, and may therefore be used in Mohr's burette. Its great disadvantage, however, is that when used in the estimation of ferrous salts the end reaction can only be found by using an external indicator. The indicator which must be used is freshly A TEXT-BOOK OF VOLUMETRIC ANALYSIS. prepared potassium ferricyanide T. S., a drop of which is brought in contact with a drop of the solution being tested, on a white slab, at intervals during the titration, the end of the reaction being the cessation of the production of a blue color, when the two liquids are brought together. Thus the estimation by potas- sium dichromate is cumbersome, and very exact results are not easily obtained. If potassium-permanganate solution is used for the estimation of these salts the end of the reaction is easily found without the use of an indicator. The permanganate is decomposed the instant it is brought in contact with a ferrous salt in an acid solu- tion ; therefore as long as any ferrous salt remains in solution the permanganate is decolorized, and when it ceases to lose its color the reaction is complete. Preparation of Standard Solution Decinormal Potassium Dichromate V. S., K 3 Cr 2 O 7 = j ^93 7^ *4*Q [ ms ' * n l ^ tre- 4" 8 96 gms. (*4-9gms.) of pure potassium dichromate are dissolved in sufficient water to make, at the ordinary temperature of the atmos- phere, exactly 1000 cc. Pure Potassium Dichromate for use in volumetric analysis should respond to all the tests for purity given in the text of the U. S. P. (under Potassii Dichromate}, as well as to the following: A solution of 0.5 gm. of the salt in 10 cc. of water, rendered acid by 0.5 cc. of nitric acid, should produce no visible change when treated with barium chloride T. S. (absence of sulphate), nor with silver nitrate T. S. (absence of chloride). If a mixture of 10 cc. of an aqueous solution of the TUTITBRSITT' 130 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. salt (1-20) with I cc. of ammonia water be treated with ammonium oxalate T. S., no precipitate should be pro- duced (absence of calcium). Standard solution of potassium dichromate is some- times used as a neutralizing solution for estimating alkalies, phenolphthalein being used as indicator. When used for this purpose the decinormal solution contains 14.689 gms. in I litre (one half the molecular weight in grammes). It is then the exact equivalent of any decinormal acid V. S. Decinormal potassium dichromate V. S. may also be used in conjunction with potassium iodide and sul- phuric acid for standardizing sodium thiosulphate V. S. Iodine is liberated from potassium iodide in this reac- tion. The reaction is expressed by the equadon = 4 K 5 SO ( + Cr/SO,), + 7 H,0 + 3!,. When used as an oxidizing agent to convert ferrous into ferric salts, or to liberate iodine from potassium N iodide, the solution of potassium dichromate must contain 4.689 gms. in I litre. If the decinormal solution containing 14.689 gms. in I litre is used, it has the effect 3N of a -- solution. 10 The decinormal solution which is used as an oxidizing agent is chemically equivalent to decinormal potassium permanganate. When used for the purpose of liberating iodine from potassium iodide, it is the equivalent of an equal volume of decinormal sodium thiosulphate. For titrating ferrous salts the decinormal solution of dichromate is used in the following manner: A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 131 Make an aqueous solution of the ferrous salt, intro- duce it into a flask, and acidulate it with sulphuric or hydrochloric acid. Now add gradually from a burette the decinormal potassium dichromate V. S. until a drop taken out upon a white slab no longer shows "a blue color with a drop of freshly prepared potassium ferricyanide T. S. Note the number of cc. of the standard solution used, multiply this number by the factor, and thus obtain the quantity of pure salt in the sample taken. Ferrous salts strike a blue color with potassium ferricyanide T. S ; but as the quantity of ferrous salts gradually diminishes during the titration, the blue be- comes somewhat turbid, acquiring first a green, then a gray, and lastly a brown shade. The process is finished when the greenish-blue tint has entirely disappeared. The reaction of potassium dichromate with ferrous salts always takes place in the presence of free sul- phuric or hydrochloric acid at ordinary temperatures, Nitric acid should not be used. If it is desired to estimate ferric salts by this standard solution it is necessary to first reduce them. This may be done by metallic zinc, magnesium, sul- phurous acid, the alkali sulphites, or by stannous chloride. Standard potassium dichromate may be checked in the same way as standard permanganate, with pure metallic iron, as described below. Decinormal Potassium Permanganate V. S., 2KMn0 4 = | ^|' 34 . It contains * 3 '^ 34 1 gms. in I litre. This solution may be prepared by dissolving the pure crystals in fresh distilled water. If the salt can be 132 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. obtained perfectly pure and dry, a decinormal solution will be obtained if 3.1534 gms. are dissolved in distilled water, sufficient to make looocc. at the ordinary atmos- pheric temperature ; but nevertheless it is always well to verify it as described below. The solution will retain its strength for several weeks if well kept, but it should always be checked by titration before it is used. The standardization of permanganate solution may be effected as follows : With Metallic Iron Thin annealed binding-wire, free from rust, is one of the purest forms of iron. O.I gm. of such iron is placed in a flask which is provided with a cork through which a piece of glass tubing passes, to the top of which a piece of rubber tubing is at- tached, which has a vertical slit about one inch long in its side, and which is closed at its upper end by a piece of glass rod (see Fig. 24). Diluted sulphuric acid is added and gentle heat ap- plied. The iron dissolves and the steam and liberated hy- drogen escape through the slit under slight pressure. The air is thus prevented from enter- ing and the ferrous solution protected from oxidation. When the iron is completely dissolved a small quan- tity of cold, recently boiled, distilled water should be added, and the titration with potassium permanganate at once begun and continued until a faint permanent FIG. 24. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 133 red color is produced. If the solution is decinormal, exactly 17.85 cc. will be required to produce this result. The iron is converted by the sulphuric acid into ferrous sulphate, Fe 2 + 2H 2 SO 4 '= 2FeSO 4 + 2H 2 . This ferrous sulphate is easily oxidized by the air, and therefore it is directed that access of air should be prevented, and the distilled water, with which the solu- tion is diluted, previously boiled in order to drive off any dissolved free oxygen. ioFeS0 4 + 2KMn0 4 + 8H,SO 4 IO )5 100)315.34 N _ 5.588 gms. 3.1534 gms. or 1000 cc. V. S. = 5Fe a (S0 4 ) 3 + K 3 S0 4 + 2MnS0 4 + 8H 2 O. N This equation, etc., shows that each cc. of 2KMnO 4 V. S. represents .005588 gm. of metallic iron. With Oxalic Acid. 0.063 gm. of the pure crystal- lized acid is weighed (or 10 cc. of decinormal oxalic acid V. S. carefully measured) and placed in a flask, with some dilute sulphuric acid and considerable water, the mixture warmed to about 60 C. (140 F.), and the permanganate added from a burette. The action is in this case less decisive and rapid than* in the titration with iron, and more care should be used. The color disappears slowly at first, but afterwards more rapidly. Note the number of cc. of the permanganate solu- tion used, and then dilute the remainder so that equal volumes of decinormal oxalic acid and decinormal permanganate solution will exactly correspond. 134 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Example. Assuming that 9 cc. of the perman- ganate solution first prepared had been required to produce a permanent pink tint when titrated into 10 N cc. of oxalic-acid solution, then the permanganate must be diluted in the proportion of 9 of permanganate and i of distilled water, or 900 and 100. The U. S. P. gives the following method for the preparation of this solution : A stronger and a weaker solution is made and mixed in certain proportions to form a solution of the proper strength. It is said that when thus prepared the solu- tion will keep its titre for months if properly preserved. The Stronger Solution. 3.5 gms. of pure crystallized permanganate are dissolved in 1000 cc. of water by the aid of heat, and the solution then set aside in a closed flask for two days, so that any suspended mat- ters may deposit. The Weaker Solution. Dissolve 6.6 gms. of the salt in 2200 cc. of water in the same manner as above, and set this solution aside for two days. These two solutions are then separately titrated in the following manner: Introduce 10 cc. of decinormal oxalic-acid solution into a flask, add I cc. of pure concentrated sulphuric acid, and before the mixture cools add the perman- ganate solution slowly from a burette, shaking the flask after each addition, and towards the end of the operation reducing the flow to drops. When the last drop is no longer decolorized, but imparts a pinkish tint to the liquid, the reaction is completed. Note the number of cc. consumed. Finally, mix the two solu- tions in such proportions that equal volumes of the A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 135 N mixture and of oxalic acid V. S. will exactly corre- spond. To obtain the accurate proportions for mixing the two solutions, deduct 10 from the number of cc. of the weaker solution consumed in the above titration ; with this difference multiply the number of cc. of the stronger solution consumed : the product shows the number of cc. of the stronger solution needed for the mixture. Then deduct the number of cc. of the stronger solu- tion consumed in the titration from 10, and with the difference multiply the number of cc. of the weaker solution consumed : the product shows the number of cc. of the weaker solution needed for the mixture. Or, designating the number of cc. of the stronger so- lution by S, and the number of cc. of the weaker solu- tion by W, and using the following formula, the proportions in which the solutions must be mixed are obtained: Stronger Solution. Weaker Solution. (J^--io)S + (io Example. Assuming that 9 cc. of the stronger and 10.5 cc. of weaker had been consumed in decomposing N IO cc. of oxalic acid V. S. ; then, substituting these values in the above formula, we obtain (10.5 - 10)9 + (10-9)10.5, or 4.5 + 10.5, making 15 cc. of final solution. 136 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. The bulk of the two solutions is now mixed in the same proportion : 450 cc. of the stronger and 1050 cc. of the weaker, or 900 cc. of the stronger and 2100 cc. of the weaker. After the solutions are thus mixed a new trial should be made, when 10 cc. of the solution should exactly N decompose 10 cc. of oxalic acid V. S. The reaction between potassium permanganate and oxalic acid is illustrated by the following equation : 2KMn0 4 + 5(H 2 C 2 4 .2H 2 0) + sH 2 SO 4 - K S SO 4 + 2MnSO 4 + ioCO a + i8H 2 O. ESTIMATION OF FERROUS SALTS WITH POTASSIUM BICHROMATE. One molecule of potassium dichromate yields, under favorable circumstances, three atoms of oxygen for oxidizing purposes. This is shown by the following equation : 6FeO + K.Cr.0, = 3 Fe,O, + Cr,O, + K,O. Here it is seen that the three liberated atoms of oxygen combine at once with the ferrous oxide, con- verting it into ferric oxide : 6FeO + 3 = Fe 6 9 or 3 Fe a O 3 . In the oxidation of a ferrous salt, the reaction takes place only in the presence of an acid. The dichromate then gives up its oxygen. Four of A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 137 its oxygen atoms combine at once with the replaceable hydrogen of the accompanying acid, the other three being liberated. The three oxygen atoms thus set free are available either for direct oxidation or for combination with the hydrogen of more acid. In the latter case a corresponding quantity of acidulous radi- cals is set free. The following equation indicates this reaction : K,Cr.O, + 4H,S0 4 = K.SO, + Cr,(SO.). + 4 H,O + O,. In this case four of the liberated atoms of oxygen combine with eight of the atoms of hydrogen of sul- phuric acid and liberate four SO 4 radicals, which at once combine with the K 2 and Cr 2 of the dichromate. The other three atoms are set free. If seven sulphuric- acid molecules are used instead of four molecules, the three free atoms of oxygen will liberate 3(SO 4 ): K a CrA+7H,SO,=K 5 SO < +Cr,(SO.) s +7H,0 + (SO.) s . If this liberation of 3(SO 4 ) takes place in the pres- ence of a ferrous salt, the 3(SO 4 ) will combine with six molecules of the ferrous salt, converting it into a ferric salt: 6FeS0 4 + 3 S0 4 = Fe,(SO.), = 3 Fe,(SO 4 ), ; 6FeSO, + K,CrA + 7 H 5 SO 4 = K,S0 4 + Cr,(S0 4 ) 3 + 7 H,0 + (3Fe,(S0 4 ),). If in the above case hydrochloric acid is used instead of sulphuric, fourteen molecules of the former must be taken to supply the necessary hydrogen. 138 A TEXT-BOOK OF VOLUMETRIC ANALYSIS, The seven liberated atoms of oxygen must have fourteen atoms of hydrogen to combine with. Three of these atoms of oxygen liberate six uni- valent, or three bivalent, acidulous radicals. Therefore, since one molecule of K 2 Cr a O 7 will give up for oxidizing purposes three atoms of oxygen, which are equivalent chemically to six atoms of hydro- gen, one sixth of the molecular weight in grammes of the dichromate, dissolved in sufficient water to make one litre, constitutes a normal solution, and one tenth of this quantity of K 2 Cr 2 O 7 in a litre, a decinormal solution. Thus the estimation of ferrous salts is effected by oxidizing them to ferric with an oxidizing agent of known power, the strength of the ferrous salt being determined by the quantity of the oxidizing agent required to convert it to ferric. Ferri Carbonas Saccharatus (Saccharated Ferrous Carbonate), FeCO, = { *j J5.73._*,. l6 (l . I5;3) gms of saccharated ferrous carbonate are dissolved in 10 cc. of diluted sulphuric acid and the solution diluted with water to about 100 cc. The decinormal potassium dichromate is carefully added, until a drop of the solu- tion taken out and brought in contact with a drop of freshly prepared solution of potassium ferricyanide ceases to give a blue color. The number of cc. of the dichromate solution is read off and the following equations applied : 6FeC0 8 + 6H a SO, = 6FeSO 4 + 6H a O + 6CO a U5-73 I5I-7 6 6 694.38 910.2 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 139 then 6FeCO 3 or 6FeSO 4 + K 2 Cr 2 O 7 + ;H 2 SO 4 = 6)694.38 6)910.2 6)293.78 10)115.73 10)151.7 10) 48.96 TI -573 S ms - *5' 1 7 S ms - 4.896 gms., or looo cc..?[ K 2 Cr 2 O 7 V.S. K,S0 4 + Cr 2 (S0 4 ) 3 + 7 H 2 + 3 Fe 2 (SO 4 ) 3 . N Thus each cc. of K 2 Cr 2 O 7 represents 0.011573 gm. of pure ferrous carbonate or 0.005588 gm. of metallic iron. The U. S. P. saccharated ferrous carbonate requires N about 15 cc. of K 2 Cr 2 O 7 V. S. for complete neutral- ization, corresponding to about 15$. .011573 X 15 =0.173585 gm. 0.173585 X IPO = I-I573 If strong sulphuric acid is added to saccharated fer- rous carbonate it will char the sugar, and a black mass of burnt sugar is obtained. This may be prevented by adding water first and then, slowly, the sulphuric acid. Instead of sulphuric acid, hydrochloric acid may be used. This will not char the sugar ; but the ferrous chloride which is then formed is too readily oxidized by the air. It has also been suggested that as hydrochloric acid so rapidly converts ordinary sugar into invert sugar as to render it easily attacked by the dichromate, it should be cautiously used, if at all. Phosphoric acid 140 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. has none of these disadvantages, and may be employed with good results. In making estimations of ferrous salts with potas- sium dichromate, care should be taken to avoid atmos- pheric oxidation. It is good practice to calculate approximately how much of the standard solution will probably be required to complete the oxidation, and then add almost enough of the standard solution at once, instead of adding it slowly. A white porcelain slab is then got ready, and placed alongside of the flask in which the titration is to be performed. Upon this slab is placed a number of drops of the freshly prepared solution of potassium ferricyanide, and at intervals during the titration a drop is taken from the flask on a glass rod and brought in contact with one of the drops on the slab. The glass rod should always be dipped in clean water after having been brought in contact with a drop of the indicator. When a drop of the solution ceases to give a blue color on contact with the indicator, the reaction is complete. Ferrous Sulphate, FeSO 4 + ;H 2 O = j ^g' 42 .- Dissolve about one gramme of crystallized ferrous sul- phate in a little water, add a good excess of sulphuric or hydrochloric acid, titrate with the decinormal potas- sium dichromate V. S. as directed under Ferrous Car- bonate, and apply the following equation : 6(FeSO ( .7H a O) + K.Cr,O, + 7 H,SO 4 = 6)1668 6)293.78 10) 278 io) 48.96 27.8 gms. 4.896 gms.,or 1000 cc. K 2 Cr 2 O, V. S. 3Fe,(SOJ. + K.SO. + Cr,(SOJ, + 49 H,O. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 14! N Thus each cc. of the K 2 Cr 2 O 7 V. S. represents 0.0278 gm. of crystallized ferrous sulphate or 0.0152 anhydrous. If I gm. of the salt is taken and dissolved as above, it should require about 37 cc. of the standard solution, equivalent to about ioo#. Anhydrous Ferrous Sulphate. 6FeS0 4 + K 2 CrA + / 6)912 6)293.78 10)152 10)48.96 N 15.2 gms. 4.896 gms., or 1000 cc. K 2 Cr 2 O 7 V. S. SFe/SO,), + K 2 SO. + Cr,(SO 4 ). + ;H 2 O. Each cc. of the standard solution represents 0,0152 gm. of real ferrous sulphate or ^.0056 gm. of metallic iron. Dried (Exsiccated) Ferrous Sulphate of the U. S. P. has the approximate composition FeSO 4 -|- 3H 2 O. It is tested in the same manner as the anhydrous ferrous sulphate. Granulated Ferrous Sulphate, FeSO 4 + 7H,O, is tested in the same manner as crystallized ferrous sul- phate, with which it should correspond in strength. ESTIMATION OF FERROUS SALTS WITH POTASSIUM- PERMANGANATE SOLUTION. The action of potassium permanganate in oxidation is very similar to that of the dichromate. The molecule 2KMnO 4 has 8 atoms of oxygen, which it gives up in the process of oxidation. These 8 atoms of oxygen unite with the replaceable hydrogen 142 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. of an accompanying acid, liberating an equivalent amount of acidulous radical. Three of these atoms of oxygen liberate sufficient acidulous radical to combine with the potassium and manganese of the permanganate, while the other five atoms are available either for direct oxidation or 2KMn0 4 + 3H 2 S0 4 =K 2 S0 4 + 2MnSO 4 + 3H 2 O + 50. For combination with the hydrogen of more acid, more acidulous radical being liberated to combine with the salt acted upon, 2KMnO 4 +8H a SO 4 =K 2 SO 4 +2MnSO 4 +8H 2 O+5(SO 4 ). 5(SO 4 ) when combined with ioFeSO 4 forms Fe 10 (SO 4 ) 15 or 5Fe 2 (SO 4 ) 3 , ferric sulphate. It is thus seen that one molecule of potassium per- manganate 2KMnO 4 has the power of converting 10 molecules of a ferrous salt into the ferric state. The equation in full is ioFeSO 4 + 2KMnO 4 + 8H 2 SO 4 = K 2 S0 4 + 2MnS0 4 + 8H 2 + 5 Fe 2 (SO 4 ), We have seen that 2KMnO 4 has 5 atoms of oxygen available for oxidizing purposes, and that each of these will combine with 2 atoms of hydrogen. 2KMnO 4 is consequently chemically equivalent to 10 atoms of re- placeable hydrogen, and a normal solution of this salt when used as an oxidizing agent is one that contains in I litre one tenth of the molecular weight of 2KMnO 4 , and a decinormal solution one which contains one hundredth of the molecular weight. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 143 When potassium permanganate is brought in contact with a ferrous salt or other oxidizable substance, it is decomposed and decolorized. When titrating with a standard solution of this salt it is decolorized so long as an oxidizable substance is present ; as soon, however, as the oxidation is com- pleted the standard solution retains its color. The end of the reaction, therefore, when permanga- nate is used, is the appearance of a permanent faint-red color. This is the principal advantage which permanganate has over dichromate. When titrating with standard permanganate solution a glass stop-cock burette should be used, as the solu- tion is slightly affected by the rubber, on Mohr's bu- rette. Ferrum Reductum is estimated for metallic iron, according to the U. S. P., in the following manner: 0.56 (0.559) S m ' f reduced iron is introduced into a glass-stoppered bottle, 50 cc. of mercuric chloride T. S. are added, and the bottle heated on a water-bath for one hour, agitating frequently, but keeping the bottle well stoppered. 2HgCl 2 + Fe a = 2FeCl 2 + 2Hg. Then allow it to cool, dilute the contents with water to loo cc., and filter. Take 10 cc. of the filtrate, add to it locc. of diluted sulphuric acid, introduce the mixture into a glass-stoppered bottle (having a capacity of about 100 cc.), and titrate the mixture with decinormal potassium permanganate V. S. until a permanent red color is produced. 144 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Each cc. of the standard solution represents ^0.0056 gm. of metallic iron, or io#. ioFeSO 4 + 2KMnO 4 + 8H 2 SO 4 = K 3 S0 4 + 2MnS0 4 + 5Fe 2 (S0 4 ) 3 + 8H 2 O. To confirm the assay, add a few drops of alcohol to decolorize (or decompose) the excess of permanga- nate, then add I gm. of potassium iodide, and digest for half an hour at a temperature of 40 C. (104 F.). Fe,(S0 4 ) 3 + 2KI = 2FeS0 4 + I 2 + K 2 SO 4 . 2)112 2)254 10) 56 10)127 5-6 "l2T7 The cooled solution is mixed with a few drops of starch test solution, which gives it a dark-blue color, because of the formation of iodide of starch. Then add carefully, from a burette, decinormal sodium thiosul- phate V. S. until the blue color is discharged. I, + 2(Na 1 S 1 0,.5H f O) = 2NaI + Na,S 4 O.+ ioH 2 O. 2)254 2)495-28 10)127 10)247.64 N 12.7 gms. 24 76 gms. or 1000 cc. Na 2 S 2 O 3 V. S. Thus each cc. of the standard thiosulphate repre- sents 0.0127 gm. of iodine, or 0.0056 gm. of metallic iron. In both of these estimations the quantity of standard solution used should be the same. The U. S. P. requirement is 8 cc. 0.0056 X 8 = 0.0448 gm. ^.0448 X IPO _ 0.056 A TEXT-BOOK OF VOLUMETRIC ANALYSIS, 145 Ferrous Sulphate (Crystallized), FeSO 4 + 7H 2 O = > 2 774 2 i *i.39 (1.3871) gms. of ferrous sulphate are dissolved in about 25 cc. of water, and the solution acidulated with sulphuric acid. Decinormal potassium permanganate V. S. is then delivered in from a burette until a permanent pink color is obtained, indicating the complete oxidation of the ferrous salt. io(FeSO 4 + 7H 2 0) + 2KMnO 4 + 8H 2 SO 4 = 100)2774.2 100)315.34 N 27. 742 gms. 3.1534 gms. or 1000 cc. stand- ard solution. 5Fe 3 (SO 4 ) 3 + K 2 S0 4 + 2MnSO 4 + 8H 2 O. Thus each cc. of the standard solution represents 0.027742 gm. of crystallized ferrous sulphate. Not less than 50 cc. should be used before the po- tassium permanganate ceases to be decolorized. 0.027742 X 50 = i. 387100 gms. 1.387100 X ioo _ 1.3871 Granulated Ferrous Sulphate, FeSO 4 + 7H 2 O, is esti- mated in the same way as the foregoing, and should correspond with it in strength. Exsiccated (Dried) Ferrous Sulphate. This salt is tested in the same manner as the other two sulphates. It contains a larger percentage of ferrous sulphate than the other two, having less water of crystallization. Its composition is approximately FeSO 4 + 3H,O. In estimating ferrous sulphate in this salt the water of crystallization is not taken into account. Then by 146 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. deducting the percentage of ferrous sulphate from 100 the percentage of water of crystallization is obtained. N ioFeSO 4 + 2KMnO 4 + 8H 3 SO 4 100)1520 103)315.34 15.20 gms. 3-1534 gms. or 1000 cc. standard solution. = 5Fe 3 (SO 4 ) 3 + K 2 SO 4 + 2MnSO 4 + 8H 2 O. Each cc. of the standard solution represents 0.0152 gm. of anhydrous (real) ferrous sulphate. If one gm. of the dried salt, treated as above described, requires N 48 cc. of permanganate solution, it contains 0.0152 X 48 = 0.7296 gm., or 72.96$ of real ferrous sulphate, and 100.00 72.96 = 27.04$ of water of crystallization. Any salt may be analyzed in this way for water of crystallization. If the salt is ptire, the difference be- tween the percentage of real salt and 100 always repre- sents the percentage of water of crystallization. ESTIMATION OF HYPOPHOSPHOROUS ACID, HYPOPHOS- PHITES, AND OTHER OXIDIZABLE SUBSTANCES. Acidum Hypophosphorosum Dilutum. An aque- ous solution containing about 10 per cent, by weight, of absolute hypophosphorous acid. (H 3 P0 3 ) HPH.O. = N This acid may be tested by neutralization with potassium hydrate V. S., as described on page 79. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 147 The U. S. P. also directs the estimation by residual titration, given below. 0.5 gm. of diluted hypophosphorous acid is mixed with 7 cc. of sulphuric acid, and 35 cc. of decinormal potassium permanganate V. S., and the mixture boiled for fifteen minutes. The potassium permanganate, in the presence of sulphuric acid, oxidizes the hypophosphorous acid to phosphoric, as the equation shows : 5HPH 2 O 2 + 6H,SO 4 -f 2(2KMnO 4 ) 2)32940 2)630.68 100)164.7 100)315.34 N 1.647 gms. 3-1534 gms. or 1000 cc. V. S. = 5H 3 P0 4 + 6H 2 + 2K 2 S0 4 + 4MnSO 4 . Each cc. of the decinormal V. S. represents 0.001647 gm. of absolute hypophosphorous acid. The quantity of permanganate solution directed to be added is slightly in excess. The excess is then ascertained by retitration with decinormal oxalic acid V. S. Each cc. of oxalic acid required corresponds to one cc. of deci- normal permanganate V. S., which has been added in excess of the quantity actually required for the oxida- tion. The excess of permanganate colors the solution red, and the oxalic acid V. S. is then added until the red color just disappears, which indicates that the excess of permanganate is decomposed. 2KMn0 4 + 5 (H 2 C 2 0,2H 2 0) + 3H 2 SO 4 = K 2 S0 4 + 2MnS0 4 + i8H 2 + ioCO 2 . If 4.7 cc. of decinormal oxalic acid V. S. are required, it indicates that 35 cc. 4.7 cc. = 30.3 cc. of deci- 148 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. normal permanganate were actually used up in oxidizing the hypophosphorous acid. Therefore 0.001647 gm. X 30.3 = 0.0499 gm., or ?^99_ X _^ = 9.98^ of HPH A- In the above process the boiling facilitates the oxida- tion, but if the acid is boiled before it is completely oxidized it will decompose. Hence the necessity for adding an excess of the permanganate and retitrating. Calcium Hypophosphite, Ca(PH 2 O 2 ), = j *j^* 67 . O.I gm. of the salt is dissolved in 10 cc. of water, then 10 cc. of sulphuric acid and 50 cc. of decinormal potassium permanganate V. S. are added, and the mix- ture boiled for fifteen minutes. The excess of permanganate is then found by reti- trating with decinormal oxalic-acid solution. The reactions which take place are expressed by the following equations : 5Ca(PH Q 2 ) 2 + 5H 2 S0 4 = 5CaSO 4 + ioHPH 2 O a ; (i) ioHPH 2 O 5 = ioH 8 PO 4 + i2H 2 O + 4K 2 SO 4 + 8MnSO 4 . (2) These two reactions may be written together thus : S Ca(PH,0,), + I7H,SO. + 4 (2KMn0 4 ) 4)848.35 4)1261.36 100)212.08 IOQ) 3I5-34 [ard V. S. 2.1208 gms. 3-1534 gms. or 1000 cc. stand- = 5CaS0 4 + 4K,S0 4 + 8MnSO 4 + ioH 3 PO 4 +I2H 2 O. A TEXT-BOOK OP VOLUMETRIC ANALYSIS. 149 Thus each cc. of the standard permanganate repre- sents 0.0021208 gm. of pure Ca(PH 2 O 2 ) a . 50 cc. of decinormal potassium permanganate are about 3 cc. more than is necessary to oxidize O.I gm. of pure cal- cium hypophosphite. Therefore not more than 3 cc. of the standard oxalic-acid solution should be required to decolorize the solution to which 50 cc. of perman- ganate has been added. Then 0.0021208 gm. x 47 .09968 gm. 0.9968 X ioo O.I = 99.68$ pure salt. Ferric Hypophosphite, Fe,(PH.OJ. = j ^\- 04 . This salt is estimated in the same manner as the fore- going. o.i gm. is dissolved in 10 cc. of water, then 10 cc. of sulphuric acid and 50 cc. of decinormal potassium per- manganate V. S. are added, and the mixture boiled for 15 minutes. The quantity of permanganate solution here added is slightly in excess of the quantity actually required to oxidize the hypophosphite. The excess is deter- mined by retitrating with decinormal oxalic acid V. S., which corresponds volume for volume with the per- manganate. Not more than 3 cc. of the standard oxalic acid so- lution should be required to decolorize the excess of permanganate, which means that 47 c.c. of the per- 150 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. manganate should actually be required to oxidize the O.I gm. of hypophosphite taken. The reaction is illustrated by the following equation : SFe/PH A),+5 1 H,SO.+ i2(2KMn0 4 ) 12)2505.20 12)3784.08 ioo)~gc8.77 IPO) 315.34 N v s 2.0877 gm. 3.i534gms.or looocc.io =5Fe J (SO ) ),+K 1 SO.+24MnSO < +3oH J PO.+ 3 6HA N This ?hows that each cc. of potassium permanga- nate V. S. represents 0.0020877 gm. of ferric hypophos- phite. If 47 cc. are required to oxidize O.I gm. of the salt, the latter contains 0.0020877 X 47 0.0981+ gm., or 98.1+ % of pure salt. Potassium Hypophosphite, KPH,O 2 = j *j 3 " 91 . o.i gm. of dry potassium hypophosphite is dissolved in about to cc. of water, then 7.5 cc. of sulphuric acid and 40 cc. of decinormal potassium permanganate V. S. are added, and the mixture is boiled for 15 minutes. Decinormal oxalic acid is then carefully delivered into the mixture until the red color, due to the excess of permanganate, is discharged. The number of cc. of the standard oxalic acid required for this purpose, sub- tracted from the 40 cc. of permanganate originally added, gives the quantity of permanganate which was actually required for the oxidation of the hypophos- phite. If the salt conforms in purity to the U. S. P. requirement, not more than 2 cc. of the oxalic acid V. S. will be required. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. !$! The following equation illustrates the reaction which takes place in this operation : 5KPH 2 O a + 6H a S0 4 + 2(2KMn0 4 ) 2)519-55 2)630.68 100)259.77 100)315.34 2.5977 gm. 3.i534gms. or looo cc. permaganate V. S. = 5KH 2 PO 4 + 2K a SO 4 + 4MnSO 4 + 6H 2 O. Each cc. of decinormal permanganate V. S. required for the oxidation of the hypophosphite, represents 0.0025977 gm. of the pure salt. If 38 cc. are required, then 0.0025977 X 38 = .0987126 gm., or 98.7+ %. Sodium Hypophosphite, NaPH 2 O s + H 2 O = \ # i" . o.i gm. of the dry salt is dissolved in 10 cc. of water and mixed with 7.5 cc. sulphuric acid and 40 cc. of decinormal potassium permanganate V. S. The mixture is then boiled for 15 minutes, and titrated with decinormal oxalic-acid solution to determine the excess of permanganate. Not more than 3 cc. of the oxalic acid V. S. should be required to discharge the red color, which means that O.I gm. of the salt should require 37 cc. of N - permaganate solution for its oxidation. The following equation shows the reaction : 5(NaPH 2 2 .H 2 O) + 6H 2 SO 4 + 2( 2 KMnO 4 ) = 2)529.2 2)630.68 100)264.6 100)315.34 2.646 gm. 3.1534 gm., N or 1000 cc. V.S. 5NaH 2 P0 4 + 2Na a S0 4 + 4 MnSO 4 + 1 1 H 2 O. 152 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Thus each cc. of the decinormal permanganate repre- sents 0.002646 gm. of NaPH 2 O 2 . Therefore 0.002646 X 37 = 0.097902 gm. or 97.9$. Aqua Hydrogenii Dioxidi U. S. P. (Solution of Hydrogen Peroxide). It is described in the U. S. P. as an aqueous solution of hydrogen dioxide, H 2 O 2 = j * , slightly acid and containing about 3$, by weight, of pure dioxide, corresponding to 10 volumes of available oxygen. This substance is official for the first time in the 'U.S. P. 1890, in which methods for its preparation, preservation, and assay are given. Solution of hydro- gen peroxide is an important commercial product, being used in the arts as well as in medicine. It is sold as containing 5, 10, 15, or 20 volumes of oxygen, in solution. This should mean that a given volume of the solution yields from itself 5, 10, 15, or 20 times its own volume of oxygen. Thus, I cc. of a 5-volume solution yields 5 cc. of oxygen ; a lo-volume solution is one of which I cc. will yield 10 cc. of oxygen ; etc. Many solutions of hydrogen dioxide are sent into the market under false pretences, being labelled as con- taining 10, 15, or 20 volumes of oxygen. It is true a given volume of these solutions will yield the specified volume of oxygen when decom- posed with potassium permanganate, but half of this oxygen comes from the permanganate itself. There- fore the peroxide of hydrogen solution contains only half as much available oxygen as is given off in this decomposition. Freshly bought samples of the five largest manufac- A TEXT-BOOK OF VOLUMETRIC ANALYSIS. I $3 turers, according to the analyses of Dr. Edward R. Squibb (Ephemeris, vol. IV. No. 2), gave 9.2, 8.7, 8.4, 10.9. 9.7, 8.6, 8.5, 7.3, and 7.4 volumes. All of these were labelled as being of 15 volumes strength. The author has had a similar experience. In its purest and most concentrated form peroxide of hydrogen is a syrupy colorless liquid, having an odor resembling that of chlorine or ozone. One cc. of this concentrated hydrogen peroxide when decomposed at o C. evolves 330.3 times its own volume of oxygen, at a pressure of 760 mm. at 45 N. latitude. At a temperature of 100 C. (212 F.) H 2 O a decom- poses rapidly into water and oxygen. This change also takes place at ordinary temperatures, but more slowly. In diluted solutions it is more stable, and may be concentrated by boiling without suffering much de- composition. Dr. Squibb made a series of experiments in order to prove this, as well as the fact that solutions of hydro- gen peroxide when kept in open vessels at the ordi- nary temperature become stronger, instead of weaker as was generally supposed. The water evaporates more rapidly than the peroxide decomposes. Part of the results of these experiments as published in the Ephemeris, vol. IV. No. 2, is as follows : A freshly made solution that yielded 10.3 volumes of available oxygen was taken as the basis of the ex- periment. The evaporation was done on a water-bath, at temperatures varying from 55 to 62 C. (131 to 143.6 F.) ; one cc. of the concentrated solution being taken out for testing after each evaporation. 200 cc. evaporated in 2 hours to 100 cc. tested 20.6 volumes : no apparent loss. 154 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 100 cc. of the io.3-volume solution were added, and evaporated in 2 hours to 100 cc., tested 29.6 volumes: 1.3 volumes loss. IOO cc. of the io.3-volume solution were added, and evaporated in 2 hours to 100 cc., tested 36.5 volumes: 4.7 volumes loss. IOO cc. of the io.3-volume solution were added, and evaporated in 2.5 hours to 23 cc., tested 146.8 volumes. Another series of evaporations were made at higher temperatures, which also showed an increase in strength, but the loss was a little larger. The Assay of Hydrogen Peroxide, H 2 O 2 = j* 33 ' 92 . The U. S. P. method is as follows: ' 34 10 cc. of the solution are diluted with water to make IOO cc. Transfer 17 cc. of this liquid (containing 1.7 cc. of the solution of H a O 3 ) to a beaker, add 5 cc. N of diluted sulphuric acid, and then from a burette potassium permanganate V. S. until the liquid just retains a faint-pink tint after being stirred. The reaction is expressed by the following equation : 5 H 2 2 100)169.6 100)315.34 N 1.696^015. 3.1534 gms. or 1000 cc. - permanganate V. S. *ioo)i70. 1.70 = K 2 SO 4 + 2MnS0 4 + 8H 2 + sO 2 . N Thus each cc. of the potassium permanganate represents .001696 (^.0017) gm. of absolute hydrogen dioxide. A TEXT-BOOK OK VOLUMETRIC ANALYSIS. -155 The U. S. P. requires that 1.7 cc. of the solution of N peroxide should decolorize 30 cc. of permanganate solution. This corresponds to 3 per cent, by weight, of H 2 O 2 . .0017 X 30 = .051 gm. .051 X 100 Estimation of Volume Strength. Let us look at the above equation in a different light. We there see that when potassium permanganate and hydrogen peroxide react, 10 atoms of oxygen are liberated. The permanganate itself when decomposed liberates five atoms of oxygen. Therefore of the above ten atoms only five come from the peroxide of hydrogen. 2KMn0 4 +sH 1 S(5 4 = K 2 SO 4 + 2MnSO 4 + 3H 2 O + 50. In order to find the factor for volume of available oxygen, see the following equation, etc.: 100)315.34 N 3.i534gms. or 1000 cc. of ~ V. S. = K 2 SO 4 + 2MnSO 4 + 8H 2 O + SO + 5O. 100)79.8 .798 gm. 100) 80 *.8o gm. 156 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. N Thus it is seen that each cc. of - potassium per- manganate represents .000798 (*.ooo8) gm. of oxygen. But we wish to find the volume of oxygen, not the N weight represented by I cc. of the permanganate. 1000 cc. of oxygen at o C. and 760 mm. pressure weigh 1.424488 grammes, *(i.43 gms.). Therefore, if 1.43 gms. measure 1000 cc., .0008 gm. will measure x. looo X .0008 - 0.5594 cc. The factor, then, for volume of oxygen, liberated N when peroxide of hydrogen is titrated with potas- sium permanganate is 0.5594, and the number of cc. N of the potassium permanganate consumed in the titration gives the volume of oxygen liberated by the quantity of hydrogen peroxide taken. N Thus if 30 cc. of the V. S. were required, 0.5594 X 30 = 16.782 cc. of oxygen, 1.7)16.782 9.87 volume strength. or the number of cc. of oxygen liberated by I cc. of the peroxide solution tested. It is convenient to operate upon I cc. hydrogen- peroxide solution. Then each cc. of potassium per- A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 157 manganate V. S. used will represent 0.5594 cc. of avail- able oxygen, or 0.0008 gm'. of oxygen, and it is only necessary to multiply the cc. by these numbers to obtain the volume or weight of available oxygen. Hydrogen-peroxide solution may also be volumetri- cally assayed by Kingzett's method, which is described in the chapter on lodimetry. The gasometric estimation is also described further on. Barium Dioxide (Barium Peroxide), BaO 2 = I *tf68* ' This substance is assayed by treating it with an acid, and then estimating the liberated hydro- gen dioxide, as follows : Weigh off 2.11 gms. of the coarse powder, put it in a porcelain capsule, add about 10 cc. of ice-cold water, then 7.5 cc. of phosphoric acid, U. S. P., and sufficient ice-cold water to make 25 cc. Stir and break up the particles with the end of the stirrer until a clear or nearly clear solution is obtained, and all that is soluble is dissolved. 5 cc. of this solution (which corresponds to 0.422 gm. of barium dioxide) is measured off for assay. Drop into this from a burette, with constant stirring, decinormal potassium permanganate V. S. until a final drop gives the solution a permanent pink tint. Not less than 40 cc. of the decinormal permanganate V. S. should be required to produce this result. In this process, the first step is the formation of hy- drogen peroxide by treating the barium peroxide with phosphoric acid, as illustrated by the following equa- tion : BaO a + H 8 P0 4 = BaHP0 4 + H a O a - 158 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. The hydrogen peroxide is then estimated with deci- normal permanganate V. S. 5H 2 2 + 2KMn0 4 + 3 H 2 S0 4 100)169.6 100)315.34 1.696 gms. 3.1534 gms. or 1000 cc. 5 permanganate V. S. 100)1.70 10 = K 2 SO 4 + 2MnSO 4 +8H 2 O + sO 2 . N Thus each cc. of - - potassium permanganate V. S. represents 0.001696 gm. (^0.0017 gm.) of H 2 O 2 ; and since 169.6 gms. of H 2 O 2 are equivalent to 844.1 gms. of BaO 2 , (8 44 .i a g ms.~i69.6 gin's.)' l cc - of the perman- ganate solution corresponds to 0.008441 gm. of BaO 2 . Not less than 40 cc. of the decinormal solution should be required. Thus .008441 X 40 = 0.3376 gm. .3376 X ioo .422 = Sofo of pure BaO 2 . Oxalic Acid, H 2 C 2 O 4 + 2H 2 O = j *\ 2 ^ 7 . Oxalic acid may be estimated either by neutralization with an alkaline V. S., or by oxidation with potassium perman- ganate V. S. The permanganate is generally used when the acid is in combination as oxalate. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 159 The reaction is illustrated as follows : 5(H.CA + 2 H,0) + 3 H,SO + 2KMnO. IOOJ628.5 6.285 gms. 3.1534 gms. or 1000 cc. permanganate V, S. 10 = K a SO 4 + 2MnSO 4 + i8H a O + loCO,. N Thus each cc. of the - - permanganate represents 0.006285 gm. of pure oxalic acid (crystallized). Note. It must be remembered, in titrating with per- manganate, that an excess of sulphuric acid is always necessary, in order to keep the resulting manganous compound in solution, by forming a soluble manganous sulphate. If hydrochloric acid is used the solution must be very dilute, and the temperature not raised too high, other- wise chlorine will be liberated, which will spoil the analysis. It should be borne in mind that the solution of potassium permanganate should not be filtered through paper, as it is decomposed by organic matter. It may, however, be filtered through gun-cotton or glass-wool. It should never be used in a Mohr's burette. l6o A TEXT-BOOK OF VOLUMETRIC ANALYSIS. TABLE OF SUBSTANCES WHICH MAY BE ESTIMATED BY OXIDATION, BY POTASSIUM BICHROMATE OR POTASSIUM PERMANGANATE, SHOWING FORMULA, MOLECULAR WEIGHT, FACTOR, ETC. Name of Substance. Formula. Mole- cular Wt. Sv.s. 10 Used. Factor (exact). Ac. hypophosphorous HPH 2 O 2 65.88 aKMnO.* Ac oxalic (crystallized) 2 KMnO 4 Barium dioxide BaOj 168.82 2KMnO 4 008441 Calcium hypophosphite Ca(PH 2 O 2 ) Q Fe 2 (PH 2 O 2 ) 6 169.67 2 KMn0 4 * 2 KMnO 4 * .0021209 Ferrous carbonate FeCO 3 1 2K.IVfnO 4 f Ferrous oxide . . . .... FeO 71.84 J K 2 Cr 2 O-r 1 j 2KMnO 4 I Ferrous sulphate (anhydrous) FeSO 4 151-58 S K 2 Cr 2 7 J 1 2KMnO 4 | -015170 Ferrous sulphate (dried) 2FeS0 4 -HH 2 357-28 K 2 Cr 2 7 J 1 2 KMnO 4 f .017864 Ferrous sulphate (crystallized). . . FeS0 4 + 7 H 2 277.42 j K 2 Cr 2 7 I ) 2 KMn0 4 f .027742 Ferrum (metallic) Fe 2 111.76 j K 2 Cr 2 7 | .005588 1 2KMnO 4 \ Hydrogen dioxide Oxygen,wt. of available, in H 2 O 2 H < 33-Q2 2 KMnO 4 2 KMn0 4 .001696 .000798 '* volume Potassium hypophosphite KPH^Oo 103.91 2 KMnO 4 2 KMnO 4 * 5S94 .002598 Sodium hypophosphite NaPH 2 O 2 -i-H 2 O 105.84 2 KMnO 4 * .002646 * Determined by residual titration with decinormal oxalic acid V. S. The factors given in this table are calculated upon the revised atomic weights, which are indorsed by the U. S. P. A TEXT BOOK OF VOLUMETRIC ANALYSIS. l6l CHAPTER XII. ANALYSIS BY INDIRECT OXIDATION. THIS method of analysis is based upon the oxidizing power of iodine. Iodine acts upon the elements of water, forming hydriodic acid with the hydrogen, and liberating oxy- gen in a nascent state. Nascent oxygen is a very active agent, and readily combines with and oxidizes many substances, such as arsenous oxide, sulphurous acid, sulphites, thiosul- phates, etc. As.0, + 2H,0 + 21, - 4 HI + AsA; H 2 S0 3 + H 2 + I, = 2HI + H 3 S0 4 . Therefore iodine is said to be an indirect oxidizer, and may be used for the estimation of a great variety of substances, with extreme accuracy. The end of the reaction in an analysis by this method is ascertained by the use of starch test solution, which produces, with the slightest trace of free iodine, a dis- tinct blue color. In making an analysis with standard iodine solution, the substance under examination is brought into dilute solution, the starch solution added, and then the iodine, in the form of a decinormal solution, is delivered in from a burette, stirring or shaking constantly, until a final 1 62 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. drop colors the solution blue a sign that a slight ex- cess of iodine has been added. Decinortnal Iodine V. S., I = { ,|*| .J^-gS J in i litre. 12.653 gms. of pure iodine are dissolved in 300 cc. of distilled water containing 18 gms. of pure potassium iodide. Then enough water is added to make the solution measure, at 15 C. (59 F.), exactly 1000 cc. The solution should be kept in small glass-stoppered vials, in a dark place. The potassium iodide used in this solution acts merely as a solvent for the iodine. If pure iodine be not at hand, it may be prepared from the commercial article as follows : Powder the iodine and heat it in a porcelain dish placed over a water-bath, stirring constantly with a glass rod for 20 minutes. Any adhering moisture, to- gether with any cyanogen iodide, and most of the iodine bromide and iodine chloride, is thus vaporized. Then triturate the iodine with about 5 per cent, of its weight of pure, dry potassium iodide. The iodine bromide and chloride are thereby decomposed, potas- sium bromide and chloride being formed, and iodine liberated from the potassium iodide. The mixture is then returned to the porcelain dish, covered with a clean glass funnel, and heated on a sand- bath. A pure resublimed iodine is then obtained. If pure iodine is. used in making this solution, there is no necessity for checking (standardizing) it. But if desired, the solution may be checked against pure arsenous acid or pure sodium thiosulphate. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 163 ESTIMATION OF ARSENOUS ACID. Arsenous Anhydride, Arsenic Trioxide, As 2 O, = I # r 9g When arsenous acid is brought in contact with iodine in the presence of water and an alkali, it is oxidized into arsenic acid, and the iodine is decolorized. The reaction is: As,0 3 + 2l 2 + 2H 2 = As 2 O B + 4HI ; NaHC0 3 + HI = Nal + H 2 O + CO 2 . The alkali should be in sufficient quantity to combine with the hydriodic acid formed, and must be in the form of potassium or sodium bicarbonate. The hydroxides or carbonates should not be used, as they interfere with the indicator. Starch solution is used as the indicator, a blue color being formed as soon as the arsenous acid is entirely oxidized into arsenic acid. o.i gm. of arsenous acid is accurately weighed and dissolved, together with about I gm. of sodium bicar- bonate, in 20 cc. of water heated to boiling. Allow the liquid to cool, add a few drops of starch T. S., and allow the decinormal iodine V. S. to flow in, shaking or stir- ring the mixture constantly, until a permanent blue color is produced. The following equation illustrates the reaction : As,0 3 + 5H 2 + 2l 2 = 4HI + 2H 3 As0 4 . 4)197.68 4)506 10) 49-42 10)126.5 N 4.942 gms. 12.65 gms. or 1000 cc. Iodine V. S, 164 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. N Thus it is seen that each cc. of the standard so- 10 lution represents 0.004942 gm. of pure As 2 O,. If 20 cc. are consumed, then 0.004942 X 20 = 0.09884 gm. .09884 X ioo ~ = 9 The U. S. P. requirement is 98.8$ of As 3 O 3 . The starch T. S. is not used in the U. S. P. process, and the end of the reaction is known by the iodine being no longer decolorized. But with starch the indication is exceedingly delicate, and it should always be used. Liquor Acidi Arsenosi, U. S. P. Measure accu- rately 10 cc. of the solution, add to it I gm. of sodium bicarbonate, and boil for a few minutes. Then allow the liquid to cool, and dilute it to 50 cc. with water. A little starch T. S. is then added and the decinormal iodine V. S. run in from a burette, until a final drop produces the blue color of starch iodide. N Each cc. of I. V. S. represents 0.004942 gm. of As 2 O 3 . (See Estimation of Arsenous Acid.) The U. S. P. requirement is that 24.7 cc. of the liquor acidi arsenosi, when treated as above, will con- sume 49.4 to 50 cc. of decinormal iodine V. S. Use 2 gms. of the bicarbonate. 0.004942 X 50 0.2471 gm. 0.2471 X ioo =. \% 24.7 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 165 Liquor Potassii Arsenitis, U. S. P. (Fowler's Solu- tion). The process is exactly the same as the fore- going. 24.7 cc., diluted and treated with 2 gms. of sodium bicarbonate, should require 49.4 to 50 cc. of N the I. V. S., corresponding to \ of As 2 O 3 . Sulphurous Acid (Acidum Sulphurosum, U. S. P.) This is an aqueous solution of sulphur dioxide, SO 2 = *^'^, containing 6.4 per cent., by weight, of the gas. Sulphurous acid when brought in contact with iodine is oxidized into sulphuric, the iodine being decolorized because of its union with the hydrogen of the accom- panying water, forming hydriodic acid. Two gms. of sulphurous acid are taken and diluted with distilled water (recently boiled and cooled) to about 25 cc. The decinormal iodine V. S. is then delivered into the solution (to which a little starch T. S. had been previously added) until a permanent blue color is produced. At least 40 cc. of the standard iodine solution should be consumed before this color appears. The following equations, etc., show the reaction that takes place : H 2 S0 3 + H,0 + I, = 2HI + H 9 SO 4 . 2)8 i. S6 2)253 10)40.93 10)126.5 4.093 gms. 12.65 S ms ' or 1000 cc. V. S. N Thus each cc. of the V. S. represents .004093 gm. of pure H 2 SO 3 . Sulphurous acid being, however, looked upon as a l66 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. solution of SO 2 in water, the quantity of this gas is generally estimated in analysis. H 2 0,S0 2 + H a O + I 2 = 2HI + H 2 S0 4 . 2)63.9 2)253 10)31.95 10)126.5 3 195 gms. 12.65 gms. N Thus each cc. of - - V. S. consumed before the blue 10 color appears represents 0.003195 gm. of SO 2 . If 40 cc. are consumed in the above analysis, the 2 gms. contain 0.003195 X 40 = 0.1278; then 0.1278 X ioo ; = 6.39^ Of S0 a . The sulphurous acid should be diluted with distilled water to below 0.04 per cent before titrating it ; for if it is not sufficiently diluted there is a risk of the sul- phuric acid formed, being again reduced to sulphurous, with liberation of iodine, thus causing irregular results. This may, however, be obviated by adding at once N a measured excess of iodine V. S. and titrating back N with sodium thiosulphate V. S. The direction to boil the distilled water is given for the purpose of freeing it from air, which would have a tendency to partially oxidize the sulphurous acid. Sodium Sulphite, Na 2 SO 3 + 7H 3 O = j ^ sS .- One gm. of the salt is dissolved in 25 cc. of distilled A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 167 water recently boiled to expel air, a little starch T. S. is added, and then the decinormal iodine V. S. de- livered in from a burette, until the blue color of starch iodide appears, which does not disappear upon shaking or stirring. The reaction is expressed as follows : Na a SO 3 + ;H 2 O + I a = 2NaI + H a SO 4 + 6H a O. 2)251.58 2)253 10)125.79 io)i26.5_ N ' 12.579 g ms 12.65 g ms - r 1000 cc. - iodine V. S. Thus each cc. of the standard solution represents .012579 gm. of crystallized sodium sulphite. If i gm. of the salt is taken, to find the percentage multiply the factor by the number of cc. of standard solution consumed, and the result by 100. The U. S. P. requirement is 96 per cent. 0.63 gm. of salt should require for complete oxidation 48 cc. of the standard solution. Therefore .012579 X 48 = .603792 gm. 0.603792 X 100 0.63 = 95.8^ Potassium Sulphite, K a SO 3 + 2H,O = ^194.- Operate upon 0.5 gm. in the same manner as for sodium sulphite. KSO, + 2H 3 + I 2 = 2KI + H a S0 4 + H 2 0. 2)194 2)253 10) 97 10)126.5 9.7 gms. 12.65 gms. or 1000 cc. of standard V S. l68 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Each cc. of the decinormal iodine V. S. used rep- resents 0.0097 gm. of crystallized potassium sulphite. If 46 cc. are used, the salt is over 89$ strength. Sodium Bisulphite, NaHSO 3 = j x 1 ^'* 6 . Dis- solve 0.26 gm. of the salt in 20 cc. of distilled water which has been previously boiled to expel air, add a little starch T. S., and pass in the decinormal iodine V. S. from a burette, until a permanent blue color appears. At least 45 cc. should be required. Apply the following equation : NaHS0 3 + I, + H,0 = Nal + HI + H 2 SO 4 . 2)103.86 2)253 10) 5I.Q3 10)126.5 N 5.193 gms. 12.65 gms. or 1000 cc. V. S. Thus each cc. of decinormal iodine V. S. represents 0.005193 gm. of sodium bisulphite. 0.005193 X 45 = 0.23368 gm. 0.23368 X ioo 0.26 Sodium Thiosulphate (Sodium Hyposulphite), Na 2 S 2 3 +5H 2 - | ^47- when brought in contact with iodine, is converted into tetrathionate of sodium, and the iodine is decolorized. It is estimated as follows : 0.25 gm. of the salt is dissolved in 10 cc. of water, a few drops of starch T. S. N . are added, and then the iodine V. S. is delivered in from a burette, until the appearance of blue starch iodide indicates an excess of iodine. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 169 At least 9.9 cc. of the standard solution should be added before a final drop produces a permanent blue color. The reaction is expressed as follows : 2(Na,S,0..5H,0) + I, 2)495-28 2)253 10)247.64 10)126.5 N 24.764 gms. 12.65 g ms - or I0 o cc - L V. S. = 2NaI + Na,S 4 O 6 + ioH 2 O. Thus each cc. represents .024764 gin. of crystallized thiosulphate. 9.9 cc. contain 0.024764 gm. X 9-9 = .2451636 gm. 0.2451636 X ioo 0.25 = 98.1; Iodine may also be used for estimating antimonous compounds. The reaction is similar to that with arsenous compounds ; thus Sb 2 0, + 2H a O + 2l 3 = Sb,0 5 + 4 HI. Antimony and Potassium Tartrate (Tartar Emet- ic), 2(K(SbO)C 4 H 4 8 ) + H 2 = j464 42 .-This is the only antimonial salt, a process for the volumetric estimation of which is given in the U. S. P. The U. S. P. directs that 0331 gm. of the crystal- lized salt or 0.322 gm. of the salt dried at 110 C. (230 F.) be taken for analysis. The salt is dissolved in 10 cc. of water, and about 20 cc. of a cold saturated solution of sodium bicarbonate and a little starch T. S. added. The decinormal iodine V. S. is then delivered in from a burette, until the blue color of the starch A TEXT-BOOK OF VOLUMETRIC ANALYSIS. iodide makes its appearance, indicating that the salt has been completely oxidized and that the iodine solu- tion has been added in slight excess. Not less than 20 cc. of decinormal iodine V. S. should be consumed before the blue color appears. The reaction is illustrated by the following equa- tion : 2(K(SbO)C 4 H 4 0.)+H,0 + 2l 9 + 3 H a O 4)662.42 4)506. 10)165.605 IO )l_ 6 jJL N 16.5605 gms. 12.65 gnis. or 1000 cc. I.V.S. = 4HI + 2KHC 4 H 4 6 + 2HSb0 8 . N Thus each cc. of iodine V. S. represents 0.0165605 gm. of pure crystallized tartar emetic. 2(K(SbO)C 4 H 4 O.) (anhydrous). 4)644.46 10)161.115 N 16.1115 gms. = 12.65 gms. of iodine or 1000 cc. V. S. N Thus each cc. of iodine V. S. represents 0.0161115 gm. of anhydrous tartar emetic. Thus 20 cc. = 0.0165605 gm. X 20 = .33121 gm. ; 0.33121 X ioo -H - = 100$ crystallized salt ; 0.331 and 0.0161 115 X 20 = 0.32223 gm. 0.32223 X ioo * = 100$ anhydrous salt. 0.322 * A TEXT-BOOK OF VOLUMETRIC ANALYSIS. I? I The operation should be quickly conducted or a precipitate of antimonous hydrate will be formed, upon which the iodine has little effect. The antimony must be in solution to be properly attacked. TABLE OF SUBSTANCES ESTIMATED BY IODINE. ^J Name of Substance. Formula. It Factor. ^ Antimony and potassium | tartrate f 2 (K(SbO)C 4 H 4 8 )+H 2 662.42 j Cryst. .016560 1 Anhydr. .016111 Arsenic trioxide As 2 3 197.68 .004942 Potassium sulphite 103.84 Sodium bisulphite 2 Na S HSO 3 2 103.86 .005193 Sodium sulphite NcioSOa -4- yHoO 251.58 .012579 Sodium thiosulphate | (hyposulphite) J Na 2 S a 3 + 5 H a O 247.64 .024764 Sulphur dioxide SO, 63 90 Sulphurous acid H 2 SO 3 81.86 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. CHAPTER XIII. ESTIMATION OF SUBSTANCES READILY REDUCED. ANY substance which readily yields oxygen in a defi nite quantity, or is susceptible of an equivalent action, which involves its reduction to a lower quantivalence, may be quantitatively tested, by ascertaining how much of a reducing agent of known power is required by a given quantity of the substance for its complete re- duction. The principal reducing agents which may be em- ployed in volumetric analysis are sodium thiosulpJiate, sulphurous acid, arsenous acid, oxalic acid, metallic zinc, and magnesium. The sodium thiosulphate is the only one which is employed officially in the U. S. P. in the form of a volumetric solution. It is used in the estimation of free iodine, and indirectly of other free halogens, or compounds in which the halogen is easily liberated, as in the hypochlorites, etc. This method of analysis is called lodometry. It depends upon the fact that iodine is an indirect oxidizer, as shown by its action upon water, the hydro- gen of which it abstracts, forming hydriodic acid, thus liberating the oxygen in a nascent state. When sodium thiosulphate acts upon iodine, sodium tetrathionate and sodium iodide are formed, and the solution is decolorized. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 173 This reaction takes place in definite proportions: one molecular weight of the thiosulphate, absorbs one atomic weight of iodine. 2Na t SA + I, = 2NaI + Na a S 4 O 8 - Chlorine cannot be directly titrated with the thiosul- phate, but by adding to the solution containing free chlorine an excess of potassium iodide, the iodine is liberated in exact proportion to the quantity of chlorine present, atom for atom. Cl, + 2KI = 2KC1 + I,. Then by estimating the iodine, the quantity of chlorine is ascertained. All bodies which contain available chlorine, or which when treated with hydro- chloric acid evolve chlorine, may be estimated by this method. Also, bodies which contain available oxygen, and which when boiled with hydrochloric acid evolve chlorine, such as manganates, chromates, peroxides, etc., may be estimated in this way. Solutions of ferric salts, when acidulated and boiled with an excess of potassium iodide, liberate iodine in exact proportion to the quantity of ferric iron present. Thus sodium thiosulphate may be used in the esti- mation of a great variety of substances with extreme accuracy. Preparation of Decinormal Sodium Thiosluphate (Hyposulphite), Na 2 S 2 3 + 5 H 2 0- | ^ >64 contains ^ 2 4-74 ( gms. in I litre. Sodium thiosulphate is a salt of thiosulphuric acid in which two atoms of hydrogen have been replaced by sodium ; it therefore seems that a 174 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. normal solution of this salt should contain one half the molecular weight in grammes in one litre. But this salt is used chiefly for the estimation of iodine, and, as stated before, one full molecular weight reacts with and decolorizes one atomic weight of io- dine; and since one atom of iodine is chemically equiv- alent to one atom of hydrogen, a full molecular weight of sodium thiosulphate, must be contained in a litre of its normal solution. Sodium thiosulphate is easily obtained in a pure state, and therefore the proper weight of the salt, re- duced to powder and dried between sheets of blotting- paper, maybe dissolved directly in water, and made up to one litre. The U. S. P. directs that a stronger solution than necessary be made, its titer found by iodine, and then the solution diluted to the proper measure. 30 gms. of selected crystals of the salt are dissolved in enough water to make, at or near 15 C. (59 F.) uoo cc. Transfer locc. of this solution into a flask or beaker, add a few drops of starch T. S., and then gradually deliver into it from a burette decinormal iodine solu- solution, in small portions at a time, shaking the flask after each addition, and regulating the flow to drops toward the end of the operation. As soon as a blue color is produced which does not disappear upon shak- ing, but is not deeper than pale blue, the reaction is completed. Note the number of cc. of iodine solution used, and then dilute the thiosulphate solution so that equal volumes of it and the decinormal iodine V. S. will exactly correspond to each other, under the above- mentioned conditions. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 1/5 Example. The 10 cc. of sodium thiosulphate, we will assume, require 10.7 cc. of decinormal iodine V. S. The sodium-thiosulphate solution must then be diluted in the proportion of 10 cc. to 10.7 cc., or 1000 cc. to 1070 cc. After the solution is thus diluted a new trial should be made, in the manner above described, in which 50 cc. of the thiosulphate solution should require exactly 50 cc. of the decinormal iodine V. S. to produce a faint-blue color. The solution should be kept in small dark amber- colored, glass-stoppered bottles, carefully protected from dust and air. One cc. of this solution is the equivalent of : Iodine ..... . ........ ... 0.012653 gramme. Bromine ................ 0.007976 " Chlorine .......... . ..... 0.003537 " Iron in ferric salts ....... 0.005588 " Iodine, 1= V*i' Dissolve 0.32 gm. of iodine in 20 cc. of water, in a beaker or flask, with the aid of i gm. of potassium iodide; the solution is mixed with a few drops of starch T. S., and then the deci- normal sodium thiosulphate V. S. gradually delivered in from a burette, in small portions at a time, shaking the flask after each addition, and regulating the flow to drops toward the end of the reaction, until a final drop just discharges the blue color. Note the number of cc. of decinormal sodium thio- sulphate V. S. consumed, and multiply this number by the factor for iodine. 1 76 A TEXT-BOOK OP^ VOLUMETRIC ANALYSIS. 2(Na 3 S 2 O 3 + 5H,O) + I, = Na 2 S 4 O 6 + 2NaI + ioH,O. 2)496 2)253 10)248 10)126.5 24.8 gms. or 12.65 ms ' looo cc. V. S. 10 Thus the factor for iodine, that is, the quantity equiv- N alent to I cc. of thiosulphate V. S., is 0.01265 gm. 0.32 gm. of iodine, which answers to the tests of the N U. S. P., requires at least 25 cc. of the V. S. 0.01265 X 25 = 0.31625 gm. .31625 X TOO -2-Cl - = 98.8$ pure iodine. Liquor lodi Compositus (Lugol's Solution). This is an aqueous solution of iodine and potassium iodide. It is estimated for iodine in the same way as the foregoing. The potassium iodide acts merely as a sol- vent for free iodine, and does not enter into the re- action. 10 or 12 gms. of the solution is a convenient quan- tity to operate upon. Starch T. S. is the indicator. The U. S. P. states that 12.66 gms. of the solution should require for complete decoloration from 49.3 to so cc. of decinormal sodium thiosulphate V. S. N As shown by the above equation, each cc. of the V. S. represents 0.01265 gm. of pure iodine. There- fore 50 cc. represent 0.01265 X 50 = .6325 gm. .6325 X ioo = 5$ pure iodine, about. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 1 77 Tinctura lodi (Tincture of Iodine). This is an al- coholic solution of free iodine, and must be diluted with a solution of potassium iodide, before titration, in order to provide sufficient liquid to keep the resulting salts in solution. Aqua Chlori (Chlorine Water) This is an aqueous solution of chlorine, Cl = j ?f i containing at least \ J'T" 0.4$ of the gas. The estimation of chlorine is effected in an indirect way, namely, by determining the quantity of iodine which it liberates from potassium iodide. A definite quantity of chlorine will liberate a definite quantity of iodine from an iodide ; these quantities are in exact proportion to their atomic weights, as the equation shows : 2KI + Cl, = 2KC1 + I a 2)70.74 2)253 10)35.37 10)126. s_ 3-537 gms. 12.65 gms. Thus it is seen that by estimating the liberated io- dine the quantity of chlorine may be determined with accuracy. Ten gms. is a convenient quantity to operate upon. To this about half a gramme of potassium iodide is added. A little starch T. S. is then introduced, and the titration is begun, with decinormal sodium thiosul- phate V. S. When the blue color of starch iodide has entirely tLs.ippeared the reaction is finished. The reaction between iodine and sodium thiosulphate is illustrated by the following equation * 178 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. I 2 + 2(Na 2 S a O s + 5H 1 0) 2)253 2)496 10)126.5 10)248 12.65 gms. 24.8 gms. or 1000 cc. - V. S. 10 N Thus we see that 1000 cc. of Na 2 S,O 3 ,5H a O repre- sent 12.65 g ms - of iodine, which are equivalent to 3.537 gms. of chlorine. Each cc. therefore is equivalent to .003537 g m f chlorine. This number is the factor which, when mul- N tiplied by the number of cc. of thiosulphate V. S. used, gives the weight in grammes of chlorine, contained in the quantity of chlorine water acted upon. The U. S. P. requirement is that 17.7 gms. of chlorine water, when mixed with I gm. of potassium iodide dis- N solved in 10 cc. of water, and titrated with -- sodium thiosulphate V. S. should consume not less than 20 cc. of the latter in decolorizing the solution. 003 5 37 X 20 = . 07074 gm .07074 X ioo 17.7 of chlorine. Chlorinated Lime (Calx Chlorata, Chloride of Lime, Bleaching-powder). This substance was formerly sup- posed to be a compound of lime and chlorine, CaOCl 3 , and hence the name chloride of lime. It is now gener- ally considered to be a mixture of calcium chloride and A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 179 calcium hypochlorite, CaCl, + Ca(ClO), or 2(CaOCl a ). The hypochlorite is the active constituent. This is a very unstable salt, and is readily decomposed even by carbonic acid. When treated with hydrochloric acid it gives off chlorine. The value of chlorinated lime as a bleaching or dis- infecting agent depends upon its available chlorine, that is, the chlorine which the hypochlorite yields when treated with an acid. In estimating the available chlorine, the latter is liberated with hydrochloric acid. This liberated gas, then, acting upon potassium iodide, sets free an equiva- lent amount of iodine. The quantity of iodine is then determined, and thus the amount of available chlorine found. .1 to .2 gm. is a convenient quantity to oper- ate upon. The U. S. P. directs to weigh off ^0.35 (0.354) gm. of chlorinated lime. This is to be thoroughly triturated with 50 cc. of water and carefully transferred, together with the washings into a flask. 0.8 gm. or more of po- tassium iodide and 5 cc. of diluted hydrochloric acid are then added, and into the resulting reddish-brown liquid, N the sodium thiosulphate V. S. is delivered from a burette. Towards the end of the titration, when the brownish color of the liquid is very faint, a few drops of starch T. S. are added and the titration continued until the bluish or greenish color produced by the starch has entirely disappeared. Not less than 35 cc. of the volumetric solution should be required to pro- duce this result. The reactions which take place in this process are illustrated by the following equations : 180 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. , + 4HC1 = 2CaCl a + 2H a O + 2C1 9 2d a + 4KI = 4KC1 + 2l f . 4)141-48 4)506 10) 35-37 10)126.5 3-537 gms. 12.65 gms. 2l a 40)506 12.65 gins. 24.8 gms. or 1000 cc. thiosulphate V. S. We thus see that I cc. of the decinormal volu- metric solution represents 0.01265 gm. of iodine, which is equivalent to 0.003537 g m - f available chlorine. Then 0.003537 X 35 = o.i237grgm. 0.12379 x ioo 35 = 35$ of available chlorine. This is a very rapid method for estimating chlorine ; but when calcium chlorate is present in the bleaching- powder (and it often is, through imperfect manufact- ure) the chlorine from it, is recorded, as well as that from the hypochlorite, the chlorate being decomposed into chlorine, etc., by hydrochloric acid. The chlorate, however, is of no value in bleaching; its chlorine is not available. Hence, unless the powder is known to be free from chlorate, the analysis should be made by means of arsenous-acid solution. The Arsenous-acid Process. 0.35 gm. of the bleaching-powder is rubbed to a smooth paste with 50 cc. of water, as described above. A measured excess of decinormal arsenous acid V. S. is then added ; this A TEXT-BOOK OF VOLUMETRIC ANALYSIS. l8l is followed by a little starch T. S., and then decinormal iodine V. S. added until the blue color appears. De- duct the number of cc. of the standard iodine solution used from those of standard arsenous-acid solution, and the quantity of the latter which went into combi- nation is found. N Each cc. of As,O, V. S. represents .003537 g m - f available chlorine. 2CaOCl, + As 2 O 3 = As 2 O 6 + 2CaCl, 4)141-48 4)198 10) 35-37 10) 49-5 N 3-537 gms. 4.95 gms. or 1000 cc. ^ V. S. Decinormal Arsenous-acid Solution is made by dissolving 4.95 gms. of the purest sublimed arsenous anhydride (As,O,) in about 250 cc. of distilled water with the aid of about 20 gms. of pure potassium bicar- bonate. The acid should be in fine powder, and the mixture warmed, to effect complete solution. The solution is checked with decinormal iodine V. S., using starch as indicator. Decinormal arsenous-acid solution and decinormal iodine solution should correspond, volume for volume. Liquor Sodae Chloratae (Solution of Chlorinated Soda ; Labarraque's Solution). This is an aqueous solution of several chlorine compounds of sodium, principally sodium chloride and hypochlorite, contain- ing at least 2.6$ of available chlorine. In this solution, as in chlorinated lime, it is the available chlorine which is estimated. The chlorine is first liberated with hydrochloric or sulphuric acid ; this then liberates iodine from potassium iodide, and the 1 82 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. free iodine is then determined by standard solution of thiosulphate. ^6.7 (6.74) gms. of chlorinated soda solution are mixed with 50 cc. of water, 2 gms. of potassium iodide, and 10 cc. of hydrochloric acid, together with a few drops of starch T. S. Then pass into the mixture from a burette sufficient decinormal sodium thiosulphate V. S. to just discharge the blue or greenish tint of the liquid. The reaction is illustrated by the following equation. Hydrochloric acid liberates chlorine from the salts in the solution : NaCl,NaC10 + 2HC1 = 2NaCl + H a O + Cl a . 70.74 The chlorine then liberates iodine from potassium iodide: Cl a + 2KI = 2KC1 + I,. 20)70.74 20)253 3-537 12.65 The iodine is then determined by sodium thiosul- phate V. S. : I, + 2(Na a S 2 O 3 +5H a O)=2NaI+Na a S 4 O fl +ioH 2 0. 2)253 2)496 10)126.5 10)248 12.65 gms. 24.8 gms. or 1000 cc. V. S. Thus each cc. of standard solution represents .01265 gm. of iodine, which is equivalent to .063537 gm. of available chlorine. In practice the potassium iodide should always be added before the hydrochloric acid is, so that the A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 183 chlorine has potassium iodide to act upon, as soon as it is itself liberated, and thus any loss of chlorine is obviated. In the pharmacopceial test above given not less than CO cc. of the V. S. should be required. 10 0.003537 X 50 = 0.17785 gm. 0.17785 X IPO = 2>6 ^ available ci. 6.7 Instead of weighing off the U. S. P. quantity, any other convenient weight may be taken. ESTIMATION OF FERRIC SALTS. When a ferric salt in an acidulated solution is di- gested with an excess of potassium iodide the salt is reduced to the ferrous state, and iodine is set free. Fe 2 Cl 6 + 2KI = 2FeCl 3 + 2KC1 + I 2 . One atom of iodine is liberated for each atom of iron in the ferric state. The liberated iodine is then determined by sodium thiosulphate, in the usual way. 12.65 gms. of iodine = 5.6 gms. of metallic iron. This is the method of the U. S. P. ; it is given in detail here. ^0.56 (0.5588) gm. of the salt is dissolved in 10 or 15 cc. of water and 2 cc. of hydrochloric acid in a glass- stoppered bottle having a capacity of about 100 cc. i gm. of potassium iodide is then added, and the mix- ture digested for half an hour at a temperature of 40 C. (104 F.). During the digestion the stopper should be 1 84 A TEXT-BOOK OF VOLUME-TRIG ANALYSIS. left in the bottle, and the heat not allowed to rise too high, otherwise the liberated iodine will be volatilized. When cool a few drops of starch T. S. are added. N It is now ready for titrating with sodium thiosul- 10 phate. Each cc. corresponds to I per cent, of metallic iron. When the quantity of metallic iron and the chemical formula for the ferric salt under estimation are known, the quantity of pure salt is easily found by calculation. In all the estimations of ferric iron it is convenient to take 0.56 gm. of the salt. Each cc. of the volumetric solution used will then represent \ of metallic iron, assuming the atomic weight of iron to be 56. Ferric salts may be tested in many, other ways ; for instance: A ferric salt in solution may be filtered through a column of zinc dust, which reduces it to the ferrous state. This is then estimated with potassium perman- ganate V. S. in the usual method, or the ferric solution is treated with a few small pieces of zinc or magnesium coarsely powdered, until complete reduction is effected. When a red color is no longer produced by sulphocy- anate of potassium the ferric salt is completely re- duced, and may be estimated with potassium perman- ganate V. S. Stannous chloride, ammonium bisulphite, and other substances may also be used as reducing agents. Ferric Chloride, Fe 2 Cl 6 + I2H 2 O == j J 39 ' 5 . ( 54-4 ^0.56 (0.5588) gm. of the salt is dissolved in a glass- stoppered bottle (having a capacity of about 100 cc.) in 10 cc. of water and 2 cc. of hydrochloric acid, and A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 185 after the addition of I gm. of potassium iodide, is kept for half an hour at a temperature of 40 C. (104 F.), then cooled, mixed with a few drops of starch T. S., and titrated with decinormal sodium thiosulphate V. S. until the blue or greenish color of the liquid is dis- charged. Each cc. represents ^0.0056 gm. or \% of metallic iron, or 0.026975 gm. of pure ferric chloride. The following equations illustrate the reactions: Fe 2 Cl B +i2H 2 O+2KI = 2FeCl 2 -[- 2KC1 + I, + I2H 2 O. 20)539-5 20)253 26.975 gms. 12.65 g ms - Then 2(Na i S 1 I 20)253 20)496 N 12.65 gms. 24.8 gms. or 1000 cc. V. S. 10 = 2NaI + Na 2 S 4 O 6 + ioH 2 O. 20 cc. of the V. S. should be required, which rep- 10 resents 20$ of metallic iron, or 96.34$ of pure ferric chloride (crystallized) : 0.026975 X 20 = 0.5395 gm. = 93- Liquor Ferri Chloridi (Solution of Ferric Chloride). This is an aqueous solution of ferric chloride, Fe 2 Cl 6 = | * > containing about 37.8 per cent, of the an- hydrous salt or about 13 per cent, of metallic iron. 1 86 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 0.56 (or .5588) gm. of the solution is introduced into a glass-stoppered bottle (having a capacity of about 100 cc.), together with 1 5 cc. of water and 2 cc. of hydro- chloric acid, i gm. of potassium iodide is then added, and the mixture kept for half an hour at 40 C. (104 F.), then cooled, and mixed with a few drops of starch N T. S. and titrated with - - sodium thiosulphate V. S. until the blue or greenish color of the liquid is dis- charged. 0.56 gm. of the solution having been taken, each cc. of the standard solution represents I per cent, and 13 cc. should be required. If 1.12 gms. are taken, as the U. S. P. directs, each cc. represents 0.5 per cent, and 26 cc. should be required. The reactions are the same as in ferric chloride, each cc. representing 0.026975 gm. of crystallized ferric chloride, or 0.016199 gm. of anhydrous ferric chloride, or .0056 gm. of metal- lic iron. To find percentage : Multiply by number of cc. used, then multiply the result by 100 and divide by the quantity of solution taken. Tinctura Ferri Chloridi (Tincture of Ferric Chlo- ride). A hydro-alcoholic solution of ferric chloride, Fe a Cl 6 = | *?Jr? > containing about 13.6 per cent. of anhydrous ferric chloride, and corresponding to about 4.7 (4.69) per cent, of metallic iron. To estimate this tincture follow the directions given for liquor ferri chloridi. Ferric Citrate, Fe,(C.H.O T ). = ) ,^; 4 *.-*o.56 (0.5588) gm. of the salt is dissolved in a glass-stop- pered bottle (having a capacity of 100 cc.) in 15 cc. of water and 2 cc. of hydrochloric acid, with the aid of gentle heat. I gm. of potassium iodide is then added, A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 187 and the mixture kept for half an hour at a temperature of 40 C. (104 F.). It is then cooled, and a few drops of starch T. S. added. The decinormal sodium thio- sulphate V. S. is then delivered in from a burette, until the blue or greenish color of the liquid just disappears. Each cc. of the decinormal solution represents I per cent, or 0.0056 gm. of metallic iron, corresponding to 0.024424 gm. of ferric citrate. 3Fe,(C.H i O,),+6KI=2Fe s (C.H ( 0,),+2K 8 C,H.O,+3l,. Ferric citrate. Ferrous citrate. s 3 Fe 2 \ 4 88. 4 8 6)335.28 3 10)126.5 10) 55-88 6)1465.44 12.65 5.588 gms. 10) 244.24 (*5.6 gms.) 24.424 gms. !,.+ 2(Na 2 S 2 3 , 5H a O) = 2NaI+Na 2 S 4 6 + ioH 2 0. 2)253 2)496 10)126.5 10)248 12.65 g ms - 2 4-8 gms. or looo cc. sodium thiosulphate. Thus each cc. represents 0.01265 gm. of iodine, which corresponds to 0.024424 gm. of ferric citrate or ^0.0056 gm. metallic iron. 16 cc. = 16 X 0.0056 = .0896 gm. metallic iron. .0896 X IPO = l6 , 0.56 16 X 0.024424 = 0.390784 gm. ferric citrate. 0.390784 X IPO = 6 , 0.56 Liquor Ferri Citratis (Solution of Ferric Citrate). This is an aqueous solution of ferric citrate, corre- sponding to about 7.5 per cent, of metallic iron. 188 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. ^0.56 (0.5588) gm. of the solution is introduced into a glass-stoppered bottle (having a capacity of about IOO cc.), together with I 5 cc. of water and 2 cc. of hy- drochloric acid, i gm. of potassium iodide is then added, and the mixture kept at a temperature of 40 C. (104 F.) for half an hour; it is then cooled and mixed with a few drops of starch T. S., and deci- normal thiosulphate V. S. delivered in from a burette until the blue or greenish color of the liquid is dis- charged. Each cc. of the volumetric solution indicates \% of metallic iron. If *I.I2 (1.1176) gms. of the liquor are taken, as the U. S. P. directs, each cc. of the V. S. used represents 0.5$ of metallic iron. Iron and Ammonium Citrate (Ferri et Ammonii Citras). The precise chemical constitution of this preparation is not determined. Therefore the metallic iron only is estimated, of which it should contain 16 per cent. Ammonio-ferric Tartrate (Ferri et Ammonii Tar- tras). The exact chemical composition of this com- pound is not known. It is, theoretically, 2(FeO)- NH 4 C 4 H 4 O 6 .3H 2 O). It should contain 17 per cent, of metallic iron. Potassio-ferric Tartrate (Ferri et Potassii Tartras). There is some difference of opinion as to the com- position of this salt. It is probably a double salt, con- sisting of one molecule of ferric tartrate, Fe a (C 4 H 4 O e ) 3 and one of potassium tartrate, K 2 C 4 H 4 O 6 , with one of H 2 O. It should contain 15 per cent, of metallic iron. Soluble Ferric Phosphate (Ferri Phosphas Solu- bilis). This salt is called soluble ferric phosphate in order to distinguish it from the true ferric phosphate. It is not a definite chemical compound, but a mixture A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 189 of citrate and phosphate of sodium and iron It should contain 12 per cent of metallic iron. The foregoing four salts being of indefinite chemical composition, are tested for metallic iron only, as follows : -5 6 (0.5588) gm. of the salt is dissolved in a glass- stoppered bottle (having a capacity of 100 cc.) in 15 cc. of water and 2 cc. of hydrochloric acid. I gm. of po- tassium iodide is then added, and the mixture kept at 40 C. (104 F.) for half an hour, then cooled, a few drops of starch T. S. added, and decinormal sodium thiosulphate V. S. delivered in slowly from a burette until the blue or greenish color of the liquid is com- N pletely discharged. Each cc. of V. S. represents I per cent, of metallic iron, if 0.56 (0.5588) gm. of the salt is taken. Iron and Quinine Citrate (Ferri et Quininae Cit- ras). The U. S. P. gives an assay process for quinine and one for iron to be applied to this salt* ESTIMATION OF THE QUININE. 1. 12 (1.1176) gms. of the salt are dissolved in a capsule in 20 cc. of water, with the aid of gentle heat. The solution is poured into a separator, the capsule is rinsed with a little water, and the rinsings added to the liquid in the separator ; when this has become cool, add 5 cc. of ammonia water and 10 cc. of chloroform, and shake. Allow the liquids to separate, draw off the chloroformic layer, and add to the residual liquid a second and a third portion of 10 cc. of chloroform added, shaking after each addition, and drawing off the chloroformic solution. The combined chloroformic A TEXT-BOOK OF VOLUMETRIC ANALYSIS. solutions are evaporated spontaneously in a tared cap- sule, and the residue dried at 100 C. (212 F.) to a constant weight. It should weigh not less than 0.1288 gm. 0.1288 X TOO 1.1176 = 11.5$ of dried quinine. In the above assay the ammonia water precipitates the quinine and the chloroform dissolves it. Then by evaporating the chloroformic solution the quinine is obtained. ESTIMATION OF THE IRON. The aqueous liquid from which the quinine has been removed, as above described, is heated on a water-bath until the odor of chloroform and ammonia has disap- peared; allow the liquid to cool, and dilute it with water to the volume of 50 cc. Take 25 cc. of this, put it in a glass-stoppered bottle (having a capacity of 100 cc.), add 2 cc. of hydrochloric acid and I gm. of potassium iodide, and digest at 40 C. (104 F.) for half an hour. Allow it to cool, add a few drops of starch T. S. and titrate with decinormal sodium thiosulphate V. S. until the blue or greenish color is discharged. Each cc. of the volumetric solution represents 0.0056 (.005588) gm. of metallic iron, or I per cent. 14.5 cc, should be required. 0.0056 X 14.5 = 0.0812 gm. 0.0812 X IPO _ ^ 0.56 Soluble Citrate of Iron and Quinine (Ferri et Quininae Citras Solubilis). This salt is assayed for A TEXT-BOOK OF VOLUMETRIC ANALYSIS, 19! quinine and iron in the manner above described under Ferri et Quinince Citras, and should respond to the requirements for the latter. Iron and Strychnine Citrate (Ferri et Strychninae Citras). This salt should be tested quantitatively for strychnine and iron. ESTIMATION OF THE STRYCHNINE. *2.24 (2.2352) gms. of the salt are dissolved in a sep- arator in 15 cc. of water, 5 cc. of ammonia water are then added and 10 cc. of chloroform, and the mixture shaken. Set aside so as to allow the liquids to separate, draw off the chloroformic layer, add a second and a third portion of 10 cc. of chloroform, shaking each time and drawing off the chloroformic solution. The chloroformic extracts are then mixed, and allowed to evaporate spontaneously in a tared capsule. The resi- due is then dried at 100 C. (212 F.) to a constant weight. This residue should not weigh less than 0.02 gm. nor more than 0.0224 gm., corresponding to not less than 0.9 nor more than i per cent, of strychnine. .0224 X IQO _ , 2.24 ESTIMATION OF THE IRON. The aqueous liquid from which the strychnine has been removed in the manner described above, is heated on a water-bath until the chloroform and ammonia are entirely volatilized. This is then allowed to cool, and diluted with water to the volume of 100 cc. 25 cc. of I Q2 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. this are transferred to a glass-stoppered bottle (having a capacity of 100 cc.), 2 gms. of hydrochloric acid and I gm. of potassium iodide are then added, and the mix- ture kept at a temperature of 40 C. (104 F.) for half an hour. After it has been allowed to cool add a few drops of starch T. S., and titrate with decinormal so- dium thiosulphate V. S. until the blue or greenish color N of the liquid is entirely discharged. 16 cc. of the V. S. should be required to produce this result, each cc. corresponding to I per cent, or 0.0056 gm. of metallic iron. 0.0056 x 1 6 = 0.0896 gm. ao8 9 6 * I0 = ,6* of Fe. O.5O Ammonio-ferric Sulphate (Ferri et Ammonii Sul- phas ; Ammonio-ferric Alum), Fe 2 (SO 4 ) 3 .(NH 4 ) 3 SO 4 2 ' 1 . This salt has a definite chemical 24H 2 O = | composition, and therefore by determining the quan- tity of metallic iron the quantity of pure salt may be found by calculation. The U. S. P. process for assay is as follows : 0.56 (0.5588) gm. of the salt is dissolved in a glass- stoppered bottle (having a capacity of 100 cc.) in 15 cc. of water and 2 cc. of hydrochloric acid, I gm. of potas- sium iodide is then added, and the mixture kept at a temperature of 40 C. (104 F.) for half an hour. It is then allowed to cool, and mixed with a few drops of starch T. S., and titrated with decinormal sodium thio- sulphate V. S. until the blue or greenish color of the A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 193 N liquid is discharged. Not less than n.6 cc. of the V. S. should be required, each cc. corresponding to i per cent, or .0056 gm. of metallic iron, or 0.0482 gm. of the salt. See the following equations : (Fe,) Fe.(SO ) ),(NH.) a SO,2 4 H i O + 3 KI 2)112 2)*g64 10) 56 10) 482 5.6 gms. 48.2 gms. S0 4 + I 2 + 2 4 H a O. 2)253 10)126.5 12.65 gms. Then I 2 + 2(Na 9 S a O 8 .5H 2 O)=2NaI+Na a S 4 6 +ioH 3 O. 2)253 2)496 10)126.5 10)248 12.65 gms. 24.8 gms. or 1000 cc. V. S. 10 N Thus it is seen that I cc. of V. S. represents 0.01265 gm. of iodine, and this corresponds to 0.0482 gm. of ammonio-ferric sulphate, or 0.0056 gm. of metallic iron. 0.0482 X n.6 = 0.55912 gm. ^ 99^ of the pure salt. 0.0056 X 1 1.6 = .06496 gm. *X 194 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Soluble Ferric Pyrophosphate (Ferri Pyrophos- phas Solublilis). This is estimated according to the U. S. P. in the following manner: 0.56 (0.5588) gm. of the salt is dissolved in a glass- stoppered bottle (having a capacity of 100 cc.) in 10 cc of water, then 10 cc. of hydrochloric acid and subse- quently 40 cc. of water are added. Then I gm. of potassium iodide is put into the solution and the tem- perature kept at 40 C. (104 F.) for half an hour. The liquid is then cooled and a few drops of starch T. S. N added, and the sodium thiosulphate V. S. delivered in from a burette, until the blue or greenish color is N completely discharged. Each cc. of the V. S. repre- sents I per cent, or 0.0056 gm. of metallic iron. True ferric pyrophosphate has the chemical compo- sition Fe 4 (P 2 O 7 ) 3 -f- 9H 2 O. The soluble ferric pyro- phosphate of the U. S. P. is a mixture of ferric pyro- phosphate and sodium citrate. The reaction with potassium iodide is expressed as follows : 4)746 4)506 10)186.5 10)126.5 18.65 gms. 12.65 gms. Thus 18.65 gms. of ferric pyrophosphate cause the liberation of 12.65 gms. of iodine, and since each cc. of N - sodium thiosulphate V. S. will absorb, and con- sequently represent, .01265 gm. of iodine, it corre- sponds to 0.01865 gm. of pure ferric pyrophosphate. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 195 10 cc. of the decinormal solution is the quantity which the U. S. P. requires should be used. 0.01865 X 10 = 0.1865 gm. 0.1865 X IOO _ -- of ferric pyrophosphate, which corresponds to 10$ of metallic' iron in the U. S. P. salt. Ferric Valerianate (Ferri Valerianas), Fe 2 (C 5 H 9 O a ) 6 '= {*7i8. The true ferric valerianate is illustrated by the above formula, but the U. S. P. salt is of variable composition, and should contain not less than 1556, nor more than 20$, of iron in combination. The estimation is conducted as follows : *o.56(o.5588) gm. of the salt is dissolved in a glass-stoppered bottle (having a capacity of 100 cc.) in 2 cc. of hydrochloric acid. This decomposes the salt, forming ferric chlo- ride and liberating valerianic acid. 15 cc. of water are now added, together with I gm. of potassium iodide, and the mixture heated to 40 C (104 F.) and kept at that temperature for half an hour; it is then cooled, and the liberated iodine estimated with decinormal sodium thiosulphate V. S., using starch T. S. as indi- cator. N Not less than 15 cc. nor more than 20 cc. of the - 10 V. S. should be required to discharge the color of starch iodide. Each cc. corresponds to i$ of metallic iron. The reactions are expressed by the following equations: Fe,(C 6 H.O,), + 6HC1 = Fe,Cl. + 6HC,H,O,; . . (i) 196 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Fe 3 Cl 6 + 2KI = 2FeCl 2 + 2KC1 + I, ; . . (2) I.+XNa.SA-SH.O) - 2NaI + Na 2 S 4 O 6 + ioH 3 O. (3) Liquor Ferri Acetatis (Solution of Ferric Acetate). This is an aqueous solution, containing about 31$ of anhydrous ferric acetate (Fe 2 (C 2 H 3 O 2 ) 6 = j * 4 ^' 92 , cor- responding to 7.5$ of iron. 1.12 (1.1176) gms. of the solution are introduced into a glass-stoppered bottle (having a capacity of 100 cc.), together with 15 cc. of water and 2 cc. of hydrochloric acid. I gm. of potassium iodide is then added and the mixture kept at a temperature of 40 C. (104 F.) for half an hour; then cooled, and, after adding a few drops of starch T. S., pass into it from a burette decinormal sodium thiosulphate V. S. until the blue or greenish color of the liquid has completely disappeared. Each cc. of the decinormal solution thus consumed represents 0.5$ of metallic iron. If 0.56 (0.5588) gm. of the solution is used instead of 1. 12 (1.1176) gm,, and treated as described above, N each cc. of the V. S. represents i of metallic iron, or 0.0056 gm. The principal reaction is expressed by the following equation: Fe,(C,H 3 0,). = 2Fe(C,H ,0,), + 2KC.H.O, + I, 2)464.92 2)253 10)232.46 10)126.5 23.246 gms. 12.65 gms, A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 197 N Thus each cc. of the V. S. also represents 0.023246 gm. of ferric acetate. N 15 cc. of the -- V. S. should be required if 1.12 gms. of solution are taken. 0.023246 X 15 =0.34869 gm. 0.34869 X ioo 1. 12 = 31.1$ of ferric acetate. N 7.5 cc. the -- V. S. should be consumed if 0.56 gm. is taken. 0.023246 X 7.5 = 0.17434 gm. o.i 7434 X IQQ^ . 0.56 Liquor Ferri Nitratis (Solution of Ferric Nitrate). An aqueous solution containing about 6.2$ of anhy- drous ferric nitrate (Fe a (NO 3 ) 8 = j * gl* ' anc * corre " spending to about 1.4$ of metallic iron. Introduce into a glass-stoppered bottle (having a capacity of ioo cc.) 1.12 (1.1176) gms. of the solution, together with 15 cc. of water and 2 cc. of hydrochloric acid. Then add to the mixture I gm. of potassium iodide, and keep it at a temperature of 40 C. (104 F.) for half an hour. Allow the mixture to cool, and esti- mate the liberated iodine with decinormal sodium thio- sulphate V. S., using starch T. S. as indicator. When the blue or greenish color of starch iodide has entirely disappeared, the reaction is completed. 198 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. N 2.8 cc. of the - - V. S. should be required, each cc. corresponding to 0.5$ of metallic iron. The reaction between the ferric nitrate and potas- sium iodide is as follows: Fe 2 (N0 3 ), + 2KI - 2Fe(N0 3 ) 2 + 2KNO 3 + I, 2)483.1 2)253 10)241.5 10)126.5 24.i5gms. 12.65 gms. or 1000 cc. V. S. 10 Thus each cc. of the decinormal sodium thiosulphate V. S. represents 0.02415 gm. of ferric nitrate. Liquor Ferri Subsulphatis (Solution of Basic Ferric Sulphate; Monsel's Solution). An aqueous solution of basic ferric sulphate of variable composition, chemi- cally corresponding to about 13.6$ of metallic iron. 1. 12 (i.i I7)gms. of the solution are introduced into a flask (having a capacity of 100 cc.), together with 15 cc. of water and 2 cc. of hydrochloric acid. I gm. of potassium iodide is then added and the mixture digested for half an hour at a temperature of 40 C. (104 F.). It is then cooled, and after adding a few drops of starch T. S., it is titrated with decinormal sodium thiosulphate V. S. When the blue or greenish color of the liquid disappears, the reaction is completed. 27.2 cc. should be required to complete the reaction, each cc. corresponding to 0.5$ or 0.0056 gm. of metal- lic iron. 0.0056 X 27.2 = 0.15232 gm. 0.15232x100 6 1. 12 * ' A TEXT-BOOK OF VOLUMETRIC ANALYSIS. IQQ Liquor Ferri Tersulphatis (Solution of Ferric Sul- phate). An aqueous solution of normal ferric sulphate Fe 2 (SO 4 ) 3 = | *' containing about 28.7 per cent. of the salt, and corresponding to about 8 per cent, of metallic iron. 1. 12 (i.i 176) gms. of the solution are introduced into a loo-cc. glass-stoppered bottle, together with 15 cc. of water and 2 cc. of hydrochloric acid ; I gm. of potas- sium iodide is then added, and the mixture kept at a temperature of 40 C (104 F.) for half an hour, then allowed to cool, and the liberated iodine estimated in N the usual way with sodium thiosulphate V. S., using starch T. S. as indicator. N About 16 cc. of the V. S. should be required. The following equation illustrates the reaction : pe 2 (S0 4 ) 3 + 2KI = 2FeS0 4 + K 2 SO 4 + I,. 2)399-2 2)253 I o)i99.6 10)126.5 19.96 gms. 12.65 S ms - or N the equivalent of 1000 cc. of thiosulphate V. S. Thus each cc. represents 0.01996 gm. of ferric sul- phate, which corresponds to 0.5 per cent, or 0.0056 gm. of metallic iron. If 16 cc. are used, the solution of ferric sulphate con- tains 0.01996 X 16 = 0.31936 gm. X IPO = 1. 12 200 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. of pure ferric sulphate, and 0.0056 X 16 = 0.0896 gm., .0896 x 100 ' 1. 12 of metallic iron. Hydrogen Peroxide, H 2 O a = j J3.9 2 .__ The iodo- metric method, which originated with Kingzett, is based upon the fact that iodine is liberated from po- tassium iodide by hydrogen peroxide, in the presence of sulphuric acid, and that this liberation of iodine is in direct proportion to the available oxygen contained in the peroxide. Then by determining the amount of iodine liberated, the available oxygen is readily found. H 2 2 + H,S0 4 + 2KI - K 2 S0 4 + 2H.O + I, 2)34 2) , 6 2)253 17 = I available O = ~ 126.5 This shows that 126.5 gms. of iodine are liberated by 17 gms. of absolute peroxide, which are equivalent to 8 gms. of available oxygen. N Thus 1000 cc. of sodium thiosulphate V. S., which absorb and consequently represent 12.65 g ms - of iodine, are equivalent to 1.7 gms. of H.^O 2 or 0.8 gm. of avail- able oxygen. N Each cc. of this V. S., then, represents, of H a O 2 ^0.0017 gm., of available oxygen *o.ooo8 gm. The coefficients for weight of H 3 O 3 and of oxygen, it is seen, are identical with those used in the perman- ganate process. Therefore the coefficient for volume is also the same in this method as in the other, namely, 0.5594. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 2OI The process is carried out as follows : Take 2 or 3 cc. of sulphuric acid, dilute it with about 30 cc. of water, add an excess of potassium iodide (about I gm.), and then I cc. of hydrogen peroxide. After the mixture has been allowed to stand five minutes starch T. S. is added, N and the titration with sodium thiosulphate begun. Note the number of cc. required to discharge the blue color, and multiply this number: by 0.0017 gm. to find the quantity, by weight, of H.,O 2 ; by 0.0008 gm. to find the weight of available oxygen ; by 0.5594 cc. to find the volume of available oxygen. If 18 cc. are required, the solution is of 0.5594 X 18= 10.0683 volume strength. 0.0017 X 1 8 = .0306 or 3.06$ H 2 O 2 . 0.0008 X 1 8 = .0144 or 1.44$ of oxygen. With this method the author has always obtained satisfactory results. The lack of uniformity in the re- action, which is frequently reported, is doubtless due to the use of insufficient acid. TABLE OF SUBSTANCES, ESTIMATED BY DECINORMAL SODIUM TRIO- SULPHATE V. S. Name. Formula. Molecular Weight. Factors. Chlorine Clo 70.68 Gm. Fe 2 (C 2 H 3 O 2 ) Ferric chloride Fe 2 Cl 6 + i2H 2 O Feo = X - = = 37$ gms. That is, .02 2 .04 I gm. of this pepsin is capable of digesting 375 gms. of egg-albumen in 3 hours, or 750 gms. in 6 hours. As egg-white contains about 12.2 per cent, of dry albumen, I gm. of this pepsin will digest 45-7 gms. of dry albumen in 3 hours. This method gives an exact statement of results, re- quires little if any skill in manipulation, requires no shaking, and the results are uniform. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 28 1 CHAPTER XXVII. DETERMINATION OF THE DIASTASIC VALUE OF MALT EXTRACTS AND PANCREATIC EXTRACTS. A ONE-PER-CENT. solution of starch-mucilage is em- ployed. This is prepared by boiling lo gms. of pure starch in water, cooling and making up to 1000 cc. 10 cc. of this standard mucilage is mixed in a beaker with 90 cc. of water. The mixture is then warmed to about 40 C. (104 F.), and a measured amount of the malt extract or pancreatic extract is added, the exact time of adding it being noted. At short intervals, say every half-minute, a drop of the mixture is placed upon a plate or white slab, with a drop of a dilute aqueous solution of iodine. As long as starch is present in the solution a blue color will be produced when brought in contact with a drop of iodine solution. When all the starch is converted by the pancreatic extract into erythro-dextrin, the blue color no longer appears and a pink or brown color is produced ; when all the erythro-dextrin disappears, no color is produced with the iodine. This is termed the achromic point. This point should be reached at the end of not less than six minutes, in order that the end reaction may be determined with sharpness. When it takes longer, the change is too gradual to be exactly determined. 282 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. In the statement of results we employ the following formula : 1>=-X i ~p x r In this, 5 = the weight of the starch employed ; P = the weight of pancreatic extract or malt extract employed ; T = the observed time from the addition of the pan- creatic or malt extract to the a chromic point \ 5 = the arbitrarily chosen standard of time in minutes. Example. 10 cc. of starch-mucilage were taken and o.i gm. of pancreatic extract was added, and the time required to reach the achromic point was three minutes. The above formula would become of the starch-mucilage digested by I gm. of the extract in five minutes. As 10 cc. of the solution of starch contains o.i gm. of dry starch, 166.66 cc. contain 1.666 gm. This method is equally applicable to malt diastase, salivary diastase, or pancreatic diastase. As malt extract is not official, no standard of strength has been fixed. A good dry extract of malt, however, should digest its own weight of starch in twelve minutes. Attfield says that 1.5 gm. malt should digest I gm. of starch within hour, with the usual quantity of water, at 60 C. The following standard of a recent German authority is to be preferred. To 0.6 gm. of starch gelatinized with 60 cc. of water and heated to 40 C. there is A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 283 added 0.5 gm. of the extract, dissolved in about 12 cc. of water. No color should be produced by iodine in a drop of the solution, at the end of fifteen minutes. If we substitute these numbers in the above formula, we have 0.6 5 3.0 - X = -- = 0.4 gm.; 0-5 15 7-5 or i gm. of a fairly good extract by this test should digest 0.4 gm. of starch in five minutes. This is equivalent to the statement that I gm. should digest I gm. of starch in twelve minutes. The method used in the laboratory of Parke Davis & Co. is as follows : Fill six or more two-ounce vials with two ounces of distilled water and two drops of iodine solution. The iodine solution is prepared from 2 gm. of iodine, 4 gm. potassium iodide, 250 gm. of water. 5 gm. of corn-starch are now mixed with 30 gm. of water, and after thoroughly stirring the mixture, in order to have all the starch in suspension, it is poured into 150 cc. boiling water and the mixture brought to the boiling-point, and the boiling continued for a min- ute, until all the starch-granules have burst, forming a uniform mucilaginous solution ; it is then cooled to 100 F. 5 gm. of malt extract are dissolved in 50 cc. of water. \2\ cc. of this solution, representing i gm. of malt extract, are added to the starch solution, which is placed on a water-bath, and maintained at a tem- perature of 100 F. during the test. At the expiration of the first five minutes two drops of the mixture are transferred by means of a nipple pipette to one of the 284 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. two-ounce vials containing the iodine. The bottle is shaken and the result noted. This is repeated at in- tervals of one minute, until two drops of the solution no longer produce a blue coloration with the dilute iodine solution, nor more than a faint purple from the formation of intermediate products following the con- version of the starch. The requirement is that the malt shall convert, according to this test, four times its weight of starch in ten minutes. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 285 CHAPTER XXVIII. ESTIMATION OF ALKALOIDS (VOLUMETRICALLY). IN making alkaloidal assays of drugs it has long been the custom to evaporate the final ethereal or chloroformic extract, and to weigh the residue as alka- loid. This residue seldom if ever consists of the pure alkaloid, and the amount of impurity is very variable ; consequently gravimetric results were in many cases very wide of the truth, and hence unreliable. The volumetric methods are in most cases much more satisfactory. While the results of the titration of the total alka- loids of drugs cannot be called absolutely accurate, nevertheless experience has shown that they are nearer the truth than those obtained by the gravimetric method. In estimating an alkaloid by titration, it is essential to know the formula and molecular weight of the alka- loid, as well as the equivalent of acid with which it will combine. In the case of drugs where two or more alkaloids are present, accurate results can only be obtained by determining how much of each alkaloid is present by a separate assay. But as a rule it is assumed that the alkaloids are present in equal quantities, and the mean of their molecular weights is taken as the basis for the calculation. 286 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. If the alkaloid be from a recent extraction, and is in the form of a free alkaloid, it is dissolved in a measured N quantity of - - hydrochloric-acid solution, and the ex- cess of acid solution then determined by residual titra- N tion with sodium hydroxide solution, using as an indicator a decoction of Brazil wood or some other suitable reagent. Then by deducting the quantity of the alkali solu- tion used, from the quantity of acid solution first added, the quantity of the latter which combined with the alkaloid is obtained, and from this the quantity of alkaloid present may be calculated. A molecular weight of a monobasic acid, or half of a molecular weight of a dibasic acid, will combine with and neutralize a molecular weight of an alkaloid, pro- vided the alkaloid is a monacid base. If the alkaloid is a diacid base, one molecular weight will combine with two molecules of a monobasic acid or one molecular weight of a dibasic acid. Sparteine and Emetine are diacid alkaloids; most of the others are monacid bases. N Thus 1000 cc. of hydrochloric acid will combine with -g- 1 ^ of the molecular weight of a monacid alkaloid, or ^L of the molecular weight of a diacid alkaloid, as the following equations show : (Quinine.) C M H,,NA + HC1 = C,H B N,0,HCL 20)324 gms. 20)36.4 gms. or 1000 cc. acid. i6.2gms. N 1.82 gms. or looo cc. i acid. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 287 (Sparteine.) C B H Q6 N, + 2HC1 = C 5 H 26 N 2 (HC1), 2)114 2)72.8 20) 57 2.85 gms. 1.82 gms. or 1000 cc. acid. A. H. Allen states : " In titrating an alkaloid with methyl-orange as indicator it is rarely convenient to employ an aqueous solution of the base. "A solution in proof-spirit can be employed, but the indicator is much less sensitive under sucli conditions. "I have found it preferable, especially when an alka- loid is much colored, as is frequently the case in assay- ing bases directly extracted from their sources, to dis- solve the alkaloid in a little chloroform, ether, amylic alcohol, or other suitable immiscible solvent. " The solution is placed in a small stoppered cylinder, together with a few cc. of water colored with a drop or two of methyl-orange. Then on gradually running in the standard acid from a burette, and agitating thor- oughly after each addition, it is easy to observe the end of the reaction, as the coloring matter remains in the immiscible layer, and presents a marked contrast to the red color of the aqueous liquid." Allen has obtained satisfactory results with aconitine and its allies, even when working on as little as 0.030 N gm., by using ether as a solvent, and titrating with hydrochloric acid. Prof. P. C. Plugge estimates the alkaloid by titrat- ing the acid of the salt of the alkaloid with standard alkali, and from the result calculates the quantity of alkaloid present. He first determines the uncombined (free) acid by titrating with standard alkali in the pres- ence of litmus. 288 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. He then titrates another portion of the solution in the presence of phenolphthalein to determine the total quantity of acid (both free and combined) pres- ent, and from this, indirectly, the quantity of alkaloid is calculated. For the estimation of the alkaloid in a commercial salt, such as quinine sulphate, strychnine sulphate, etc.: Dissolve the salt in hot water, and titrate with 20 sodium-hydroxide solution, using phenolphthalein, methyl-orange, or some other suitable indicator. The acid in combination with the alkaloid acts as though it were a free acid, and may be readily esti- mated by this method. Methyl-orange is the best indicator for alkaloids, as it shows an alkaline reaction with most of them. Phenolphthalein should be used with caution, as an indicator, in titrating morphine, as this alkaloid has a faint acid reaction with it. It is sometimes preferable to titrate the solution of N alkaloidal salt with - NaOH in the presence of 20 phenolphthalein to exact neutrality. The alkaloid is now in a free state in a neutral liquid, and may be N titrated with HC1 in the presence of methyl- orange. Prof. Plugge made a number of experiments with a view to determine the possibility of estimating volu- metrically, the amount of acid contained in alkaloidal salts, and from this determining the amount of alkaloid. He finds (i) That in the salts of the weak opium bases narco- A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 289 tine, papaverine, and narceine the amount of acid can be volumetrically estimated with either litmus or phe- nolphthalein, the reaction being as precise and well de- fined as if no alkaloid were present. (2) That in the salts of alkaloids in general, the acid can be readily determined by the use of phenol- phthalein, the volatile alkaloids coniine and nicotine being exceptions ; and that in the case of morphine, brucine, codeine, and thebaine, phenolphthalein may be used with certain restrictions. (3) That the free acid in solutions of alkaloidal salts can be determined by the use of litmus, but in solutions of weak opium bases litmus cannot be used. The entire quantity of acid, both free and combined, may be determined by the use of phenolphthalein. The difference between the two titrations gives the quantity of acid united to the base. TABLE SHOWING BEHAVIOR OF SOME OF THE ALKALOIDS WITH INDICATORS. Name. Formula. Methyl- orange. Phenolphthalein Litmus. Aconitine Atropine.. Brucine . C 8 sH 4 6N0 12 C 17 H 23 N0 3 C 23 H 28 N 2 4 C 17 H 21 N0 4 C 18 H 31 N0 3 C 8 H 16 N C 17 H 19 N0 3 C 5 H 7 N C 20 H 24 N 2 0, C 21 H 22 N 2 O a Alk aline Neutral Alkaline Neutral Alkaline Faintly acid Alkaline Neutral Alk; iline Cinchona bases.. Cocaine Codeine Coniine . Morphine . . Nicotine Quinine Strychnine Urea is neutral to methyl-orange, phenolphthalein, and litmus. Caffeine is neutral to phenolphthalein and litmus. Antipyrine is neu- tral to phenolphthalein and litmus. Pyridine is neutral to phenolph- thalein and alkaline to litmus. 290 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. TABLE SHOWING THE FACTOR FOR VARIOUS ALKALOIDS WHEN TITRATING WITH ACID OR ALKALI. 20 Name. Formula. Molecular Weight.* Factor. C 33 H45NOu 6j.7 O O^21C Atropine < r,,Ho,NO 280 U U J^^D Brucine . ... C,,H,NoOA Cinchonine CioHaaNaO JV4 2OJ. Cinchonidine daHaaNaO *y4 2OJ. OOI.17 Ci 7 H 3 ,NO 4 ^y4 qo^ o 015 15 Codeine . . . . CiaHo.NOo 2QQ C 8 H 1B N *y joe Emetine j C 30 H 4 4N 3 O4 (Glenard) 496 0.0124 ( C 3 oH 4 oN 2 O 5 (Kunz) Ci 7 H 3 iNO 4 508 1O1 0.0127 o 01515 Hyoscyamine C, 6 H 33 NO 3 261; O OI "325 Morphine Ci 7 H 19 NO 3 >8 o 01425 Nicotine C 6 H,N 81 o 00405 Pilocarpine C,iHi,N 2 O a 208 o 0104 Quinine . C 20 H 3 4N 3 O a aoj. o 0162 Sparteine .... . Ci 6 H 3 N 2 lid. o 00285 Strychnine CaiH 32 NaO a 5-74 o 0167 ESTIMATION OF ALKALOIDS BY MAYER'S REAGENT. The results of titrating with Mayer's solution have only an approximate value, being influenced to a large extent by various conditions, such as degree of dilution, mode of conducting the ojferation, and the length of time allowed for precipitation after each addition of the reagent. The Mayer's solution is added from a burette, and the precipitate allowed to subside after each addition until no further precipitation takes place, which can be seen by bringing a drop of the clear supernatant liquid in contact on a watch-glass, with two or three drops of the reagent. A more common practice is to filter the solution after each addition of the reagent, using the same filter. A TEXT BOOK OF VOLUMETRIC ANALYSIS. 2QI When 10 cc. of the filtered liquid are no longer affected by two drops of the reagent, the titration is complete. If a considerable length, of time is allowed to elapse after each addition of reagent, it is found that the re- sults of a titration will coincide more nearly with what theory requires; but the principal advantage which vol- umetric analysis has over gravimetric, namely, rapidity of execution, is thereby forfeited. The presence of alcohol, free acetic acid, or ammonia vitiates the result ; but gum, albumen, glucose, or extrac- tives in moderate quantities have no effect upon the reaction. In all comparative titrations with this reagent the dilution of the alkaloidal solution should be the same. The solution should be slightly acid, and its strength about 1-200. In titrations where the end reaction can only be ascertained by the cessation of the formation of a pre- cipitate, it is often necessary to filter a por- tion of the turbid solution at intervals dur- ing the titration, and test it to see whether the process is completed. In such cases Beale's filter, Fig. 29, may be used. Over the lower end of this instrument a piece of filter-paper is tied, and over that a piece of thin muslin to keep the paper from being broken. When dipped into a turbid mixture the clear liquid rises, and may be poured out of the little spout for testing. If the proces 8 is shown to be unfinished, the contents are washed back to the bulk of the liquid, and small portions filtered out at intervals until the process is found to be completed. 2Q2 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. The Decinormal Mayer's Solution is made as fol- lows : N Mercuric Potassium Iodide V. S., U. S. P. Hgl a -f 2KI = 783.98. 39.2 gms. in a litre. Dissolve 13.546 gms. of pure mercuric chloride in 600 cc. of water, and 49.8 gms. of potassium iodide in 100 cc. of water. Mix the two solutions, and then add enough water to make the mixture measure at or near 15 C. (59 F.) exactly 1000 cc. The reaction which takes place when these two solu- tions are mixed is HgCl, + 4 KI == Hgl, + 2 KI + KC1. A. B. Lyons and many others prefer to use a solution of half the above strength. Each cc. of the decinormal solution, according to Dr. Mayer, precipitates of gnr Aconitine. . . 0.0267 Atropine.. . . 0.0145 Brucine .... 0.0233 Cinchonine . 0.0102 gm. Coniine. . . 0.00416 Morphine.. 0.0200 Narcotine.. 0.0213 Nicotine. . . 0.00405 gm. Quinidine. . . 0.0120 Quinine . . . 0.0108 Strychnine. . 0.0167 Veratrine. . . 0.0269 The precipitates are hydriodates of the alkaloids, re- spectively, with iodide of mercury; but Lyons finds that they are not of definite composition, though the variation is very slight. This reagent will give similar precipitates with all of the alkaloids, except perhaps colchicine, caffeine, and digitaline. ALKALOIDAL ASSAY BY IMMISCIBLE SOLVENTS. Many alkaloids are soluble in certain liquids in which their salts are insoluble, while in other liquids the case A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 293 is reversed. When such liquids are not miscible the separation may be effected by the so-called " shaking- out process." In many cases the extraction or separation may be effected by adding to the concentrated aqueous extract, a suitable alkaline precipitant, such as ammonia water or sodium-carbonate solution, which liberates the alka- loid, then shaking up with some solvent, such as chloro- form, ether, benzine, benzol, or amylic alcohol. The liberated alkaloid is thus dissolved or washed out of the aqueous solution. The alkaloid may be again abstracted from this solu- tion by the addition of a dilute acid, which forms again a salt of the alkaloid. In the U. S. P. chloroform is exclusively used as a solvent for alkaloids. The extraction is directed to be performed in a glass separator or separatory funnel, which consists of an elongated (globular, cylindrical, or conical) glass vessel, provided .with a well-fitting stopper and an outlet-tube containing a well-ground glass stop-cock. (See Fig. 30.) When the alkaloidal solution, suitably prepared, is introduced into the separator, and the chloroform subsequently added, the latter, owing to its higher specific gravity, will form the lower layer. If the two are violently shaken together there will often result an emulsion, which will separate slowly, and often imperfectly. FIG. 30. This is particularly liable to happen if the aqueous liquid containing the alkaloid, either in solution or suspension, is strongly alkaline, or has a high specific 2Q4 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. gravity. To avoid this formation of an emulsion it is better to frequently invert the separator or to rotate it rapidly than to shake it violently. The emulsion may sometimes be destroyed by the addition of more of the solvent, and, if necessary, aided by the application of gentle heat, or by the introduc- tion of a small quantity of alcohol or hot water. On withdrawing the chloroform solution of an alka- loid from the separator, a small amount of the solution will generally be retained in the outlet-tube by capil- lary attraction. If this were lost the results of the assay would be seriously vitiated. To avoid this loss, several successive small portions of chloroform should be poured into the separator without agitation, and drawn off through the stop-cock to wash out the out- let-tube. Another source of loss is due to the pressure gener- ated in the separator by the rise of temperature caused when an alkaline and an acid liquid are shaken together. Some of the liquid adheres to the juncture of the stopper and neck, and when the stopper is loosened some of the liquid is ejected. When an alkaline carbonate is used instead of caus- tic alkali for liberating the alkaloid, the liquids should be cautiously and gradually mixed by rotation, and the separator left unstoppered until gas is no longer given off. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 295 CHAPTER XXIX. ESTIMATION OF THE ALKALOIDAL STRENGTH OF SCALE SALTS. FOUR gms. of the scales are dissolved in 30 cc. of water in a capsule with the aid of gentle heat. The solution is cooled and transferred to a glass separator ; an aqueous solution of 0.5 gm. of tartaric acid is then added, followed by an excess of solution of sodium hydroxide. The tartaric acid prevents the precipita- tion of Fe 2 (OH) 8 , and the NaOH sets free the alkaloid. The alkaloid is then extracted by shaking up the mix- ture with successive portions of chloroform, 15 cc. each time. The chloroformic layers are separated each time and mixed, evaporated in a tared capsule on a water-bath, and the residue dried 100 C. (212 F.), and weighed. Or the residue may be titrated by add- ing sufficient decinormal sulphuric or hydrochloric acid to dissolve the salts and still remain in excess, then titrating residually with decinormal NaOH or KOH to determine the excess of acid. GENERAL METHOD FOR THE ESTIMATION OF THE ALKALOIDAL STRENGTH OF EXTRACTS. One gm. of the extract is dissolved in 20 cc. of water, heating gently if necessary. 20 cc. of a solution con- taining 6 gms. of sodium carbonate are added, followed 296 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. by 20 cc. of chloroform. Agitate, warm gently, and separate the chloroform. Add to this 20 cc. of dilute sulphuric acid with an equal bulk of water, again agi- tate, warm, and separate the acid liquor from the chloroform. To this acid liquor add an excess of am- monia, and agitate with 20 cc. of chloroform. When the liquors have separated, transfer the chloroform to a weighed dish, and evaporate over a water-bath. Dry the residue for one hour at 100 C. (212 F.), and weigh. This process may be extended to almost any extract containing alkaloids, except opium. If the resi- due consists of only one alkaloid, the formula and molecular weight of which are known, it may be titrated instead of weighed. Assay of Extract of Nux Vomica. Extract of nux vomica dried at 100 C., 2 gms. ; alcohol ; ammo- nia-water sp. gr. 0.960, water, chloroform, decinormal sulphuric acid V. S., centinormal potassium hydroxide V. S., of each q. s. Put 2 gms. of the dried extract of nux vomica into a glass separator. Add to it 20 cc. of a previously prepared mixture of 2 volumes of alcohol, I volume of ammonia-water, and I volume of water. Shake the separator until the extract is dissolved. Then add 20 cc. of chloroform and agitate during five minutes. The chloroform dissolves the alkaloids which the ammonia liberated. Allow the chloroformic solution to separate, remove it as far as possible, pour a few cc. more of chloroform into the separator, and without shaking draw this off through the stop-cock to wash the outlet-tube. Repeat the extraction with two further portions of chloroform of 15 cc. each, washing the outlet-tube each time as just directed. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 297 Collect all the chloroformic solutions in a wide beaker ; expose the latter to a gentle heat on a water- bath until the chloroform and ammonia are completely dissipated. Add to the residue 10 cc. of decinormal sulphuric acid measured accurately from a burette, stir gently, and then add 20 cc. of hot water. When solution has taken place add 2 cc. of Brazil-wood T. S. (The sulphuric acid combines with the alkaloids, and forms sulphates of the alkaloids.) Now carefully run into this solution centinormal potassium hydroxide V. S. until a permanent pinkish color is produced, showing a slight excess of the alkali. Divide the number of cc. of centinormal po- tassium hydroxide used by 10. Subtract the number N found from 10 (the 10 cc. of acid first used), and 10 N the number of cc. of the acid which went into com- 10 bination with the alkaloids is found. The two principal alkaloids of nux vomica are strychnine and brucine, and it is assumed that they are present in equal proportions ; and thus the factor for total alkaloids is found by taking the mean of their re- spective molecular weights: Strychnine, 334 2)7:28 Brucine, 394 364 364 gms. of the total alkaloids of nux vomica will neutralize 1000 cc. of normal sulphuric acid. 36.4 gms. will neutralize 1000 cc. of decinormal sulphuric acid. Hence each cc. of decinormal sulphuric acid used in the above assay represents 0.0364 gm. of an equal mixture of strychnine and brucine. And by multi- 298 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. plying the number of cc. used by this factor, the quantity of these alkaloids in the 2 gms. of extract taken is obtained, and this quantity multiplied by 50 will give the percentage. The extract should contain 15 per cent of total alka- loids by the above assay. Fluid Extract of Nux Vomica is evaporated to a solid extract, and then assayed by the above pro- cess. Tincture of Nux Vomica is assayed by evaporating IOO cc. to dryness, and the residue then tested by the above process. It should contain 0.3 gm. of alkaloids. Assay of Extract of Opium. Extract of opium dried at 100 C.,4 gms. ; ammonia-water, 2.2 cc. ; alco- hol, ether, water, of each a sufficient quantity. Dissolve the extract of opium in 30 cc. of water, filter the solution through a small filter, and wash the filter and residue with water until all soluble matters are extracted, collecting the washings separately. Evaporate in a tared porcelain capsule first the wash- ings to a small volume, then add the first filtrate, and evaporate the whole to a weight of 10 gms. Rotate the concentrated solution about in the capsule until the rings of extract are redissolved. Pour the liquid into a tared flask, and rinse the capsule with a few drops of water at a time until the entire solution weighs 15 gms. Then add 8.5 cc. of alcohol, shake well, add 20 cc. of ether, and shake again. Now add the ammonia-water, stopper the flask with a sound cork, shake it thoroughly during ten minutes, and set it aside in a moderately cool place for at least six hours, or overnight. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. At the expiration of this time remove the stopper carefully, and brush into the flask any crystals which may adhere to the cork. Place two rapidly acting, plainly folded filters, one within the other, in a small funnel, wet them well with ether, and decant upon the inner one, the ethereal solution, as completely as pos- sible. Add 10 cc. of ether to the contents of the flask, rotate, and again decant upon the filter; repeat this operation with another 10 cc. of ether. Then pour the liquid in the bottle upon the filter in small portions at a time, in such a way as to transfer the greater portion of the crystals to the filter. When the liquid has passed through transfer the remaining crystals to the filter by rinsing the flask with several small portions of water, using not more than 10 cc. in all. Apply water to the crystals drop by drop, until they are practically free from mother-liquor, and afterwards wash them with a saturated alcoholic solution of mor- phine, added drop by drop. When this has all passed through displace the remaining alcohol by ether, using about 10 cc. or more if necessary. Dry to a constant weight at a temperature not ex- ceeding 60 C., and carefully transfer the crystals to a tared watch-glass and weigh them. The weight multi- plied by 25 gives the percentage of crystallized mor- phine present in the extract. Instead of drying and transferring the crystals to a watch-glass as above directed, the filter containing them may be immersed in some boiling water in a beaker, and an excess of decinormal sulphuric acid added to dissolve the crystals (the quantity being noted) ; a few drops of methyl-orange are then added, 3OO A TEXT-BOOK OF VOLUMETRIC ANALYSIS. and the mixture titrated with decinormal potassium hydroxide. Deduct the quantity of the latter used from the quantity of decinormal acid first added, and the quantity of decinormal acid which combined with the morphine is found. 1000 cc. of normal acid represents one molecular weight of the alkaloid. 1000 cc. of decinormal acid represents one tenth of a molecular weight of the alkaloid (30.3 gms.) ; thus each N cc. of acid represents 0.0303 gm. of crystallized morphine. The number of cc. used, multiplied by this factor gives the quantity of morphine present in the 4 gms. of extract taken. This multiplied by 25 gives the per cent, of crystal- lized morphine; it should contain 18 per cent. Assay of Tincture of Opium (Laudanum). Tinc- ture of opium, loo cc. ; ammonia-water, 3.5 cc. ; alco- hol, ether, water, each a sufficient quantity. Evaporate the tincture to about 20 cc., add 40 cc. of water, mix thoroughly, and set the liquid aside for an hour, stirring occasionally and disintegrating the resinous flakes ad- hering to the capsule ; then filter, and wash the filter and residue with water, collecting the washings sepa- rately. Evaporate first the washings to a small vol- ume, then add the first filtrate and evaporate to 14 gms. Pour the liquid into a tared flask; rinse the cap- sule, and add the rinsings until the entire solution weighs 20 gms. Then add 12.2 cc. of alcohol; shake well; add 25 cc. of ether; shake again. Now add the ammonia-water, cork well, shake for ten minutes, and A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 301 set aside for at least six hours or overnight, so that the crystals may form. At the expiration of this time decant the ethereal layer upon a double, plain, rapidly acting filter pre- viously wet with ether; add 10 cc. of ether to the con- tents of the flask, rotate, and again decant. Repeat this operation with another 10 cc. of ether. Then pour the liquid in the bottle upon the filter, in small portions at a time, so as to transfer the greater portion of the crystals to the filter, and wash the remaining crystals on to the filter with the aid of a small quantity of water, using not more than 10 cc. Then wash the crystals, first with a few drops of water, then with an alcoholic solution of morphine, and finally with ether to displace the alcohol. Dry the crystals to a con- stant weight and weigh on a tared watch-glass. If 100 gms. of tincture have been operated upon, the weight of the crystals is at once the per-cent. of crys- tallized morphine. The yield should be 1.3 to 1.5 gms. of morphine from 100 cc. of tincture. Assay of Opium. Opium, in any condition to be valued, 10 gms.; ammonia-water, 3.5 cc. ; alcohol, ether, water, each a sufficient quantity. Introduce the opium (which, if fresh, should be in very small pieces, and if dry, in very fine powder) into a bottle having a ca- pacity of 300 cc. ; add 100 cc. of water ; cork well. Agitate the bottle frequently during twelve hours ; then pour the whole as evenly as possible upon a wetted filter having a diameter of 12 cm., and when the liquid has drained off wash the residue with water carefully dropped upon the edges of the filter and contents until 150 cc. of filtrate are obtained. Then carefully trans- fer the moist opium back to the bottle by means of a 302 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. spatula, add 50 cc. of water, agitate thoroughly and repeatedly during fifteen minutes, and return the whole to the filter. When the liquid has drained off, wash the residue as before until the second filtrate measures 150 cc., and finally collect about 20 cc. more of a third filtrate. Evaporate in a tared capsule, first the second filtrate to a small volume, then add the first filtrate, rinsing the vessel with the third filtrate, and continue the evaporation until the residue weighs 14 gms. From this point proceed exactly as in the assay of tincture of opium. The weight of the crystals obtained, when multiplied by 10, represents the percentage of crystallized mor- phine present in the sample of gum. Opium should contain 9$; the powdered not less than 13$ nor more than 15$. Assay of Cinchona, U. S. P. (a) For Total Alka- loids. Cinchona, in No. 80 (or finer) powder and com- pletely dried at 100 C, 20 gms. ; alcohol, ammonia- water, chloroform, ether, normal sulphuric acid V. S., potassium hydroxide V. S., each a sufficient quantity. 20 gms. of the cinchona in very fine powder is intro- duced into a bottle provided with an accurately fitting glass stopper, and to this is added 200 cc. of a pre- viously prepared mixture of 19 volumes of alcohol, 5 volumes of chloroform, and i volume of ammonia- water; the bottle is stoppered, and thoroughly and frequently shaken during four hours. The liquid is then passed through a plug of cotton in a funnel into another bottle, being careful that there occurs no loss by evaporation. IOO cc. of the clear filtrate (representing 10 gms. of A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 303 cinchona) are transferred to a beaker and evaporated to dryness. The crude alkaloids thus obtained are dissolved in 10 cc. of water and 4 cc. of normal sul- phuric acid with the aid of gentle heat. The cooled solution is then filtered into a separator, and the beaker and filter washed with water until the washings no longer have an alkaline reaction, using as little water as possible. Now add 5 cc. of potassium hydroxide V. S., or suf- ficient to render the liquid alkaline. The alkaloids are thereby reliberated, and may be shaken out by chloro- form. 20 cc. of chloroform are first added, and the extraction repeated, using 10 cc. at a time, until a drop of the last chloroform extraction leaves no residue when evaporated on a watch-glass. The chloroformic extracts are then mixed, evapo- rated in a tared beaker, the residue dried at 100 C. (212 F.), and weighed. The weight multiplied by 10 will give the percentage of total alkaloids in the specimen tested. The volumetric method cannot very well be em- ployed here, as the alkaloids exist in varying propor- tions and are very numerous, thus making it difficult to find a factor which will answer for all cases. (b) For Quinine. Transfer 50 cc. of the clear filtrate remaining over from the preceding process (and repre- senting 5 gms. of cinchona) to a beaker, evaporate it to dryness, and proceed as directed in the assay for total alkaloids, using, however, only half the amounts of volumetric acid and alkali there directed. Add the united chloroformic extracts containing the alkaloids in solution, gradually and in small portions at a time, to about 5 gms. of powdered glass contained in 304 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. a porcelain capsule placed over a water-bath, so that when the contents of the capsule are dry all or nearly all of the dry alkaloids shall be in intimate admixture with the powdered glass, and the chloroform com- pletely expelled. Now moisten the residue with ether, and having placed a funnel containing a filter (7 cm. in diameter) and well wetted with ether over a small graduated tube (A), transfer to the filter the ether- moistened residue from the capsule. Rinse the latter, several times if necessary, with fresh ether, so as to transfer the whole of the residue to the filter ; then percolate with ether, drop by drop, until exactly 10 cc. of percolate are obtained. Then collect another 10 cc. by similar slow percolation with ether in a second graduated tube (B). Transfer the contents of the tubes to two small tared capsules, properly marked (A and B), and evaporate to a constant weight at 100 C. (212 F.) and weigh them. (The residue in (A) will con- tain practically all of the quinine, together with a por- tion of the alkaloid less soluble in ether ; the residue in (B) will consist almost entirely of these alkaloids.) From the amount of residue obtained in (A) deduct that contained in (B). This will give approximately the amount of quinine present in the 5 gms. of sample. Multiply this by 20 a-nd the percentage of quinine containing one molecule of water is obtained. Cinchona calisaya should contain not less than 5 per cent, of total alkaloids, and at least 2.5 per cent, of quinine. Cinchona succirubra should contain not less than 5 per cent, of its peculiar alkaloids. Assay of Fluid Extract of Ipecac. 8 gms. of the fluid extract are diluted with 8 gms. of water in an A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 305 ordinary vial, 32 gms. of chloroform and 48 gms. of ether are added and shaken up ; 4 gms. of ammonia water are now introduced, and the mixture frequently agitated during half an hour. Fifty gms. of the chloroform-ether solution (repre- senting 5 gms. of the fluid extract) are separated, poured into a tared flask, and the solvent distilled or evaporated off; the varnish-like residue is twice treated with 5 to 10 cc. of ether, and evaporated by forcing a current of air into the flask by means of a rubber bulb ; the residue is then dried in a water-bath and weighed. For the titration, the residue may be dissolved in a known quantity of decinormal hydrochloric acid ; the solution may be assisted by a gentle heat, or the addi- tion of a small quantity of alcohol; 10 or 12 drops of Brazil-wood T. S. are then added_ and the excess of acid determined by means of decinormal alkali, the latter being added until the liquid becomes cardinal to purplish red in color. The quantity of decinormal alkali used is then sub- tracted from the quantity of decinormal acid first added. This gives the quantity of the decinormal acid which was used to neutralize the alkaloids present. Emetine, according to Kunz, is diacid, and has the formula C 30 H 40 N 2 O 5 , molecular weight 508. Therefore one molecule of emetine will neutralize two molecules of hydrochloric, or, half a molecular weight, 254 in grammes, will neutralize I litre of normal hydrochloric, acid, while 25.4 gms. will neutralize 1000 of decinormal acid. Thus each cc. of decinormal acid represents 0.0254 N gm. of emetine. If acid is used, each cc. represents 0.0127 gm. of emetine. 306 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. C,.H, NA + 2HC1 = C M H. N,0 6 (HC1) 11 . Emetine (Kunz). 2)508 2)72.79 10)254 gms. 10)36.37 gms. or 1000 cc. _ y g i 2) 25. 4 gms. 2) 3 637 gms. or 1000 cc. _V. S. 10 12.7 gms. 1.818 gms. or 1000 cc. V. S. 20 Thus if decinormal acid is employed, the number of cc. which were neutralized by the alkaloid when mul- tiplied by .0254 gm. gives the quantity of emetine present in 5 gms. of the fluid extract ; and when this is multiplied by 20 the percentage is obtained. Assay of Ipecac Root. 10 gms. of the finely powdered and dried root are placed in a bottle having a capacity of about 150 cc. ; 40 gms. of chloroform and 60 gms. of ether are added, and shaken well for several minutes ; 10 gms. of ammonia-water are now added ; this liberates the emetine, which immediately dissolves in the chloroform and ether, while the suspended powder settles to the bottom of the bottle. The bottle is frequently shaken during one hour, and 5 gms. more of ammonia-water added ; the powder then agglu- tinates in a lump, and the liquid becomes perfectly clear. 50 gms. of the chloroform-ether solution are now taken (representing 5 gms. of the root) and transferred to a tared flask, and the process completed as described under the assay of the fluid extract. The titration is in this case a little more difficult be- cause of the presence of fat from the root. It is advis- able to extract the fat from the root before subjecting it to this assay. Estimation of the Strength of Resinous Drugs. Take 5 to IO gms. of the drug in powder, and A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 307 place it in a strong glass flask with 100 cc. of pure alcohol (U. S. P. and free from resin). Close the flask with a good cork, and digest it in a warm place at about 49 C. (120 F.) for 12 hours, shaking from time to time. Pour or filter off 80 cc. (representing -f^ of the total drug taken), place it in a weighed beaker, and evaporate to 25 cc. on the top of the water-bath. Now add 50 cc. of distilled water, and boil gently over a low gas flame till all the alcohol is driven off. Let it cool and perfectly settle, pour off the supernatant liquor, wash the deposited resin by decantation with hot dis- tilled water, and then dry the beaker and its contents in the air-bath at 105 C. (220 F.) and weigh, deduct- ing the tare of the beaker. Thus treated, jalap, for example, should show 12 per cent of resin, of which not over 10 per cent should be soluble in ether. Scam- mony should show 75 per cent resin, which is entirely soluble in ether and in solution of potassa. From the latter it is not reprecipitated by dilute hydrochloric acid in excess. For other resinous drugs no official standard has yet been laid down. 308 A TEXT-BOOK OF VOI I ! METRIC ANALYSIS. CHAPTER XXX. GLUCOSIDES. GLUCOSIDES are proximate vegetable principles, which when boiled with a dilute acid, or subjected to some other method of decomposition, take up the ele- ments of water, and yield glucose and some other sub- stance, this other substance differing in each case according to the particular glucoside operated upon. Upon this property of these bodies is based a method for their estimation. This method depends upon converting the glucoside into glucose, and then estimating the glucose by Feh- ling's solution in the usual way, and from the amount of glucose formed calculating the quantity of the gluco- side. The conversion of glucosides into glucose is shown by the following equations : C,,H,A + H,0 = C.H.(OH)CH, + C.H.A. Salicin, 286. Saligenol. Glucose, 180. Thus it is seen that 180 gms. of glucose are derived from 286 gms. of salicin. C,,H,A> + 2H,o = C IS H,A + 2C.H,,o, Digitalin. Digitaliretin. Glucose. C a ,H 6 A, + 5H,0 = C,.H M 0, + 3C.H.A- Jalapin. Jalapinol. Glucose. Q.H.A + H,0 = C,,H,A + C.H.,0.. Glycyrrhizin. Glycyrrhetin. Glucose. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 309 CHAPTER XXXI. MILK. MlLK is the nutritive secretion of glands (the mam- mary glands) which are characteristic of the mam- malia. This secretion takes place as a result of pregnancy and delivery, and continues for a variable period, con- stituting the entire food of the young animal until it is able to live upon other foods. The milk of 'different animals contains qualitatively identical or analogous ingredients to that of the cow, namely, fat (which is held in suspension), nitrogenous matters such as casein and albumen, milk sugar, in- organic salts, and water. The average composition of cow's milk is as follows : Fat 3.65 per cent. Proteids 4.40 " " Lactose 4.25 " " Inorganic salts 0.75 " " Total solids 13.05 " " Water 86.95 " " 100.00 In the milk of different animals, however, these in- gredients are in different proportions, as the following table shows : 0* TOT TJFITlESltT! 310 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Human. Goat. Mare. Ass. Fat per cent. 34O per cent. c 2 per cent. I j per cent. Proteids 2 AC 1 8 2 2 o 7 Lactose 5?c 1 O e Inorganic salts . O "3S O 7 O0 tv CO ON I ON I O iiSj lff II dl *| "I 21 Jl 2131 Sh?|*|*j SI'S I < I I I N I I ! N I N l N I N I N I co I ml Sj| ^1 ml s,l a s m I ro I co co I en 8 I S ?! ft 3,1 ml si a N ***$ ON O M