ill... Edmund O'Neill QUALITATIVE ANALYSIS A MANUAL FOR THE USE OF STUDENTS OF CHEMISTRY IN SCHOOLS AND COLLEGES BY L. M. DENNIS PROFESSOR OF ANALYTICAL AND INORGANIC CHEMISTRY AND THEODOEE WHITTEI^EY INSTRUCTOR IN ANALYTICAL CHEMISTRY CORNELL UNIVERSITY BOSTON, U.S.A. GINN & COMPANY, PUBLISHERS &t&en*tim JJreaa 1902 Pf COPYRIGHT, 1902, BY L. M. DENNIS AND THEODORE WHITTELSEY ALL RIGHTS RESERVED IN MEMORIAii ' PREFACE THE purpose of the authors has been to prepare a work on qualitative analysis that shall be both exact and compen- dious, avoiding on the one hand the diffuseness of the larger treatises and on the other the incompleteness of the smaller manuals. Full statement has been made of the behavior of the bases toward certain reagents, but extraneous details have been omitted. Under the headings Methods of Analysis explicit directions have been given for the separation and detection of the elements, and these directions are followed in every case by a full discussion of the reasons for the different steps and of the difficulties that may arise in the course of analysis. It is hoped that this method of presentation, together with the occasional references to such articles in chemical journals as deal with debated points or new methods, may serve to arouse the inter- est of the student and to discourage on his part a blind and mechanical style of work that is so common in courses in qualitative analysis and is so greatly to be deplored. The Introduction discusses in considerable detail the prin- ciples and operations involved in qualitative analysis, but it does not include the consideration of the dissociation theory. However valuable this concept may have proven to chem- istry, the authors feel convinced that it should not be made the basis of instruction in qualitative analysis, and that its iii iv PREFACE consideration may profitably be deferred to some later time in the education of the student. The method of balancing equations described on pages 8-11 of the Introduction was suggested to one of us (L. M. D.) in the year 1890 by Professor Gerhard Kriiss of the University of Munich, and it has been used in the laboratory of Cornell University since that time. The method of instruction used by the authors is given in page 3 of the Introduction, the preliminary study of the ele- ments and groups as there outlined being then followed by the application by the student of the knowledge thus acquired to the analysis of mixtures whose composition is unknown to him. Most of these mixtures are given to the student in the solid form. If limited time or other reason makes it desirable to shorten the course as above outlined, time may be gained by omitting the first part of the course and proceeding directly to the study of such preliminary experiments as are printed in heavy type, following these with the study of the analysis of the different groups and of the detection of the more common acids, and concluding the course with the analysis of some mixtures of unknown composition. In many operations it is desirable to use acids of six times normal strength. The formulas of acids of that strength are printed in heavy type; thus: H^SO^. Wherever the strength of the acid is not designated either in this manner or otherwise, dilute acid (twice normal) should be used. L. M. DENNIS, THEODORE WHITTELSEY. ITHACA, NEW YORK, September, 1902. CONTENTS PART I INTRODUCTION PAGE QUALITATIVE CHEMICAL ANALYSIS ...... 1 QUANTITATIVE CHEMICAL ANALYSIS ..... 1 REACTIONS . . . . 3 EQUATIONS . .' ... . . . . 7 PRECIPITATION . x " 11 FILTRATION. . . . . . . . ... 14 SOLUTION OF THE PRECIPITATE . . . . ... . 16 EVAPORATION ... . . ... . . . 17 PART II THE BASES HYDROCHLORIC ACID GROUP . .... . . .18 Silver . . . 18 Mercury . .18 Lead 19 Method of Analysis . . . . . . . .21 Discussion . . 21 HYDROGEN SULPHIDE GROUP 23 DIVISION A . . 23 Mercury . ' . . . . . . ;. . .23 Copper . . . . . ... . . 24 Cadmium . . , .25 Bismuth . 26 Method of Analysis . 27 Discussion 28 DIVISION B . . . 30 Arsenic 30 Antimony 34 Tin 36 VI CONTENTS PAGE HYDROGEN SULPHIDE GROUP (continued) PRECIPITATION AND SEPARATION OF DIVISIONS A AND B . 38 Discussion 40 DIVISION B . . . . . / . . .44 First Method of Analysis . . . . . 44 Discussion . . . . . . . . . 45 Second Method of Analysis 48 Discussion .......... 50 AMMONIUM SULPHIDE GROUP 53 Nickel . . ; 53 Cobalt . . ,' 54 Iron . . . . . . . . ' . . . 55 Manganesa ...... .^ .. 58 Zinc . ... . . . * 59 Aluminum ......... 60 Chromium . . . . . . .60 Method of Analysis , . . . . * .62 Discussion .......... 66 AMMONIUM CARBONATE GROUP AND MAGNESIUM . . . 71 Barium . . . . . . . ... 71 Strontium . . . . . . ... 72 Calcium . . v . . . . i; -. . ' 73 Magnesium . . .... . . . . 73 Method of Analysis . . "..".-.> . .'74 Discussion 76 AMMONIA, SODIUM, POTASSIUM .79 Ammonia .^^ . . 79 Sodium , . . . . / . t .-v ' lfi *T . . 79 Potassium .... ..... . . 79 Detection . . . . . . . . .80 Discussion 81 CHLORATES CARBONATES SULPHIDES SULPHITES . CYANIDES PART III THE ACIDS 83 85 86 87 88 CONTENTS vii PAGE PHOSPHATES . 90 SULPHATES ........... 91 CHROMATES . 92 BORATES ... . . 94 OXALATES . . . . . . . . . ... . 96 TARTRATES AND ACETATES (ORGANIC MATTER) . . .97 TARTRATES $ 97 ACETATES . . . . . . . . ... .98 IODIDES . . . ' . . . . . . . . 99 BROMIDES . . . . . . 100 FERROCYANIDES . . . . v 101 FERRICYANIDES 103 CHLORIDES _^ . 104 NITRATES 106 PART IV SYSTEMATIC ANALYSIS OF A SOLID SUBSTANCE I. PREPARATION OF SOLUTIONS FOR THE DETECTION OF THE BASES 109 a. THE SUBSTANCE is NOT A METAL ; .--..,.. 109 Water Solution . . . . ' . - 4fc HO Hydrochloric Acid Solution . . . | r . . Ill Nitric Acid Solution . .. *. . -V- / ' H 2 Aqua Regia Solution . .-. .. . . 112 Residue Insoluble in Water and Acids . . . .112 b. THE SUBSTANCE is A METAL OR AN ALLOY . . 117 II. PREPARATION OF SOLUTIONS FOR THE DETECTION OF THE ACIDS 117 III. INTERPRETATION OF RESULTS . . 122 APPENDIX LIST OF APPARATUS . . . . . . . * - . . 125 LIST OF REAGENTS !'<*" * ^^ TABLE OF ATOMIC WEIGHTS OF THE ELEMENTS . . 134 THE PERIODIC SYSTEM . . 135 ABBREVIATIONS OF THE TITLES OF JOURNALS . . . 136 TABLE OF METRIC MEASURES WITH ENGLISH EQUIVALENTS . 137 CONVERSION FACTORS . . . . . . . . 138 INDEX 139 QUALITATIVE ANALYSIS PART I INTRODUCTION Qualitative Chemical Analysis deals with the operations and methods that are employed to ascertain what chemical elements or simple chemical compounds are present in more complex substances or mixtures. Quantitative Chemical Analysis treats of the methods that are employed to determine how much of each element or of each simpler compound is present in the more complex substance or mixture. It is apparent that before the chemist can proceed with the determination of the amount of each of the elements present in a substance he must know what elements the substance con- tains. For this reason the qualitative analysis of the material in hand must precede the quantitative analysis. Qualitative analysis may be said to consist chiefly of the study of the solubilities of the elements and their compounds. Thus in the systematic qualitative analysis of a chemical sub- stance it is first brought into solution, and there is then added to this solution a substance of such a nature as will cause com- pounds of certain of the elements, if the latter are present, to separate in a solid form, i.e., to be precipitated. Other elements that are present will remain in solution, and they are then sepa- rated from the precipitate by filtration through paper. To the clear liquid which passes through the filter, and which is termed the filtrate, another chemical substance of known nature is added, 1 2 QUALITATIVE ANALYSIS and a second group of elements is precipitated. This precipi- tate is also separated by filtration, and a third group of elements is precipitated from the nitrate, and so on. The chemicals of known nature that are used to cause the various separations are termed reagents. When these elements have thus been divided into groups the members of each group are separated one from the other by suitable chemical treatment, and careful tests are then made to ascertain which elements are present and which are absent. The subject of qualitative analysis has become so broad that it is now subdivided into various branches, such as the analysis of inorganic substances, the analysis of organic substances, gas analysis, spectroscopic analysis, etc. The branch of qualitative analysis treated in this manual comprises the detection of about thirty of the more common elements and their compounds. Before the chemist can ascertain by analysis what elements are present in a compound or mixture, he must first familiarize him- self with the characteristic properties and the chemical behavior of these different elements. This information may be imparted by the teacher to the student in a variety of ways. The proper- ties of the elements and their compounds may be stated outright to the pupil, who then begins at once with the actual analysis of chemical mixtures. Such a method makes it possible to give the student a slight knowledge of the subject in a comparatively short period of time, but it should be adopted only when the time available for the work is very limited, for the student will be obliged to follow mechanically the directions that he receives and will fail to understand the various steps involved in the analysis. It is unnecessary to emphasize the importance of this understanding of each and every part of the work. Without it no one may hope to become a successful chemist. The other extreme in the teaching of the subject consists in allowing the student to ascertain for himself the characteristic INTRODUCTION 3 properties of the various elements and then to devise his own plan for the qualitative analysis of mixtures. The faults of this procedure are as great as those of the first, for it is impos- sible for the pupil to study exhaustively each method of separa- tion ; and, furthermore, he loses that aid which is to be derived from the accumulated experience of the many chemists who have preceded him. A system of instruction lying between the two extremes above described is .therefore to be preferred, and where time will permit, the authors would recommend the adoption of the following plan : The student, using solutions of the compounds of the metals taken in alphabetical or other chance order, studies the action upon these solutions of (1) potassium hydroxide, (2) ammo- nium hydroxide, (3) sodium carbonate, (4) hydrogen sulphide, (5) ammonium sulphide, (6) hydrochloric acid, and (7) sulphuric acid. He next classifies the elements into groups, basing the classification upon his own experimental work. The teacher then explains the reasons for the division of the elements into groups as followed in the text-book, and the student proceeds to study in the laboratory these groups and the separation from one another of the elements in each group, this work being accompanied by lectures and recitations. REACTIONS The chemical change that takes place between two or more sub- stances when they are brought together is termed a reaction, and for the sake of brevity it is expressed by the chemist by means of an equation in which the different elements are represented by symbols. Thus, if ammonium hydroxide is added to a solu- tion of ferric chloride, a reaction takes place and there is formed ferric hydroxide, which is insoluble in the liquid that is present, and therefore separates as a precipitate. The other product of 4 QUALITATIVE ANALYSIS the reaction is ammonium chloride, which remains dissolved in the water. This reaction is expressed by the equation, FeCl 3 + 3 NH 4 OH = Fe(OH) 3 + 3 NH 4 C1. In many cases of chemical change the reaction is not as com- plete and definite in character as the equation might lead the stu- dent to suppose. Thus, if a solution of zinc chloride is treated with hydrogen sulphide, some of the zinc will be precipitated as zinc sulphide and some hydrochloric acid will be formed, the substances tending to react as in the following equation : ZnCl 2 + H 2 S = ZnS + 2 HC1. The free hydrochloric acid that is formed has, however, a solvent action upon the zinc sulphide, tending to transform it to zinc chloride with the liberation of hydrogen sulphide, thus : ZnS + 2 HC1 = ZnCl 2 + H 2 S. It is therefore evident that complete precipitation of the zinc from a neutral solution of zinc chloride cannot be effected by means of hydrogen sulphide. There is a tendency toward the precipitation of zinc sulphide, on the one hand, and the decom- position of the sulphide by the hydrochloric acid, on the other. A reaction of this nature may be represented as follows : ZnCl 2 + H 2 S z^ ZnS + 2 HC1, the two arrows showing that the reaction may proceed in both directions. Under suitable conditions, however, the reaction may be forced to proceed in but one direction ; that is, the com- plete precipitation of the zinc may be effected. This results when an alkali acetate is added to a solution of zinc chloride before the treatment with hydrogen sulphide. This acetate sodium acetate is usually employed reacts with the hydro- chloric acid as fast as the latter is set free, forming sodium chloride and free acetic acid. Zinc sulphide is insoluble in the free acetic acid, and the complete precipitation of that substance INTRODUCTION 5 will therefore result. Under these circumstances the reaction will proceed in but one direction, and the sign of equality then correctly represents the chemical change that takes place. In chemical analysis such reactions as proceed to a definite end are almost invariably used. If the work is carried on under the conditions which are necessary for bringing about definite reactions, the student may dismiss from his mind the possibility of reversal. The effect of temperature upon the course of a reaction is often quite marked. Not only is the velocity of the reaction usually increased by heating the reacting substances, but even the very nature of the product is frequently dependent upon the temperature at which the reaction takes place. Thus, if a solution of copper sulphate is treated with potassium hydroxide, both solutions being cold, light blue cupric hydroxide, Cu(OH) 2 , is precipitated. This precipitate is difficult to filter and wash. If, however, the two chemicals are brought together and then heated for some time nearly to boiling, the blue cupric hydroxide changes to black cupric oxide, which can easily be filtered. The course of a reaction is also often markedly affected by the concentration of the solutions of the reacting substances. Thus, if a mixture of the sulphides of nickel, cobalt, and manganese be treated with cold dilute hydrochloric acid, the sulphide of manganese will be dissolved, while the sulphides of nickel and cobalt will scarcely be attacked. If, however, a more concen- trated hydrochloric acid be employed, some of the sulphides of nickel and cobalt will be dissolved. The instances above cited are simple illustrations of the broad rule that the course of every reaction is dependent upon the prevailing conditions. In general it is necessary for success in analytical work that these conditions be known, so that if they do not already prevail before a certain operation is per- formed they may artificially be created. In some cases the conditions may be varied within wide limits without altering 6 QUALITATIVE ANALYSIS the course of the reaction ; in others any variation of condition is of pronounced effect. In this manual, wherever the separation of elements from one another is involved, the conditions that must prevail are discussed. In the preliminary reactions, on the other hand, a statement that the interaction of two chemicals produces a precipitate of definite composition must always be interpreted to mean that this substance will be formed under certain con- ditions, which, for the purposes of this manual, it is not deemed necessary to define more closely. In the reactions of sodium or potassium hydroxide with solu- tions of salts of the metallic elements it may be assumed that, when the conditions are so chosen as to prevent the formation of a basic salt, the precipitate that is at first produced is in every case the normal hydroxide. A few of these hydroxides break down almost immediately into water and the oxide, so that the precipitate may be said to be the oxide. Others suffer dehy- dration more slowly or only under certain conditions, while by far the greater number do not undergo this change as long as they are moist. This adherent water, however, must be removed before the precipitate can be analyzed, and in this operation we meet new conditions which are favorable to dehydration. It is legitimate, therefore, to represent these precipitates by the for- mulas of the normal hydroxides unless dehydration is known to take place rapidly under ordinary conditions. When these hydroxides dissolve readily in potassium or sodium hydroxide it is probable that the salts that are formed in solu- tion are derived from the normal hydroxide by replacement of the hydrogen of the hydroxyl groups by the alkali metal. Thus, when antimony hydroxide is treated with sodium hydroxide, we may assume that the following reaction takes place : OH HONa ONa \OH HONa \ONa INTRODUCTION 7 When an attempt is made to prepare this salt in a solid form, however, it suffers hydrolysis ; that is, it is more or less decom- posed by water, and instead of Sb(ONa) 3 there is obtained SbO(ONa) or Sb 2 O 3 , according to the extent to which hydrolysis has taken place. Since it is probable that these salts, as long as they remain in solution, are derived from the normal hydrox- ides, they will be so represented in this manual. EQUATIONS As has previously been stated, an equation is a brief form of expression of the chemical change that takes place when two or more reacting substances are brought together. An equation cannot be written unless the reacting substances and all of the products of the reaction are known. For this reason the first step in thus expressing a chemical change consists in writing on the left of the sign of equality the formulas of those sub- stances which react upon one another, and upon the right of the sign the formulas of all of the products of this reaction. When this has been done all that is necessary to complete the equation is to insert before the different formulas such num- bers as will represent the relative amounts of the substances entering into the reaction. A case of simple transposition presents no difficulty to the student who has become familiar with the formulas of the acids and their salts. For example, when barium chloride and sodium sulphate interact we know from investigation that barium sulphate and sodium chloride are formed. In representing this by means of an equation it is at once apparent that if the barium and the sodium simply change places in the two salts, one molecule of barium chloride will act upon one molecule of sodium sulphate, form- ing one molecule of barium sulphate and two of sodium chloride, thus: BaCl 2 + Na 2 SO 4 = BaSO 4 + 2 NaCl. 8 QUALITATIVE ANALYSIS When, however, a reaction involves oxidation or reduction, the insertion of the numbers directly representing the relative amounts of the reacting substances and of the products becomes more difficult. Thus, when concentrated nitric acid acts upon metallic copper, investigation has shown that the products are copper nitrate, nitric oxide, and water : Cu + HN0 3 - ->Cu(N0 3 ) 2 + NO + H 2 0. It would be possible by repeatedly trying different numbers to chance upon those which correctly express the relative amounts of copper and nitric acid that react upon each other, and the amounts of the different products. The waste of time that such a procedure would involve may be avoided in a very simple manner. Thus in the reaction just given we may assume that when copper is acted upon by nitric acid it is first oxidized to copper oxide. To change one atom of copper to copper oxide, one atom of oxygen is necessary: Cu + O - CuO. This oxygen is furnished by the nitric acid. It is known that, when nitric acid acts as an oxidizing agent under the conditions prevailing in this experiment, two molecules of the acid furnish three atoms of oxygen that are available for the oxidation of the copper. The other products of this decomposition of the nitric acid are water and nitric oxide : 2 HNO 3 = H 2 O + 2 NO + 3 O. It is therefore evident that two molecules of nitric acid can oxidize three atoms of Cu to CuO : 3 Cu + 2 HNO 3 = H 2 O + 2 NO + 3 CuO. But copper nitrate and not copper oxide is formed in the reaction. This means that more nitric acid reacts on the copper oxide, which we have assumed is formed in the beginning, changing it to copper nitrate. To transform the three molecules of CuO to INTRODUCTION 9 the nitrate, there will be needed six more molecules of nitric acid : 3 CuO + 6 HN0 3 = 3 Cu(NO 3 ) 2 + 3 H 2 O. Hence the total amount of nitric acid required by the three atoms of copper will be two molecules for the oxidation to the copper oxide, and six more for the transformation of the oxide to the nitrate, or eight in all. We are now ready to complete the equation, and it becomes 3 Cu + 8 HNO 3 = 3 Cu(NO 3 ) 2 + 2 NO + 4 H 2 O. In solving such equations it is frequently of advantage to consider the oxygen acids as made up of water and the acid anhydride, and the salts of such acids as made up of an oxide of a metal and the acid anhydride. Thus : Sulphuric acid H 2 SO 4 H 2 O-SO 3 Nitric acid HNO 3 H 2 O-N 2 O 5 Chloric acid HC1O 3 H 2 O-C1 2 O 5 Sulphurous acid H 2 SO 3 H 2 O-SO 2 Ferrous sulphate FeSO 4 FeO-SO 3 Ferric sulphate Fe 2 (SO 4 ) 3 Fe 2 O 3 -3 SO 3 Copper nitrate Cu(NO 3 ) 2 CuO-N 2 O 5 Potassium chlorate KC1O 3 K 2 O-C1 2 O 5 Sodium sulphite Na 2 SO 8 Na 2 O-SO 2 To illustrate the employment of such formulas let us consider the action of nitric acid on mercurous nitrate. The products of this reaction are known to be mercuric nitrate, nitric oxide, and water, HgN Q 3 + HNO 3 > Hg(NO 3 ) 2 + NO + H 2 O, or, representing the acid and salts in the manner just described, Hg 2 ON 2 O 5 + H 2 ON 2 5 > HgON 2 5 + NO + H 2 O. It has already been stated that when nitric acid breaks down with the formation of nitric oxide and water, three atoms of oxygen are available for oxidation, H 2 ON 2 5 = H 2 + 2 NO + 3 O. 10 QUALITATIVE ANALYSIS One atom of oxygen can change one Hg 2 O to Hg 2 O 2 or 2 HgO. Hence the 3 O from the nitric acid will oxidize 3 Hg 2 O to 6 HgO. Inserting these figures in the equation we have 8(Hg,0-N,0 8 ) + l(H,0-N,0 5 ) >6(HgON 2 O 5 )+ 2NO+H 2 O. The six molecules of mercuric nitrate contain 6 N 2 O 5 . Only 3 N 2 O 5 are furnished by the mercurous nitrate. Hence, to obtain the remainder of the N 2 O 5 , three more (H 2 O-N 2 O 5 ) are necessary in addition to the 1(H 2 O-N 2 O 6 ) that was needed for the oxidation. This makes 4(H 2 O-N 2 O 5 ) in all. The water of the acid is set free, appearing as 4 H 2 O. The completed equation is 3(Hg 2 0-N 2 5 ) + 4(H 2 ON 2 6 ) = 6(HgO-N 2 O 6 ) + 2 NO + 4 H 2 O, or, written in the usual manner, 6 HgN0 3 + 8 HN0 3 = 6 Hg(NO 3 ) 2 + 2 NO + 4 H 2 O. These numbers contain a common factor, 2. Dividing by this we have 3 HgN0 3 + 4 HN0 3 = 3 Hg(NO 3 ) 2 + NO 4- 2 H 2 O. Another instructive illustration of this method is furnished by the reaction taking place when chromium oxide is fused with sodium nitrate and sodium carbonate. Cr 2 O 3 + NaNO 3 4- Na 2 CO 3 - > Na 2 CrO 4 4- CO 2 + NO, or Cr 2 O 3 4- Na 2 O-N 2 O 5 + Na 2 O-CO 2 - > Na 2 O-CrO 3 4- CO 2 + NO. The Cr 2 O 3 is oxidized to 2 CrO 3 , the oxygen being furnished by the N 2 O 5 of the sodium nitrate. l(Na 2 O-N 2 O 5 ) furnishes 3 O. To oxidize 1 Cr 2 O 3 to 2 CrO 3 requires 3 O. Therefore, 1 Cr 2 3 + l(Na2O-N 2 5 ) + Na 2 O-CO 2 - > 2(Na 2 0-00 3 ) + CO 2 + 2 NO. CrO 3 is an acid oxide, and unites with the alkali base, Na 2 O, to form sodium chromate. The 2(Na 2 O-CrO 3 ) contains 2 Na 2 O. INTRODUCTION 11 Only one of these is furnished by the l(Na 2 ON 2 O 5 ). The other comes from the sodium carbonate which is introduced for just this purpose. When the Na 2 O of the Na 2 OCO 2 unites with CrO 3 , the CO 2 of the carbonate is set free. The equation now stands : Cr 2 3 + 2 NaN0 3 + Na 2 CO 3 = 2 Na 2 CrO 4 + CO 2 + 2 NO. This method of solving equations involving oxidation and reduction is of general applicability, and it will be employed continually throughout the work. It is of marked usefulness in quantitative analysis as well. PRECIPITATION In the usual methods of qualitative analysis an element is generally recognized by causing it to unite with other elements to form compounds of known properties. These compounds are usually insoluble in the liquid that is present, and they therefore separate as precipitates. It should be borne in mind, however, that strictly speaking no precipitation is ever entirely complete. The precipitated solid dissolves in the liquid that is present until the latter is saturated. In most of the reactions that are employed by the analytical chemist this solubility is very slight, for the condi- tions prevailing at the time of the precipitation are so chosen as to reduce it to a minimum. For example, it may be lessened (1) by varying the temperature, (2) by adding a liquid in which the precipitate is less soluble than in the original solution, or (3) by removing the substance in which the precipitate is soluble; (4) the solubility of a precipitate is also sometimes decreased by adding to the mixture a substance that contains an element or group that is also present in the precipitate. The effect of temperature upon the completeness of precipi- tation is strikingly shown by the action of hydrochloric acid 12 QUALITATIVE ANALYSIS upon a concentrated solution of a lead salt. If the acid is added to a cold solution of the lead compound, almost all of the lead will be precipitated as lead chloride, for this substance is but slightly soluble in cold water. But if the solutions, before mixing, are heated nearly to boiling, almost no lead chloride will appear because of its solubility in hot water. An illustration of the reduction of solubility by the addition of a liquid in which the precipitate is less soluble than in that originally present is to be found in the precipitation of lead by dilute* sulphuric acid. The lead sulphate is somewhat soluble in water, but if alcohol is added it is precipitated almost completely. The increase in the completeness of precipitation resulting from the removal of a substance from solution is shown in the precipitation of aluminum hydroxide by ammonium hydroxide. Aluminum hydroxide is appreciably soluble in the excess of ammonia that is present after precipitation. If this be removed by heating the liquid, more complete precipitation of the aluminum hydroxide results. The decrease in the solubility of a precipitate consequent upon the addition of a substance containing an element or group that is also present in the precipitate may be illustrated by the precipitation of sulphuric acid by a solution of barium chloride. If there is added just enough barium chloride to pre- cipitate the sulphuric acid, some of the barium sulphate will remain in solution ; but if more of the barium chloride, which contains the element barium in common with the precipitated barium sulphate, is added, the solubility of the barium sulphate in the liquid will be lessened and the precipitation will conse- quently be more complete. It is for this reason that an excess of the precipitant should usually be added, and the excess should be greater, the greater the solubility of the precipitate. Certain amorphous substances, termed colloids, tend to dissolve in water and form pseudo-solutions. Aluminum INTRODUCTION 13 hydroxide, ferric hydroxide, and arsenious sulphide are familiar examples of such bodies. These colloids are precipitated from their pseudo-solutions by the addition of a salt solution or an acid. Thus, if a solution of arsenious acid in water be treated with hydrogen sulphide, almost no precipitate appears, but there is formed chiefly a cloudy liquid. Upon the addition of an acid or a solution of a salt to this liquid the yellow sulphide of arsenic will be precipitated. Colloids become denser and less soluble at high temperatures, and it is therefore well to precipitate them from hot solutions. When a liquid reagent is added to the solution under analysis it should be put in at first drop by drop, and the student should carefully observe the effect of such addition. The solution should not be shaken as soon as some of the reagent is added, for it frequently happens that characteristic reactions, giving important information concerning the nature of the substances under examination, take place where the reagent comes in con- tact with the other liquid. Any change in the color of the solution, and the odor or color of any gas that may be evolved, should carefully be noted. The reagent may then be added in an amount sufficient to react with all of the substance in solution. It is usually impossible to ascertain, except by deli- cate chemical tests, just when this amount has been introduced. Hence an excess of the reagent (but only a slight excess) is ordinarily employed. To make sure that such an excess has been added it is well to test the filtrate with an additional drop or two of the reagent. If further precipitation results on this addition, more of the reagent must be added to the entire filtrate, and the filtration must be repeated. To ascertain whether a reagent, if employed in excess, will redissolve a precipitate that it has produced, complete precipita- tion is first effected and then more of the reagent is added and the mixture is shaken or stirred. If no action is visible, the vessel containing the liquid is then carefully heated over a Bunsen flame. 14 QUALITATIVE ANALYSIS Some precipitates dissolve more readily in an excess of the reagent if such excess is added as soon as the precipitate is formed. It is therefore well to supplement the procedure described above by placing in a test tube a few drops of the solution to be precipitated and adding at once a large excess of the reagent. When it is desired to heat more than a very small quantity of a liquid in a test tube the flame should be applied to the tube somewhat below the surface of the liquid. If the heat is applied at the lower end of the tube, bubbles of steam will form at the bottom, and, rising, will force the liquid before them and may drive it out of the mouth of the tube. The flame should not be allowed to strike the glass above the surface of the liquid, for the glass might then become so hot as to break when the liquid comes in contact with it. Wooden test-tube holders are sometimes used for holding the tube in the flame, but a less expensive and more convenient device consists of a piece of stiff paper or a strip of cloth that is wrapped around the upper part of the tube. The projecting ends of the strip are held between the thumb and finger. FILTRATION To separate a precipitate from the liquid in which it is sus- pended, it is brought upon a filter paper which is supported in a glass funnel. The student is supplied with filter paper already cut in circular form. To prepare one of these disks for use it is folded twice, so that it forms a quadrant. This is opened so that three layers of the paper are upon one side and one layer upon the other, and it is then inserted into a dry funnel. After wetting the paper with distilled water from the wash bottle it should be fitted snugly to the glass by gentle pressure with the finger, and the water remaining on the filter should be allowed to run through and should not be poured out. The funnel is INTRODUCTION 15 then placed over the vessel that is to receive the filtrate. If the funnel is of proper shape and the paper in correct position, the stem of the funnel will fill with liquid during filtration, and the process will thus be greatly hastened. The filter should never be filled to the top, for then the precipitate could not be washed with a jet of water without danger of carrying some of it over the edge of the paper and down the walls of the funnel into the filtrate. Many substances when first precipitated are so finely divided that they will pass through the pores of the filter paper. If, before filtering, the precipitate is heated with the liquid and is allowed to stand hot for some time, the particles of the precipi- tate become larger, and the substance is then easily retained by the paper. A good illustration of a precipitate of this character is barium sulphate. If this substance is precipitated from cold solutions, it passes readily through the pores of the paper, but it is easily retained by the paper if treated in the manner above described. If the precipitate settles readily or is of such a nature as to clog the pores of the filter, it may be washed by decantation before being brought upon the filter. This is done by allowing the precipitate to settle and then carefully pouring off the liquid through the filter, keeping back as much of the precipitate as possible in the dish in which it was formed. After most of the supernatant liquid has thus been removed, water is poured upon the precipitate and heated to boiling. The whole is thoroughly shaken, the precipitate allowed to settle, and the liquid again poured off through the filter. The filtrate is then set aside for further examination, and the precipitate is washed two or three times more by decantation, this wash water now being thrown away. If a precipitate does not settle readily, it is at once brought upon the filter, the liquid allowed to run through, and the precipitate is then washed with distilled water with the aid of 16 QUALITATIVE ANALYSIS a wash bottle, the jet of water being directed along the upper edge of the paper. The paper should be filled nearly to the top with water, and all of this liquid should be allowed to run through before more water is added. Unless otherwise directed, hot water should always be employed in washing precipitates. The reason for this is two- fold. In the first place the internal friction of water at 100 is less than one-sixth of what it is at 0, and consequently filtra- tion will proceed more rapidly when hot solutions are used. Moreover, the soluble salts that are to be separated from the precipitate by the filtration usually dissolve to much greater extent in hot water than in cold, so that the amount of washing necessary to carry these substances into the filtrate will be less if the wash water is hot. If a precipitate passes through the paper and causes a turbid filtrate, this filtrate should again be poured upon the filter and the operation repeated until the filtrate ' comes through clear. If a clear filtrate cannot be obtained in this manner, a new filter of two or three filter papers should be made and the liquid filtered through this thicker layer ; or the liquid may be allowed to stand until the precipitate settles, and the supernatant liquid then poured off or drawn off with a siphon. THE SOLUTION OF THE PRECIPITATE A precipitate that has been collected on a filter is sometimes dissolved by simply pouring the solvent upon the precipitate, the funnel first being placed in a test tube or flask to receive the solu- tion when it runs through. But when the solvent acts upon the paper, or when the precipitate must be heated with the solvent to effect solution, the filter paper should be removed from the funnel, spread upon a glass plate, and the precipitate scraped off with a horn spatula into a small porcelain dish. The solvent is then poured directly upon the precipitate in the dish. INTRODUCTION 17 If the amount of the precipitate is very small, it is washed into the apex of the filter, the paper is removed from the funnel, and the point containing the precipitate is torn off and placed in a porcelain dish. The paper is spread out with the precipitate uppermost, and the solvent is then added. ' EVAPORATION If a liquid is to be driven off by evaporation, it should be placed in a porcelain evaporating dish; for if a glass vessel is heated over a flame, it may break as soon as the liquid is driven off and the dish becomes dry. A porcelain evaporator may be heated directly over the free flame without the use of wire gauze. If the contents of the dish gives off acid fumes or a strong odor of ammonia during the evaporation, it should be placed in the hood. The heating of solid residues to drive off ammonium salts should always be carried on in the hood. PAET ; II THE BASES , HYDROCHLORIC ACID GROUP PRELIMINARY REACTIONS SILVER: Reactions of solutions of silver salts. KOH or NaOH precipitates Ag 2 O, brown. NH 4 OH produces in neutral solutions a white precipitate, prob- ably AgOH, which changes immediately to Ag 2 O. When more NH 4 OH is added the precipitate dissolves, forming salts, such as AgNO 3 -2NH 3 , diammonia silver nitrate. Na 2 C0 3 precipitates Ag 2 CO 3 , wfcite. or a basic, carbonate, yellow ; soluble in large excess of the concentrated reagent. H 2 S precipitates Ag 2 S, black : soluble in hot dilute HNO 3 , usually with an evolution of H 2 S, although a partial oxidation of the latter to sulphur by the HNO 3 may also take place. (NH 4 ) 2 S precipitates Ag 2 S. HC1 precipitates AgCl, white, curdy, changing on exposure to the light through various shades 'from lavender to black ; easily soluble in NH 4 OH, forming 2AgCl-3NH 3 . 1 From this solution HN0 3 reprecipitates AgCl. AgCl is somewhat soluble in concentrated HC1. - 1^804 precipitates from concentrated solutions Ag 2 SO 4 , white, crystallm^gparingly soluble in water. MERCURY: Reactions of solutions of mercurous salts.. KOH or NaOH produces a black precipitate which is a mixture 2 of Hg 2 O, HgO, and Hg. 1 Jarry : Compt. rend., 124, 288 (1897). 2 Barfoed : J. prakt. Chem., 38, 441 (1888). 18 LEAP. 19 NH 4 OH, when added to a solution of HgNO 3 , produces a blank precipitate which is a mixture 1 of Hg and Hg 2 -N-NO 3 . Na 2 C0 3 precipitates light yellow basig carbonates, changing to gray because of partial decomposition into HgO, Hg, and CO 2 . The basic carbonates are soluble irl large excess of the concen- trated reagent. H 2 S produces a blafck precipitate which is a mixture 2 of HgS and Hg. (NH 4 ) 2 S produces a precipitate which is probably a mixture of HgS and Hg. If the (NH 4 ) 2 S contains an excess of sulphur the precipitate consists of HgS. HC1 precipitates HgCl, white ; insoluble in water or dilute acids ; changed by NH 4 OH to a black mixture 3 of Hg and Hg-NH^Cl. H 2 S0 4 precipitates Hg 2 SO 4 , white, crystalline, somewhat soluble in water. K 2 Cr0 4 precipitates Hg 2 CrO 4 , red, insoluble in KOH. SnCLj added in small quantity to a dilute solution precipitates HgCl. If the amount of SnCl 2 is increased and the mixture warmed, the precipitate is reduced to black, finely divided mercury. Under ordinary conditions the reduction is incom- plete, and the mixture of mercury and unreduced HgCl has a gray color. LEAD: Reactions of solutions of lead salts. KOH or NaOH precipitates basic salts or Pb(OH) 2 according to the conditions under which precipitation takes place ; white, soluble in an excess of the reagent, forming salts of plumbous acid, as potassium plumbite, Pb(OK) 2 . 1 Barfoed : J. prakt. Chem., 39, 204 (1889), has shown that this precipitate is a mixture of metallic mercury and the white precipitate that is formed when ammonia acts on mercuric nitrate. Hof mann and Marburg have shown the latter to be H^-N-NOs (see note, page 23). 2 IIg 2 S is stable only below 0. Antony and Sestini : Gaz. chim. UaL, 24, 1, 193 (1894). 3 Barfoed : J. prakt. Chem., 39, 211 (1889). 20 HYDROCHLORIC ACID GROUP . NH 4 OH precipitates basic salts somewhat soluble in water. Na 2 C0 3 precipitates a basic carbonate soluble in large excess of the concentrated reagent. HgS precipitates PbS, black ; insoluble in (NH 4 ) 2 S X 1 ; oxidized by hot, fuming HN0 3 to PbS0 4 , white. PbS is soluble in warm dilute HN0 3 , forming Pb(N0 3 ) 2 and H 2 S ; a part of the H 2 S is at the same time usually oxidized to sulphur. From lead solutions containing a large amount o'f hydrochloric acid, H 2 S may precipitate PbCl 2 - 2 PbS, brick red, transformed into PbS by further action of H 2 S, more easily when the solution is largely diluted ; the change is brought about instantly if the red precipitate is treated with (NH 4 ) 2 S precipitates PbS. HC1 precipitates from not too dilute solutions PbCl 2 , white, flocculent ; easily soluble in hot water, from which it crystallizes in long needles when the solution is cooled ; somewhat soluble in cold water. PbCl 2 is less soluble in dilute HC1 than in water ; still less- soluble in concentrated HC1. precipitates PbS0 4 , white ; almost insoluble in water (i part in about 30,000 parts of water) or in dilute acids. PbS0 4 is somewhat soluble in HN0 3 ; readily soluble in a warm solution of ammonium acetate containing a little free acetic acid. Precipitation of PbSO 4 from dilute solutions takes place more rapidly if dilute H 2 SO 4 or alcohol is present, the sulphate being more insoluble in either of these reagents than in water. K^CrC^ precipitates PbCr0 4 , yellow ; easily soluble in NaOH, forming Pb(ONa) 2 ; less soluble in dilute HN0 3 than in NaOH. KI precipitates PbI 2 , yellow; soluble in hot water, from which it crystallizes on cooling in characteristic shining plates. 1 See list of reagents, page 127. ANALYSIS 21 METHOD OF ANALYSIS Place the neutral or acid solution in an Erlenmeyer flask and add HC1 a little at a time as long as a precipitate is formed. Shake vigorously, filter, and wash twice with cold water. Treat the precipitate as directed in the table below, If members of succeeding groups are known to be absent, the filtrate may be rejected. Otherwise the filtrate or the solution in which HC1 fails to produce a precipitate should be treated as directed on page 38. Boil the precipitate with water and filter while hot. Residue : Silver, Mercury Wash with hot water and treat the precipitate on the filter with a little NH 4 OH. Residue : Mercury A black residue proves the pres- ence of mercury. Filtrate: Silver Acidify with HN0 3 ; a white precipitate proves the presence of silver. Filtrate: Lead Divide into two portions. To one, add K 2 Cr0 4 ; a yellow pre- cipitate soluble in NaOH indicates the presence of lead. To the other portion, add KI ; a yellow precipitate soluble in hot water and recrystallizing in plates on cooling proves the presence of lead. DISCUSSION The members of the hydrochloric acid group, of metals are distinguished from the metals of the other groups by the insol- ubility of their chlorides in water or dilute hydrochloric acid. The distinction is not altogether a sharp one, for lead chloride is so soluble in water that it may pass partly or, under favor- able conditions, entirely into the filtrate from this group precipi- tate ; on the other hand, hydrochloric acid may precipitate the oxychlorides of bismuth and antimony, members of the follow- ing group, but these precipitates will dissolve if sufficient hydro- chloric acid is added. The separation of lead from the other members of this group is based on the ready solubility of its chloride in hot water, silver chloride and mercurous chloride being almost completely 22 HYDROCHLORIC . ACID GEOUP insoluble in water. It not infrequently happens that the pre- cipitate of mercurous chloride is so finely divided that some of it passes through the filter paper, producing a turbid filtrate. If, however, more than traces of lead are present in this filtrate, the presence of this small quantity of mercurous chloride does not interfere with the test for lead, as lead chromate is soluble in sodium hydroxide, while mercurous chromate is not dissolved by that reagent. The separation of silver from mercury is based on the differ- ent behavior of their chlorides toward ammonium hydroxide, silver chloride being changed into the soluble compound AgCl'2NH 8 , while mercurous * chloride is changed into an insoluble- mixture of Hg-NH 2 -Cl and metallic mercury. Yet .when the amount of mercurous chloride is large and that of silver chloride is small, it is possible that most or even all of the silver chloride may be reduced to metallic silver by the mercury in the black mixture. 1 Therefore in case a considerable quantity of the black compound is formed and the ammoniacal k filtrate is found to be free from silver, that element should be 5ted for in the residue. This may be done by heating the )lack mixture under the hood until the mercury and its com- pounds are volatilized. The residue may then be dissolved in nitric acid and the solution tested for silver. 1 Barnes : Chem. News, 51, 97 (1885). Antony and Turi : Gaz. chim. ital, 23, II, 231 (1893). MERCURY 23 HYDROGEN SULPHIDE GROUP (NOTE. The precipitation of the HsS group in the analysis of a mixture should be started half an hour before the student is ready to analyze the precipitate. For directions, see page 38.) DIVISION A PRELIMINARY REACTIONS MERCURY : Reactions of solutions of mercuric salts. KOH or NaOH precipitates from cold solutions at first reddish brown basic salts, which change to yellow HgO when the alkali is in excess. The yellow oxide when warmed with the alkalis changes to red HgO. NH 4 OH precipitates Hg-NH 2 -Cl from solutions of HgCl 2 . If a solution of Hg(NO 3 ) 2 is used, Hg 2 -N-NO 3 is precipitated. 1 Both of these precipitates are white. Na^Og precipitates a basic mercuric carbonate, ^^jfih b r * soluble in large excess of the concentrated reagent. I^S precipitates HgS, black, insoluble in hot HNO%, but changed by prolonged action of that reagent into Hg(N0 3 ) 2 -2HgS, white, insoluble in HNO y Both Hg(N0 3 ) 2 -2HgS and HgS are changed by aqua regia into sulphur and HgCl 2 which dissolves. HgS is insoluble in (NH 4 ) 2 S X . If a current of I^S is passed slowly through a solution of mercuric chloride, a white compound, HgCl 2 - 2 HgS, is first precipitated. Further treatment with H 2 S changes this into yellow and then brown compounds as the amount of HgS in the precipitate increases, until finally black HgS is produced. Some of these intermediate double compounds are soluble in HNO y They are changed by (NH 4 ) 2 S X into HgS. (NH 4 ) 2 S precipitates HgS. HC1 produces r^precipitate. I^SC^ produces no precipitate. KI precipitates HgI 2 , red ; soluble in excess, forming HgI 2 - 2 KI. KCN precipitates from concentrated solutions of mercuric chloride Hg(CN) 2 , white ; fairly soluble in water, and therefore 1 Hofmann and Marburg: Ztschr. anorg. Chem., 23, 131 (1900); Ann. Chem., 305, 196 (1899). 24 HYDKOGEN SULPHIDE GKOUP not precipitated from 'dilute solutions ; easily soluble in KCN, forming Hg(CN) 2 -2 KCN. Hg(CN) 2 is not acted upon by the hydroxides of the alkali metals. SnCl 2 reduces HgCl 2 to HgCl, white. If an excess of SnCl 2 is added, the HgCl is more or less reduced to black metallic mercury. If the super- natant liquid is poured off and the precipitate boiled for some time with HCI and a little SnCl 2 , the finely divided mercury collects in globules. Na 2 C0 3 fusion. When a compound of mercury is ground with about six times its weight of dry Na 2 CO 3 and the mixture fused in a bulb tube, metallic mercury is deposited on the walls of the tube. This may be gathered into globules by rubbing the deposit with a glass rod. (The tube should be heated gently at first, and any moisture that may condense on the walls of the tube should be removed by a roll of filter paper before fusion takes place.) COPPER: Reactions of solutions of cupric salts. KOH or NaOH precipitates Cu(OH) 2 , light blue ; changed by boiling with KOH to CuO, black. Cu(OH) 2 is soluble in a large excess of KOH, forming a blue solution. If certain other substances, such as tartaric, citric, or arsenious acid, are present, the addition of KOH imparts a deep blue color to c%pric solutions ; and if sufficient of one of the above- mentioned acids is present, no precipitation takes place, even on boiling. NH 4 OH precipitates light blue basic salts; easily soluble in excess, forming deep blue solutions of ammonia cupric salts such as CuS0 4 '4 NH 3 . KCN decolorizes these blue solutions, the soluble double salt cuprous potassium cyanide, CuCN'KCN, being formed. Na 2 C0 3 precipitates greenish blue basic cupric carbonate ; soluble in large excess of the concentrated reagent. I^S precipitates 1 CuS, black ; changed by hot HN0 3 into sulphur and Cu(N0 3 ) 2 , which is soluble. CuS is insoluble in H 2 SO. Moist CuS is 1 As to the formation of CuaS and sulphur in this reaction, see Thomsen : Ber. d. chem. Ges., 11, 2043 (1878); Coppock: Chem. News, 73, 262 (1896); Brauner: Chem. News, 74, 99 (1896); Rossing: Ztschr. anorg. Chem., 25, 413 (1900). CADMIUM 25 oxidized to CuS0 4 when exposed to the air. CuS is slightly soluble in (NH 4 ) 2 S X . From this solution dilute acids precipitate a yellowish brown compound, the composition of which is not definitely known. 1 Contrary to the statements in most text-books CuS is some- what soluble in Na 2 S I . 1 It is, however, insoluble in Na^S. CuS is soluble in KCN, forming CuCN-KCN and cyanogen. ,(NH 4 ) 2 S produces the same precipitate as H 2 S. HCI produces no precipitate. I^SC^ produces no precipitate. KCN precipitates Cu(CN) 2 , unstable, decomposing into cyano- gen and cuprous cyanide, CuCN. The latter is soluble in KCN, forming CuCN-KCN. K 4 Fe(CN) 6 , potassium ferrocyanide, precipitates Cu 2 Fe(CN) 6 , reddish brown ; soluble in water to a sufficient degree to color dilute solutions red without the formation of a precipitate ; undecomposed by acetic acid ; decomposed by alkalies. Iron. If a piece of bright iron foil is placed in a solution of a copper salt, metallic copper is deposited on the iron. CADMIUM: Reactions of solutions ofcgadmium salts. KOH or NaOH precipitates Cd(OH) 2 , white. NH 4 OH precipitates Cd(OH) 2 ; soluble in excess, forming salts such as Cd(N0 3 ) 2 -4 NH 3 . If this ammoniacal solution is treated with KCN, a soluble double salt, Cd(CN) 2 -2 KCN, is formed. Na 2 C0 3 precipitates CdCO 3 , or a basic carbonate, white ; soluble in a large excess of the concentrated reagent. H^S precipitates CdS, varying in color from light yellow to orange according to the conditions under which precipitation tpkes place ; easily soluble in HCI or in hot// 2 S0 4 ; changed by hot HNOJ into sulphur and Cd(N0 3 ) 2 , which is soluble. CdS is insoluble in (NH 4 ) 2 S X . CdS is insoluble in KCN. (NH 4 ) 2 S precipitates CdS. HCI produces no precipitate. 1 Rossing: Ztschr. anorg. Chem., 25, 407 (1900). 26 HYDEOGEN SULPHIDE GEOUP 4 produces no precipitate. KCN precipitates Cd(CN) 2 , white ; easily soluble in excess, forming Cd(CN) 2 -2 KCN. BISMUTH : Reactions of solutions of bismuth sa/ts. KOH or NaOH precipitates Bi(OH) 3 , white. NH 4 OH precipitates Bi(OH) 3 or a basic salt, white; insoluble in excess; soluble in dilute acids. Na 2 C0 3 precipitates bismuthyl carbonate, (BiO) 2 CO 3 , or other basic carbonates, white; soluble in excess of the concentrated reagent. IljS precipitates Bi^, dark brown ; changed by hot HNO Z into sulphur and Bi(N0 3 ) 3 , which is soluble. Bi 2 S 3 is insoluble in (NH 4 ) 2 S X . (NH 4 ) 2 S precipitates Bi 2 S 3 . E^O added in large quantities to solutions of bismuth salts that do not contain too much acid, precipitates basic salts, as BiOCl, 1 (BiO) 2 S0 4 , BiON0 3 , etc. The basic salts of bismuth are soluble in dilute inorganic acids. HC1 precipitates BiOCl l from a solution of Bi(N0 3 ) 3 that does not contain too much acid, the oxygen being furnished by the water which is present. Since BiOCl is soluble in HC1, the precipitate is best obtained by adding a small amount of the bismuth solution to. a large amount of water containing a little HC1. BiOCl is white ; insoluble in tartaric acid, ; changed by I^S into Bi 2 S 3 . may precipitate (BiO) 2 SO 4 under the conditions noted in the preceding paragraph ; soluble in acids. Na^nOg in alkaline solution (SnCl 2 + NaOH in excess) pro- duces a black precipitate, 2 the Na 2 SnO 2 being oxidized toNa 2 SnO 3 . 1 The precipitation of BiOCl is more complete than that of the other basic salts of bismuth, but is not quantitative. Classen : Ber. d. chem. Ges., 23, 940 (1890). 2 The composition of this black substance is uncertain. Some chemists main- tain that it is BiO, while others believe that it is metallic bismuth in a colloidal form. For a discussion of the question, see Vanino and Treubert : Ber. d. chem. Ges., 31, 1113, 2267 (1898); 32, 1072 (1899); Schneider: J. prakt. Chem., 58, 562 (1898); 60, 524 (1899); Lottermoser : J. prakt. Chem., 59, 489 (1899). Tanatar, who has made BiO in a different way, states that the black substance formed in the above reaction is not homogeneous. Ztschr. anorg. Chem., 27, 43^ (1901). ANALYSIS OF DIVISION A 27 DIVISION A METHOD OF ANALYSIS Boil the sulphides, obtained as directed on page 40, in an evaporator with a few cubic centimeters of HNO$ until brown fumes cease to be freely evolved. Dilute with a little water and filter. Filtrate: Lead, Bismuth, Copper, Cadmium Residue : Add a little // 2 S04 and evaporate carefully until dense white Mercury fumes of S0 3 appear. Add an equal volume of // 2 S0 4 and Boil with a filter. very little aqua Filtrate : Bismuth, Copper, Cadmium Add NH 4 OH until strongly alkaline. A deep blue solution proves the presence of copper. A white precipitate indicates bismuth. Filter Residue: Lead Wash, warm the precipitate regia. Filter, boil the filtrate until chlorine is expelled, and then add SnCl a and wash. with and warm. NH 4 C 2 H 3 2 A gray or Filtrate: Cop If Copper is per, Cadmium If Copper is Precipitate : Bismuth and a few drops o f HC 2 H 3 2 and filter. To black precipi- tate proves the presence of absent : present : Add two or the filtrate add mercury. Make slightly Acidify with three drops of K 2 Cr0 4 . acid with HCI HCI and pre- HCI to the pre- A yellow pre- and treat with cipitate with cipitate in the cipitate soluble H 2 S. H 2 S. Filter funnel and in NaOH proves A yellow pre- and boil the allow the fil- the presence of cipitate proves precipitate im- trate to drop lead. * the presence of mediately with into a beaker cadmium. // 2 S0 4 . Filter, of water. rejecting the A white pre- residue. Dilute cipitate proves the colorless the presence of filtrate with an bismuth. equal volume of water and treat with H 2 S. A yellow pre- cipitate proves the presence of cadmium. 28 HYDROGEN SULPHIDE GKOUP DISCUSSION Mercury is distinguished from the other members of Division A of this group by the insolubility of its sulphide in HNO y The black sulphide of mercury, HgS, may be changed to the white compound Hg(NO 3 ) 2 -2 HgS, but this also is insoluble. Lead that is present in the solution to be analyzed will not be pre- cipitated completely in the hydrochloric acid group, but will appear as lead sulphide in Division A of the hydrogen sulphide group. (If the precipitation takes place from a strong hydro- chloric acid solution, a brick-red compound, PbCl 2 -2PbS, may be precipitated. This will be changed to the black sulphide either by further treatment with hydrogen sulphide or by the action of ammonium polysulphide upon the group precipitate.) In the treatment of the sulphides of Division A of this group with nitric acid, the greater part of the lead sulphide is usually changed to the nitrate which dissolves. (A portion may be oxidized to the sulphate. The latter reaction may predominate if the boiling is prolonged or if concentrated acid is employed.) A partial precipitation of bismuth as a basic nitrate may occur when the nitric acid solution is diluted before filtration. If this is the case, it is well to redissolve the precipitate by the addition of a little dilute nitric acid. Lead is distinguished from bismuth and cadmium by the insolubility of its sulphate in // 2 $0 4 . As lead sulphate is somewhat soluble in nitric acid, this acid is expelled by heat- ing the solution with sulphuric acid. If lead is not entirely precipitated at this point, it may appear as the sulphide in the final test for cadmium. Since nitric acid is more easily decomposed by heat than is sulphuric acid, it may be assumed that the former has been completely removed when the sulphuric acid begins to decompose into water and sulphur trioxide. Dilute sulphuric acid and not water is used in diluting this solu- tion, because water might precipitate a basic sulphate of bismuth. ANALYSIS OF DIVISION A 29 The separation of "bismuth from copper and cadmium is based on the insolubility of its hydroxide in ammonium hydroxide, the hydroxides of copper and cadmium being soluble in that reagent. If a deep blue color is imparted to the solution by the addi- tion of ammonium hydroxide in excess, the presence of copper is conclusively proved. (In case it is desired to test for such very small quantities of copper l as might by this method escape detection, a portion of the ammoniacal solution may be evapo- rated to small bulk, acidified with acetic acid, and the solution tested for copper with potassium ferrocyanide.) The separation of copper from cadmium is based on the solu- bility of cadmium sulphide in // 2 S0 4 , copper sulphide being insoluble in that reagent. The sulphides must be filtered off and treated with the acid immediately or some of the copper sulphide will be oxidized by the air to sulphate which will pass into solution. Some of the double compounds that are first formed when hydrogen sulphide acts on a solution of a mercuric salt are soluble in nitric acid. If, therefore, the precipitation is inter- rupted before these compounds are completely transformed into mercuric sulphide, some mercury will pass into the filtrate upon treatment with nitric acid. In the subsequent treatment of this filtrate the mercury in such case will still be in solution after an excess of ammonium hydroxide is added, and will be precipitated by hydrogen sulphide with the sulphides of copper and cadmium. A portion of the mercuric sulphide thus formed may be dissolved when the sulphides of copper and cadmium are treated with sulphuric acid, and will then be precipitated as black mercuric sulphide in the final test for cadmium. This difficulty is not encountered in case the group precipitate has been treated with ammonium polysulphide, because this reagent transforms these intermediate compounds into mercuric sulphide. It should be remembered that a black precipitate in the final test for cadmium may be lead sulphide (see page 28). 1 See also page 43. 30 HYDROGEN SULPHIDE GROUP DIVISION B PRELIMINARY REACTIONS ARSENIC: Reactions of solutions of arsenious compounds. KOH or NaOH produces no precipitate. NH 4 OH produces no precipitate, produces no precipitate. precipitates from arsenious solutions containing hydrochloric acid As 2 S 3 , lemon yellow ; almost insoluble in warm concentrated HC1 ; oxi- dized by a crystal of KC10 3 and hot HCI, or by hot HNO^ to H 3 As0 4 which is soluble, sulphur being set free at the same time ; As 2 S 3 is sol- uble in (NH 4 ) 2 S X forming sulpho-salts, such as ammonium sulpho-arsenate, (NH 4 ) 3 AsS 4 , from which dilute HCI precipitates As 2 S 5 . (A portion of the As 2 S 3 precipitate should be dried for use in the KCN fusion test described below. To do this lay the filter containing the remainder of the precipitate upon a piece of wire gauze. Place this wire gauze high above a low Bunsen flame and dry the filter and precipitate thoroughly, but do not allow the paper to char.) (NH 4 ) 2 S precipitates As 2 S 3 from solutions of H 3 AsO 3 acidified with hydrochloric acid. The reagent here is H 2 S, formed by the action of (NH 4 ) 2 S on HCI. As 2 S 3 is soluble in (NH 4 ) 2 S, forming (NH 4 ) 3 AsS 3 . HCI produces no precipitate. HjSC^ produces no precipitate. Nascent hydrogen. If a slightly acid solution of any compound of arsenic is introduced into a hydrogen generator (see Gutzeit test), arsine, AsH 3 , a colorless, poisonous gas, is formed. When arsine 1 is led into a solution of AgNO 3 a finely divided black precipitate of metallic silver is produced, and the arsine is 1 The hydrogen generator used in this experiment consists of a 100 cc. flat- bottom flask provided with a two-hole rubber stopper. Through one opening of the stopper passes a delivery tube ending just below it and so bent above the stopper that it projects downward at an angle of 45 and extends about 4 cm. AKSENIC 31 at the same time changed to H 3 AsO 3 . The oxygen necessary for this oxidation seems to be furnished by the water. 1 The silver may be filtered off and the arsenic in the filtrate precipi- tated as As 2 S 3 by H 2 S. But this filtrate usually contains some AgNO 3 that has not been acted upon by the arsine, and conse- quently Ag 2 S would be precipitated with As 2 S 3 when the filtrate is treated with H 2 S. It is therefore necessary to remove the silver in the filtrate, by precipitating it with HC1 and filtering off the AgCl, before H 2 S is added. Gutzeit test (modified). When arsine acts upon a crystal of AgN0 3 there is first formed a yellow compound which is said to be AsAg 3 '3AgN0 3 . If the amount of arsine is large, the yellow compound appears only for an instant, and turns black almost immediately, metallic silver and arsenious acid being formed. The oxygen necessary for the production of arsenious acid seems to be furnished by the moisture in the gas. In this experiment it is convenient to use a test tube as a hydrogen gen- erator. To ascertain the purity of the reagents, a blank test should first be made as follows : Place a fragment of zinc in a dry test tube and add through a funnel a few cubic centimeters of dilute I^SC^. Introduce into the test tube a loosely fitting plug of absorbent cotton about 3 cm. long and push this down into the tube until its upper surface is about 2 cm. below the mouth of the tube. Above this place a second plug of cotton, the lower end of which has been moistened with a solution of lead ammonium acetate to absorb any H 2 S that may be evolved from the reagents. Cover beyond the neck of the flask. Through the other opening passes a funnel tube reaching nearly to the bottom of the flask. In using the generator, place in the flask some zinc and a small piece of platinum foil, insert the stopper, and add through the funnel tube from 5 to 15 cc. of dilute sulphuric acid, the volume of acid being dependent upon the amount of arsenic to be introduced. Join to the end of the delivery tube by means of a short piece of rubber tubing a straight glass tube, drawn out at the lower end and of sufficient length to reach to the bottom of a test tube. Fill a test tube half full with a solution of AgNOa and slip it over this delivery tube. Now introduce through the funnel tube the solution which is to be tested, adding but a little at a time. Never allow the gases which are evolved to pass directly into the atmosphere, and keep all flames away from the apparatus. 1 The products of this reaction are usually said to be arsenious and nitric acids a"nd metallic silver. It has been shown in Lunge's laboratory that some nitrous acid also is formed. Marchlewski : Eer, d. chem. Ges., 24, 2269 (1891). 32 HYDKOGEN SULPHIDE GEOUP the mouth of the tube with a piece of filter paper and fold this down snugly around the edge. On this paper place a small crystal of AgN0 3 and a fragment of Pb(C 2 H 3 2 ) 2 . The dry cotton is used to absorb mois- ture from the gas. In case H 2 S is formed, the crystal of Pb(C 2 H 3 2 ) 2 on the paper will show whether it has been completely absorbed by the moistened plug. If the reagents are pure and the experiment is properly performed, neither of the crystals on the paper should change color. Having thus tested the apparatus, remove the plugs and introduce a few drops of H 3 As0 3 . Quickly arrange the apparatus as before and observe the color changes in, the silver nitrate crystal. Gatehouse test (modified). This differs from the Gutzeit test only in that the source of hydrogen is aluminum and KOH. The reaction may be hastened by gently warming the tube until bubbles begin to appear on the surface of the aluminum. The lead salts are not needed, but a plug of absorbent cotton should be inserted to dry the gas. KCN. Dry As 2 O 3 or As 2 S 3 , when fused in a bulb tube with a dry mixture of three parts Na 2 CO 3 and one part KCN, is reduced to metallic arsenic which is deposited in the cooler part of the tube as a shining black mirror. If the amount of arsenic present is small, the mirror has a brownish color. KCN acts as a reducing agent here because it unites with oxygen to form KCNO, or with sulphur to form EONS. Reinsch test. If a bright strip of copper foil is introduced into an arsenious solution to which has been added from one-fourth to one-sixth of its volume of concentrated HC1, and the mixture is boiled, arsenic is deposited on the copper as a gray coating. If the foil is then removed, washed with water, dried by pressing between pieces of filter paper, introduced into a bulb tube and heated, a sublimate of crystalline As 2 3 is formed. This should further be identified by an additional test for arsenic. If the Gatehouse test is to be employed, the portion of the tube that contains the sublimate is cut off and a strip of aluminum foil is passed through it and bent over at the ends to hold the foil in place. The experiment is then performed in the manner already described (see above). Bettendorff test. When a mixture of equal parts of a concentrated solution of SnCLj and concentrated HC1 is boiled with a few drops of H 3 As0 3 , the latter is reduced to metallic arsenic. Bettendorff test (modified). If a strip of tin foil is placed in a mod- erately dilute solution of stannous chloride containing hydrochloric acid, y ARSENIC 33 and a few drops of a solution of arsenious acid are then added and the liquid heated to boiling, the arsenious acid is reduced to metallic arsenic which in part is deposited unevenly on the foil and in part remains sus- pended in the solution, imparting to it a brown color. Reactions of a solution of arsenic acid. KOH or NaOH produces no precipitate. NH 4 OH produces no precipitate. Na 2 C0 3 produces no precipitate. I^S. If a current of hydrogen sulphide is rapidly passed through a solution of arsenic acid, As 2 S 5 is slowly precipitated. 1 The precipitation is complete and much more rapid when a relatively large quantity of hydro- chloric acid is present. 2 If hydrogen sulphide is slowly passed through a hot solution of arsenic acid that contains but a moderate amount of hydro- chloric acid, a different reaction takes place : arsenic acid is reduced to arsenious acid and arsenic trisulphide is precipitated, mixed with sulphur. Arsenic pentasulphide resembles the trisulphide in color and in its behavior toward most reagents. (NH 4 ) 2 S precipitates As 2 S 5 from a solution of H 3 AsO 4 acidified with HC1. The reagent here is the H 2 S formed by the action of (NH 4 ) 2 S on HC1. As 2 S 5 is soluble in (NH 4 ) 2 S>< forming sulpho-salts such as (NH 4 ) 3 AsS 4 . HC1 produces no precipitate. H 2 S0 4 produces no precipitate. (NH 4 ) 2 Mo0 4 , ammonium molybdate, added in excess to a solution of arsenic acid containing nitric acid precipitates ammonium arseno- molybdate, yellow ; less soluble in HN0 3 than in water. Precipitation is best obtained by adding a few drops of HN0 3 and a few drops of H 3 As0 4 to a small test tube half full of (NH 4 ) 2 Mo0 4 . The mixture should then be warmed but should not be heated above 70, lest white Mo0 3 be precipitated. 1 Brauner and Tomicek : Monatshefte, 8, 624 (1887). 2 From a cold solution containing two or more parts of concentrated hydro- chloric acid (sp. gr. 1.20) to one part of water a moderate amount of arsenic acid is completely precipitated as As 2 Ss in from half an hour to an hour. This has been made the basis of a method for the quantitative determination of arsenic. Under the above conditions lead, bismuth, cadmium, and antimony are not precipitated. Neher : Ztschr. anal Chem., 32, 45 (1893). 34 HYDROGEN SULPHIDE GKOUP MgCl 2 . When this reagent is added to a solution of arsenic acid or an arsenate, the solution having first been made alkaline with NH 4 OH, there is formed MgNH 4 AsO 4 , a white crystalline precipitate. If the amount of arsenic present is small, the pre- cipitate forms only after some time. (Compare the precipita- tion of MgNH 4 PO 4 , page T4.) FeS0 4 , Na^Og, or (NH 4 ) 2 S0 3 when boiled with a solution of H 3 As0 4 that has been acidified with H 2 S0 4 or HC1 reduces it to H 3 As0 3 . The sulphites are best used in the solid form. Nascent hydrogen (zinc and sulphuric acid) reduces H 3 AsO 4 first to H 3 AsO 3 , and this is then further reduced to AsH 3 ; complete reduction, therefore, takes much more time than when an arsenious solution is introduced into the generator. Gutzeit test (modified). See preceding paragraph. Gatehouse test (modified). Nascent hydrogen evolved by the action of aluminum on potassium hydroxide does not reduce arsenic acid. In the Bettendorff test, Bettendorff test (modified), and Reinsch test reduction takes place less rapidly than when the arsenic is in the triva- lent condition. * ANTIMONY : Reactions of solutions of antimonious salts. KOH or NaOH precipitates Sb(OH) 3 ; soluble in excess of KOH, forming Sb(OK) 3 . NH 4 OH produces a similar precipitate. Na 2 C0 3 produces a white precipitate, probably Sb(OH) 3 ; soluble in large excess of the reagent. H 9 S precipitates from moderately acid solutions of antimonious salts Sb 2 S 3 , orange red ; soluble in warm, concentrated HC1 ; oxidized by hot HNOfr forming sulphur and H 3 Sb0 4 which is soluble. Sb 2 S 3 is soluble in (NH 4 ) 2 S X forming sulpho-antimonates, as (NH 4 ) 3 SbS 4 , from which dilute acids precipitate Sb 2 S 5 , orange red. The latter is also precipitated when acid solutions of antimonic salts are treated with H 2 S. Sb 2 S 5 resembles Sb 9 S 3 in its behavior toward most reagents ; when treated with hot concentrated hydrochloric acid, however, it is changed to SbCl 3 with the separation of sulphur. f (NH 4 ) 2 S precipitates Sb 2 S 3 from antimonious solutions acidified ANTIMONY 35 with hydrochloric acid. The reagent here is the H 2 S formed by the action of (NH 4 ) 2 S on HC1. Sb 2 S 3 is soluble in (NH 4 ) 2 S, forming (NH 4 ) 3 SbS 3 . HC1 precipitates, from solutions that do not contain too much free acid, SbOCl, white; soluble in dilute inorganic acids or in tartaric acid. The precipitate is best obtained by adding a small amount of the antimonious solution to a large quantity of water that has been acidified with a little HC1. H 2 0. If a few drops of a solution of SbCl 3 that does not contain too much free HC1 are added to a relatively large quantity of water, SbOCl is precipitated. This is changed directly to Sb 2 S 3 when treated with H 2 S0 4 produces no precipitate from acid or neutral solutions of antimonious salts. Nascent hydrogen. If a slightly acid solution of a compound of anti- mony is placed in a platinum spoon together with a small piece of zinc, a portion of the antimony will be deposited upon the platinum in metallic form and will appear as a black stain. This deposit is only slightly soluble in cold ///, but is easily dissolved by a few drops of fuming HN0 3 , the latter reagent oxidizing it to H 3 Sb0 4 . The excess of HN0 3 may completely be removed by adding to the solution a few crystals of tartaric acid and then warming the mixture. The antimony is precipitated in the form of a sulphide when the residual solution is diluted with a little water and treated with H 2 S. If an acid solution of a compound of antimony is introduced into a hydrogen generator (see page 30) there may be produced by the action of the nascent hydrogen both antimony and stibine. The antimony will be deposited upon the platinum foil in the generator. The properties of this metallic antimony have been described in the preceding paragraph. Stibine, SbH 3 , is a color- less, poisonous gas. If this gas is led into a solution of AgNO 3 there is formed SbAg 3 , black. If this substance is washed with water and boiled with tartaric acid, it is decomposed with the formation of silver and a soluble compound of antimony. The antimony is precipitated in the form of a sulphide when H 2 S is passed through the nitrate. i - 36 HYDKOGEN SULPHIDE GROUP [The following tests are made as described on pages 31 and 32, except that SbClo is used instead .of HoAsOo.l o o A J Gutzeit test (modified). When SbH 3 acts upon a crystal of AgN0 3 it is possible that there may first be formed a yellow compound analogous to that produced by the action of AsH 3 . In practice, however, this is seldom seen, black SbAg 3 usually forming at once. Gatehouse test (modified). Metallic antimony is deposited in the test- tube generator chiefly on the aluminum, and no SbH 3 is formed. Bettendorff test. Antimony is not precipitated. Bettendorff test (modified). Antimony is deposited as a velvety black coating which is evenly distributed over the entire surface of the tin foil. TiN: Reactions of solutions of stannous salts. KOH or NaOH precipitates Sn(OH) 2 , white ; soluble in excess, forming stannites, as Sn(OK) 2 . NH 4 OH precipitates Sn(OH) 2 ; insoluble in excess. Na 2 C0 3 precipitates Sn(OH) 2 ; soluble ^n large excess of the reagent. H 2 S precipitates SnS, dark brown ; soluble iBwarm concentrated HC1 ; soluble in (NH 4 ) 2 S X , forming sulpho-stannates, a^(NH 4 ) 2 SnS 3 , from which a dilute acid precipitates SnS 2 , yell^F (NH 4 ) 2 S precipitates SnS from stannous sdffetions acidified with HC1. The reagent here is H 2 S, formed by the action of (NH 4 ) 2 S on HC1. SnS is insoluble or nearly so in colorless (NH 4 ) 2 S. HC1 produces no precipitate. H 2 S0 4 produces no precipitate. HgCl 2 is reduced by SnCl 2 in hydrochloric acid solution to HgCl, white ; if an excess of SnCl 2 is added, some of the HgCl is reduced to black metallic mercury, and the resultant mixture usually has a gray color. Nascent hydrogen. If a solution of a compound of tin containing a small amount of HC1 is acted upon by nascent hydrogen (zinc and sul- phuric acid), it will be reduced to metallic tin, which will appear as a spongy mass adhering chiefly to the zinc. Metallic tin dissolves when boiled with HCI, forming SnCl 2 . The action is hastened by placing a small piece of platinum 'in contact with the tin. KC10 3 . When a crystal of this substance is added to a solution of SnCl 2 contajryngry/tf', and the mixture is warmed, SnCl 4 is formed. TIN 37 HN0 3 oxidizes metallic tin to SnO 2 or H 2 SnO 3 , white, amor- phous substances difficultly soluble in acids. 1 Reactions of acid solutions of stannic salts. (The following reactions are those of a solution of a stannic salt that has been prepared directly from a stannous salt by treating it with KC1O 3 and HC1. A solution of a stannic salt made from metastannic acid behaves somep'hat differently. Since hydrogen sulphide reacts in^the same way with either solution, it is considered sufficient for Jhg purposes of this manual to introduce in th&r preliminary reactions only those of such stannic soluti^^p as are usually encountered in the course of analysis.) KOH or NaOH precipitates stannic acid, Sn(OH) 4 , white, gelat- inous ; solub^r iir KOH or NaOH, forming stannates, as Sn(OK) 4 . Upon neyttnt-liziiig j^.j^olution of a stannate with a dilute ack^n(0 TTA - iJ - 1 - 1 ^ ^ - NH 4 OH precipitat ^^ Na 2 00 3 precipitat^^^Bnuc acid-; soluble in excess. H 2 S precipitates ylow SnS 2 from solutions Containing a moderate amount of HC1 ; soltrole in warm concentrated 'HC1 ; soluble in (NH 4 ) 2 S X to form sulpho-stannates, as (NH 4 ) 2 SnS 3 / * (NH 4 ) 2 S precipitates SnS 2 from stannic solutions acidified with HC1. The r^|ent here is H 2 S, formed by the action of (NH 4 ) 2 S on HC1. Sn 2 is soluble in (NH 4 ) 2 S, forming (NH 4 ) 2 SnS 3 . x HC1 produces no precipitate from soluticra of moderate con- centration. H 2 S0 4 produces no precipitate from solutions t of ordinary concentration. From very dilute solutions a basic sulphate is precipitated. HgC^ produces no precipitate. Nascent hydrogen reduces stannic solutions to metallic tin. 1 a-stannic acid is easily soluble in inorganic acids. This^acid is not formed in the treatment above described. 38 HYDROGEN SULPHIDE GROUP PRECIPITATION AND SEPARATION OF DIVISIONS A AXD B PREPARATION OF THE SOLUTION The solution from which this group is to be precipitated should be free from nitric acid and should contain about one volume of HCI to ten volumes of the water solution. If nitric acid has been used to dissolve the original substance, evaporate to srafcll bulk and boil with HCI until the nitric acid has been reduced and tlie odor of chlorine is no longer noticeable. If hydrochloric acid has been used to dissolve the original substance or lo remove nitric acid/ltf d water to the solution until the desired concentration is reacheu otherwise add to the filtrate from the hydrochloric acid group, \ c to the solution itself if it has been found to contain no members of that group, about one-tenth its volume of ///. \ PRECIOTATION | Heat the solution to boiling and pass through the hot solu- tion a slow 1 current of hydrogen sulphide! until precipitation is complete, the solution is cool, and no further change of color is observed in the precipitate. 2 To determine the point 1 The gas should pass through the solution at such a rate that the bubbles may easily be counted. 2 It has been suggested that complete precipitation may be secured in less time and with less wq(0B of H 2 S by using the following arrangement. The vessel in which precipitation takes place is an Erlenmeyer flask of such a size that it is not more than one-third filled by the solution. The flask is furnished with a rubber stopper, through which passes a glass tube reaching to the bottom of the flask and bent at a right angle above the stopper. Introduce into the flask the solution prepared as above directed and heat it to boiling. Insert the stopper loosely, having withdrawn the tube so that its lower end is just above the surface of the liquid. Attach the other end of the tube by six inches of rubber tubing to the HaS generator and pass a rapid current of gas through the flask for a mogaent to expel the air. Without shutting off the gas insert the stopper firmly and push down the tube until it reaches nearly to the bottom of PRECIPITATION 39 of complete precipitation, filter a small portion into a test tube, dilute with one-half its volume of water, and treat with hydrogen sulphide for a few minutes. In a solution known to contain no members of subsequent groups, filter and reject the nitrate. Otherwise reserve the filtrate for the analysis of the following groups. If no precipitate is produced by hydrogen sulphide, pass to the ammonium sulphide group. (If oxidizing agents have not been removed, free sulphur may be formed. See page 40.) Wash the precipitate thoroughly with hot water until a few drops of the wash water impart but a slight turbidity to a little silver nitrate solution. If Division A is known to be absent, treat the precipitate as directed on page 44. If Division B is known to be absent, 1 or if both divisions may be present, proceed as follows : SEPARATION OF DIVISIONS A AND B Transfer a small portion of the precipitate to an evaporating dish, add a few cubic centimeters of ammonium polysulphide and warm for a moment. If this portion of the precipitate dissolves completely, Division A is absent and the remainder of the precipitate should be treated as directed on page 44. If the portidn of the precipitate that is treated with ammo- nium polysulphide is not completely dissolved, the presence of Division A is indicated. In such case treat the remainder with the flask. Shake the flask occasionally. After a short time disconnect the flask from the generator and cool its contents by holding it under a stream of cold water. Now dilute the liquid in the flask with an equal volume of cold water and repeat the treatment with H 2 S in the manner described. See Graebe : Ber. d. chem. Ges., 31, 2081 (1898). Precipitation will usually be found to be complete after this second treatment with the gas has pontinued for a few minutes. 1 Even when it is known that members of Division B are absent, it is well to treat the precipitate with ammonium polysulphide in order to change any inter- mediate sulpho-chlorides of lead or mercury into the sulphides. 40 HYDROGEN SULPHIDE GROUP ammonium polysulphide (10 cc. will usually suffice) and warm for about three minutes with occasional stirring. The mixture should not be allowed to boil. Filter while warm, and analyze the residue as directed for Division A on page 27. The filtrate may contain members of Division B. Add HC1 until the solution is milk-white from the separation of finely divided sulphur, or until further acidification produces no more precipitation. If no flocculent or colored precipitate is formed, members of Division B are absent. If a precipitate appears, filter, reject the filtrate, and treat the precipitate as directed on page 44. DISCUSSION The members of the hydrogen sulphide group are distin- guished from those of the hydrochloric acifl group by the greater solubility of their chlorides in dilute hydrochloric acid, and from those of the other groups by the relative insolubility of their sulphides in the same reagent. If nitric acid is present in any considerable amount in the solution that is treated with hydrogen sulphide, the members of the hydrogen sulphide group will not be completely precipitated until most of that acid has been reduced by the hydrogen sul- phide. Other oxidizing agents, such as permanganates, chro- mates, ferric salts, etc., will also be reduced. The free sulphur that is formed by this reduction is often thrown out in a more or less flocculent form and may thus lead to a false conclusion as to the presence of members of this group. It may also make difficult the subsequent treatment of the precipitate (filtration, etc.). For this reason it is well to reduce not only nitric acid, but also, so far as may be, any other oxidizing agents that are present, before the solution is treated with hydrogen sulphide. The treatment with hydrochloric acid prescribed for the removal of nitric acid will also decompose nitrites, chlorates, and sulphites. If a permanganate or chromate is present, the reduction may be hastened by the addition of a little alcohol PRECIPITATION 41 before boiling with hydrochloric acid, but in such case the boil- ing should be continued until the excess of alcohol is removed. This reduction of permanganates and chromates is further desirable, since these salts, if present in relatively large amounts, will form black or colored precipitates when reduced by hydro- gen sulphide. Prolonged boiling of arsenious compounds with hydrochloric acid should be avoided, as arsenious chloride is volatile. The quantity of hydrochloricfacid in the solution that is to be treated with hydrogen sulphicrtPlfe important. Enough of the acid must be present to prevent the precipitation of the members of the ammonium sulphide group, but its amount must not be so great as to prevent the precipitation of those.' sulphides of the hydrogen sulphide group which are somewhat soluble in strong hydrochloric acid. Moreover, the presence of hydrochloric acid is necessary in order to insure the complete precipitation of members of Division B. From alkaline solutions these elements are pre- cipitated by hydrogen sulphide incompletely or not at all, owing to the formation of soluble sulpho-salts. When hydrogen sulphide is passed into neutral solutions or into solutions that contain these elements in the form of acids, if no other acids or salts are present, pseudo-solutions 1 of the sulphides 2 are formed. Hydrochloric acid prevents the formation of these pseudo-solutions and causes the sulphides to be precipitated in a compact form, which makes filtration easy. The concentration, one part of HCI to ten parts of solution, has been found to answer the conditions most satisfactorily. Precipitation of arsenic pentasulphide from solutions of arsenic acid takes place slowly and is complete only after several hours. By heating the solution at the beginning of the 1 Pseudo-solutions are discussed in the Introduction, page 12. 2 Schulze: J. prakt. Chem., 25, 431 (1882); 27, 320 (1883); Brauner and Tomicek : Monatshefte, 8, 614 (1887). 42 HYDROGEN SULPHIDE GEOUP treatment with hydrogen sulphide, conditions are made favorable for the reduction of arsenic acid to arsenious acid, and from the latter arsenic trisulphide is easily precipitated. (If arsenic acid is known to be present, it may be reduced with sulphur dioxide, the excess of the latter expelled by boiling, and the solution then treated with hydrogen sulphide. It must be borne in mind, however, that in this reaction sulphuric acid is formed and that this will precipitate lead and members of the ammonium carbonate group.) The solution should be cool at the end of the operation in order to secure the complete precipitation of the sulphides soluble in hot hydrochloric acid. / The members of Division B of the hydrogen sulphide group are distinguished from those of Division A by the solubility of their sulphides in ammonium polysulphide, sulpho-salts being formed. In these compounds the arsenic and antimony are pentavalent and the tin tetravalent, even if the three elements were precipitated as their lower sulphides by hydrogen sulphide. In the latter case it may be assumed that the excess of sulphur in the polysulphide unites, for example, with SnS to form SnS 2 , which then reacts with (NH 4 ) 2 S to form the sulpho-salt. SnS 4- S = SnS 2 ; SnS 2 + (NH 4 ) 2 S = (NH 4 ) 2 SnS 8 . When these sulpho-salts are treated with an acid it is pos- sible that the corresponding free sulpho-acids are at first formed, as (NH 4 ) 2 SnS 3 + 2 HC1 = 2 NH 4 C1 -f H 2 SnS 3 ; but if they are, they immediately break down into free hydro- gen sulphide and the sulphide of the metal, as H 2 SnS 3 = H 2 S + SnS 2 . The separation of Divisions A and B by ammonium polysul- phide is not always sharp. Copper sulphide is somewhat soluble SEPARATION OF DIVISIONS A AND B 43 in this reagent, and when the resulting solution is acidified a reddish brown precipitate is formed. Whether this precipitate contains also sulphides of arsenic, antimony, or tin is easily determined by treating the precipitate as directed on page 44. To detect an amount of copper so small that its sulphide would be dissolved entirely by the ammonium polysulphide, a solution may be used that has been obtained by boiling with HNO% a small portion of the group precipitate before it has been treated with the polysulphide. After this solution has been evaporated to small bulk, rendered alkaline by the addition of ammonium hydroxide, and then acid by the addition of acetic acid, it may be tested for copper with potassium ferrocyanide. HYDROGEN SULPHIDE GROUP DIVISION B FIRST METHOD OF ANALYSIS To the precipitate containing only sulphides of Division B, add concentrated hydrochloric acid and maintain at a temperature a little below the boiling point for some minutes, stirring occasionally. Filter. Residue: Arsenic Boil with HCI and a crystal of KC10 3 , pro- longing the operation until the residue con- sists evidently of little else than free sulphur. Filter and boil the fil- trate to expel chlorine. Portion 1. Boil with FeSO 4 and use the solu- tion for (a) Gatehouse test (modified), (b) Bet- tendorff test, (c)Reinsch test. Portion2. Add a few drops of the solution to 5 cc. of (NH 4 ) 2 Mo0 4 in a small test tube and warm gently. Portion 3. Make al- kaline with NH 4 OH and add MgCl 2 . Filtrate : Tin, Antimony, (Arsenic) Warm until II 2 S is expelled. Portion 1. Place a few cubic centimeters of the solution in a porcelain dish and introduce a piece of platinum foil and a strip of zinc in such a way that they are in contact with each other. Dilute with a little water if the evolution of hydrogen is violent. Place under the hood and allow the action to proceed until the evolution of hydrogen has nearly ceased, adding more zinc if necessary. Remove the zinc and wash it with a stream of distilled water from the wash bottle. With the finger rub off into a porcelain dish any coating that may still adhere to the zinc ; place in the dish a clean piece of platinum foil ; add HCI and boil for several minutes. Filter if a residue remains. Introduce the filtrate into a small test tube that has been rinsed out with HgCl2, and boil. A gray precipitate proves the presence of tin. If the precipitate is white, pour off the liquid and treat the precipitate with NH 4 OH, and if it is black- ened, the presence of tin is proved. Portion 2. To a few drops of the solution on a plati- num foil or in a platinum spoon add a granule of zinc. A black stain on the platinum indicates the presence of antimony. Wash the foil and immerse it in cold HCI as long as hydrogen bubbles appear ; then wash with water. Dissolve the stain with a drop of fum- ing HN0 3 , add a little crystallized tartaric acid, and warm gently until no more brown fumes appear. Dilute with two or tliree volumes of water ; add a few drops of HCI, and treat with H 2 S. An orange- colored precipitate, which may settle only after the solution has stood for some time, proves the presence of antimony. Portion 3. Use a little of the solution for the modi- fied Bettendorff test. Portion 4. Test a few drops of the solution directly for arsenic by the methods given under Portion 1 in the other column, in case no reaction for arsenic was obtained with the. portion of the group precipitate which was insoluble in cone,. HCI (Residue : Arsenic). ANALYSIS OF DIVISION B 45 DISCUSSION In the first method of analysis of Division B arsenic is separated from antimony and tin by taking advantage of the difference in solubility of the sulphides in hydrochloric acid, the sulphides of arsenic being but slightly attacked while those of antimony and tin are dissolved by that reagent. The separation is not altogether sharp, so that the filtrate obtained after treatment of the sulphides with hydrochloric acid must be tested for arsenic in case it is desired to detect mere traces of that element. The detection of arsenic. In the modified Gutzeit test the nascent hydrogen is produced by the action of zinc on sul- phuric acid, and both arsenious and arsenic compounds are reduced to arsine. The essential point of the test is the for- mation of the yellow compound (AsAg 3 - 3AgNO 3 ) when this gas acts upon a crystal of silver nitrate. The disadvantages of the test are : (a) when antimony is present stibine is produced and this may form a similar yellow compound or may blacken the silver nitrate crystal at once ; (b) if hydrogen sulphide is evolved from the reagents, the crystal is also blackened. Although the latter difficulty may be avoided, the test is not so simple as the modified Gatehouse test. [The Gutzeit test is sometimes applied to a mixture that may contain other substances than compounds of arsenic, anti- mony, and tin. When this is done it must be borne in mind that under the conditions of the test : (a) it is preferable that the arsenic in the substance tested should be in the trivalent condition; (b) phosphites and hypophosphites evolve phosphine which produces exactly the same color changes in the crystal of AgNO 3 as does arsenic; 1 1 See Ber. d. chem. Ges., 16, 2435 (1883). 46 HYDROGEN SULPHIDE GROUP (c) sulphides l and sulphites, if present in large amount, will evolve more hydrogen sulphide than can be absorbed by the lead ammonium acetate in the cotton.] In the modified Gatehouse test the nascent hydrogen is pro- duced by the action of aluminum on potassium or sodium hydroxide. Only arsenious compounds are reduced. It has an advantage over the modified Gutzeit test, since (a) no stibine is formed when antimony is present, and (b) there is no danger of an evolution of hydrogen sulphide from the reagents. The action in the tube should be gentle; otherwise the alkali may be carried through the cotton and blacken the crystal. [If in this test there is used a mixture that may contain other elements than those of Division B of the hydrogen sulphide group, it must be borne in mind that certain gases other than arsine produce color changes in a crystal of silver nitrate (see above).] The Bettendorff test is delicate when concentrated hydro- chloric acid solutions are used. Under these conditions any arsenic that may be present will behave like arsenious chloride and will be reduced by stannous chloride. A dilute solution of arsenious acid is, however, not reduced by stannous chloride. The Reinsch test, in which the arsenic is deposited upon cop- per, is not in itself conclusive. Deposits may also be formed on the copper by compounds of antimony and tin. It is necessary therefore to supplement the Reinsch test by other tests. If on heating the copper foil in a glass tube a sublimate is produced, and this then responds to the Gatehouse test, the presence of arsenic is proved. [If elements other than the three mentioned are present in the solution submitted to the Reinsch test, they may form a deposit on the copper. Organic matter or compounds of silver, mercury, bismuth, gold, platinum, palladium, or sulphur may i See Ber. d. chem. Ges., 16, 2435 (1883). ANALYSIS OF DIVISION B 47 cause such deposits. These deposits, however, will not obscure the test for arsenic if the Reinsch test is supplemented as above described.] Ammonium molybdate or magnesium chloride cannot be used satisfactorily in the detection of mere traces of arsenic acid. When the amount of arsenic acid is small, precipitation takes place so slowly that the rapid and accurate tests discussed above are to be preferred. The detection of antimony. In the modified Gutzeit test if the crystal of silver nitrate is blackened without the intermediate formation of a yellow compound, the presence of antimony is indicated. In the modified Bettendorff test, in which tin foil and a moderately dilute solution of stannous chloride in hydrochloric acid form the reducing agents, the presence of antimony is indi- cated if the tin is evenly covered with a black deposit. Arsenic, if present, will be deposited more unevenly on the foil. A similar deposit on platinum foil when a granule of zinc is placed upon the platinum in the acid solution to be tested is also an indication of the presence of antimony. In this case it is possible to confirm the presence of this element. Any metal- lic tin which may also have been deposited on the platinum will dissolve easily in cold HCI. The antimony is scarcely attacked if it is not exposed to the air when wet with the acid. The antimony that may have been deposited as a black coat- ing on the platinum is instantly changed to antimonic acid by the action of fuming nitric acid. Before testing for antimony with hydrogen sulphide the excess of nitric acid is removed by heating the solution with crystallized tartaric acid. This is oxidized by the nitric acid, which is itself entirely decom- posed. The detection of tin. When platinum and zinc are placed in the acid solution of a tin salt there is formed metallic tin, which is chiefly deposited on the zinc. When this tin is dissolved in 48 HYDROGEN SULPHIDE GROUP hydrochloric acid stannous chloride is produced. The test for tin is based on the reducing action of this solution on mercuric chloride. The amount of mercuric chloride taken is small in order that the stannous chloride may be present in excess and a dark precipitate of metallic mercury may be formed. In case the amount of tin is so small that the reduction does not pro- ceed beyond the formation of mercurous chloride, the addition of ammonium hydroxide and the consequent formation of the black mixture of mercury and Hg-NH 2 -Cl will show that the mercuric chloride has been reduced and that therefore tin is present. SECOND METHOD OF ANALYSIS Place the precipitate in an evaporator ; add a small quantity of concentrated HC1 and boil the liquid. If there is a residue other than sulphur, add a crystal of KC1O 3 and boil, repeating the operation until the residue consists evidently of little but free sulphur. Filter and reject the residue. (If KC1O 3 has been used, boil the filtrate until the odor of chlorine has disappeared ; then add a little FeSO 4 solution and boil.) Pour the solution, a few drops at a time, into the funnel tube of the hydrogen generator, arranged as described on page 30, and lead the evolved gases into a test tube half full of AgNO 3 solution. The solution should be added so slowly that the bubbles of gas passing through the solution in the test tube may easily be counted. The zinc should be kept in contact with the platinum foil during the whole operation. The action should be allowed to continue until the evolution of gas has almost ceased. The amount of zinc that is used should be sufficient to insure that some of the metal still remains in the generator at the end of the reaction. ANALYSIS OF DIVISION B 49 Contents of the Generator : Tin and Antimony Filter the contents of the generator, re- jecting the filtrate. Remove the pieces of zinc and the platinum foil, and wash into the filter any substance loosely adhering to them. Wash the contents of the filter into the apex, allow to drain, and tear off the apex of the filter. If a black precipitate appears, the presence of antimony or arsenic is indicated. Filter. Filtrate: Arsenic Add HCI until no further precip- itate is formed. Precipitate : Antimony Wash thorough- ly by decantation Loose Precipitate, Black Stain on chiefly on Zinc Platinum Shake vigorously with hot water Place the apex of If the platinum and filter. Pass until the wash the filter containing foil is stained black, H 2 S through the water gives no the precipitate, to- antimony is proba- clear filtrate. A turbidity with gether with a clean bly present. Immerse yellow precipitate HCI. Boil the piece of platinum the foil in cold HCI proves the pres- precipitate with a foil, in an evapora- as long as hydrogen ence of arsenic. little tartaric acid. tor, add a very little bubbles appear; then To confirm the Filter. Dilute the HCI, and heat to wash with water. presence of arse- filtrate with a little boiling for some Dissolve the stain nic, especially if water, add a few minutes. Filter, with a drop of fum- the precipitate has drops of HCI, and place the filtrate in ing HNO 3 , add a little an orange tint, treat with H 2 S. a small test tube crystallized tartaric filter, and boil the An orange-col- that has been rinsed acid, and warm gently precipitate with ored precipitate., out with HgCl 2 , and until no more brown a little HNO Z . which may appear boil. A white pre- fumes appear. Dilute Filter, add the only after the solu- cipitate, becoming with two or three filtrate to 5 cc. tion has stood for gray on warming, volumes of water, (NH 4 ) 2 MoO 4 , and some time, proves proves the presence add a few drops of warm. A yel- the presence of of tin. If the pre- HCI, and treat with low precipitate, antimony. cipitate is white, H 2 S. An orange- which may not pour off the liquid colored precipitate, appear immedi- and treat the precip- which may appear ately, proves itate with NH 4 OH, only after the solu- the presence of and if it becomes tion has stood for arsenic. blackened, the pres- some time, proves the ence of tin is proved. presence of antimony. Contents of the Test Tube Arsenic and Antimony 50 HYBEOGEK SULPHIDE GKOUP DISCUSSION Of the sulphides of Division B of the hydrogen sulphide group which are precipitated from solutions of their sulpho-salts by dilute hydrochloric acid, those of tin and antimony are much more easily soluble in hot HCI than that of arsenic. A suitable oxidizing agent, however, easily changes the latter into arsenic acid, and this is soluble in water. If, therefore, the addition of potassium chlorate is needed to dissolve the sulphides, the presence of arsenic is indicated. The first step in the separation of the members of Division B is the reduction by nascent hydrogen of a solution of their compounds. The reaction is vigorous but obviously not instan- taneous. A solution that has been prepared as above, if all the members of this division are present, contains stannic chloride, antimonic chloride, and arsenic acid. If these are first reduced to stannous chloride, antimonious chloride, and arsenious acid, the subsequent reduction in the generator will be completed in much less time. This is especially true in the case of arsenic acid. The solution also contains the oxidizing agents chlorine and potassium chlorate, which it is desirable to remove. The chlorine is expelled by heating the solution, the potassium chlorate being reduced at the same time if a sufficient quan- tity of hydrochloric acid is present. In the removal of these oxidizing agents the antimonic chloride is reduced to the trivalent condition. The reduction of arsenic acid is accom- plished by the addition of ferrous sulphate. The stannic chloride is not reduced by the above treatment, but this is immaterial since its reduction is easily effected by the nascent hydrogen. The solution should be introduced into the generator slowly and in small portions : ANALYSIS OF DIVISION B 51 (a) to avoid a too rapid passage of the evolved gases through the silver nitrate, for this would result in but partial interaction with the last-named reagent ; and (b) to avoid the presence, at any one time, of relatively large amounts of the arsenic solution in the generator, for this might result in the precipitation of metallic arsenic in the generator. The separation of arsenic and antimony from tin is based upon the fact that nascent hydrogen reduces compounds of tin to metallic tin, while compounds of arsenic and antimony are reduced to arsine and stibine, which are gases. The tin remains in the generator, and the two gases pass off with the hydrogen. Solid arsenic hydride may separate in the generator if platinum foil is present, but under ordinary conditions sufficient arsine will be formed to make the detection of arsenic possible in the gases that pass out of the generator. As tin is soluble in acids, it is necessary to allow the reaction in the generator to proceed until all of the acid is decomposed by the zinc. If the operation is carried out slowly, as directed, the reduc- tion of compounds of antimony to stibine is seldom complete, a portion of the antimony being deposited in the metallic form as a black coating upon the platinum. Metallic tin also may be deposited on the platinum. In such case most of it is but loosely adherent and may be removed by washing. The remainder, which in the absence of antimony appears as a gray coating, will easily dissolve in cold HCI and is thus separated from the antimony, since the latter is but slightly soluble in the cold acid. The antimony that may have been deposited as a black coat- ing on the platinum is instantly changed to antimonic acid by the action of fuming nitric acid. Before testing for antimony with hydrogen sulphide the excess of nitric acid is removed by heating the solution with crystallized tartaric acid. This is oxidized by the nitric acid, which is itself entirely decomposed. 52 HYDBOGEN SULPHIDE GROUP The separation of the arsenic and the antimony that have passed out of the generator as arsine and stibine is based upon the difference in the action of those gases upon a solution of silver nitrate. The arsine is oxidized to arsenious acid, which remains in solution, metallic silver being at the same time pre- cipitated; while the stibine forms SbAg 3 , which is also pre- cipitated. The formation of a black precipitate in the silver nitrate, therefore, indicates the presence of arsenic or antimony. In order that the separation may be as complete as possible it is necessary that more silver nitrate be used than is sufficient to react with the gases, since otherwise an excess of stibine might dissolve in the water and appear in the filtrate with the arseni- ous acid. The filtrate from the precipitate produced by the action of the gases on silver nitrate will therefore contain some unacted-upon silver nitrate if the operation has been properly carried out. The silver must be removed by precipitation as silver chloride before the solution is tested for arsenic with hydrogen sulphide ; otherwise the black silver sulphide which would be formed would mask the color of the yellow arsenious sulphide. The separation of arsenic from antimony by this method is not altogether sharp, as stibine also is somewhat oxidized by silver nitrate. If the amount of stibine is relatively large, a sufficient quantity of antimony may pass into solution to impart an orange color to the precipitate in the final test for arsenic, even if an excess of silver nitrate is present in the test tube at the end of the operation. In such a case arsenic may be detected by boiling the precipitate with HNO y The sulphide of arsenic will be changed to arsenic acid, and when this is added to ammonium molybdate a yellow precipitate of ammonium arseno-molybdate is formed. NICKEL 53 AMMONIUM SULPHIDE GROUP PRELIMINARY REACTIONS NICKEL : Reactions of solutions of nickel salts. KOH or NaOH precipitates Ni(OH) 2 , apple-green ; insoluble in excess. Ni(OH) 2 is oxidized by boiling with bromine water and NaOH to Ni(OH) 3 , black. If this precipitate is filtered out and boiled with NH 4 OH, it is reduced to Ni(OH) 2 , nitrogen being at the same time liberated. If NH 4 C1 is present, the Ni(OH) 2 which is formed dissolves, forming perhaps NiCl 2 '4NH 3 . NH 4 OH, if added in relatively small quantity to a solution of a nickel salt, precipitates Ni(OH) 2 ; if in larger quantity, greenish blue basic salts. The precipitates are soluble in NH 4 OH in the presence of ammonium salts, forming complex salts, as Ni(N0 3 ) 2 '4 NH 3 . These solutions and precipi- tates are changed by (NH 4 ) 2 S to NiS, black. Na 2 C0 3 precipitates basic carbonates, apple-green; soluble in large excess of the concentrated reagent. H 2 S precipitates NiS from an ammoniacal solution. No precipitate is produced in solutions containing an inorganic acid or a considerable quantity of acetic acid. From neutral solutions or those but weakly acidified with acetic acid, prolonged treatment with H 2 S partially precipitates the nickel as NiS. In dilute acetic acid solutions containing an alkali acetate the precipitation is complete and fairly rapid if the solution is warm. (NH 4 ) 2 S precipitates from neutral or alkaline solutions NiS, black; somewhat soluble in excess of (NH 4 ) 2 S, more readily in the presence of NH 4 OH, forming a dark brown solution from which NiS is reprecipitated if the solution is boiled and the solvent is thus removed. NiS is prac- tically insoluble in cold dilute HC1, normal l ; changed by aqua regia to free sulphur and NiCl 2 which dissolves. NiS is oxidized by the air to NiS0 4 . HC1 produces no precipitate. 1 An acid of this strength may be made by diluting one volume of the dilute hydrochloric acid, which is twice normal (see Appendix), with one volume of water. It may also be prepared by diluting one volume of concentrated hydrochloric acid (specific gravity 1.20) with twelve volumes of water. 54 AMMONIUM SULPHIDE GROUP H 2 S0 4 produces no precipitate. BaC0 3 when shaken with a cold neutral or but slightly acid solu- tion of MC1 2 does not precipitate a compound of nickel. BaCO 3 is most conveniently used for this purpose in a state of suspen- sion in water. The reagent bottle should therefore be shaken before using. A Borax Bead is colored brown when fused with a compound of nickel. Heat a small loop of clean platinum wire to redness and dip it into pow- dered borax. Heat gently at first and then fuse until all bubbles dis- appear and a clear bead results. Touch the solid substance to be tested with the hot bead in such a way that a small amount adheres to the bead and fuse again until clear and free from bubbles. Note the color. COBALT : Reactions of solutions of cobalt salts. KOH or NaOH precipitates from cold solutions a blue basic salt which when warmed with the alkali changes to Co(OH) 2 , pink. Co(OH) 2 may be oxidized to Co(OH) 3 , black. The reaction takes place slowly in the air, more readily under the influence of oxidizing agents, such as bromine water in the presence of an alkali (NaOH). Co(OH) 3 is not reduced by boiling it with NH 4 OH. NH 4 OH precipitates blue basic salts ; easily soluble in NH 4 OH in the presence of ammonium salts, forming complex salts, as Co(N0 3 ) 2 '4NH 3 . This solution quickly assumes a red color, being oxidized by the air to com- plex cobaltic ammonia compounds. These solutions and precipitates are changed by (NH 4 ) 2 S to CoS, black. Na 2 C0 3 precipitates pink basic carbonates; soluble in large excess of the concentrated reagent. H 2 S. The action of this reagent upon solutions of cobalt salts is similar to that upon solutions of nickel salts. (NH 4 ) 2 S precipitates from neutral or alkaline solutions CoS, black ; insoluble in excess ; practically insoluble in cold normal l HC1 ; attacked by aqua regia, forming sulphur and CoCl 2 which dissolves. CoS is oxi- dized by the air to CoS0 4 . HC1 produces no precipitate. H 2 S0 4 produces no precipitate. 1 See note, page 53. IRON 55 BaC0 3 when shaken with a cold neutral or but slightly acid solution of CoCl 2 does not precipitate a compound of cobalt. A Borax Bead is colored blue when fused with a compound of cobalt. IRON : Reactions of solutions of ferrous salts. KOH or NaOH precipitates Fe(OH) 2 , white, which is immediately oxi- dized to a dirty green compound unless special precautions are taken. The oxidation proceeds rapidly when the precipitate is exposed to the air, until finally it is transformed into Fe(OH) 3 , reddish brown. Immediate precipitation is prevented by the presence of a relatively large quantity of ammonium salts. NH 4 OH produces in neutral solutions an incomplete precipitation of Fe(OH) 2 , a portion of the iron remaining in the solution as a double salt, such as FeS0 4 -(NH 4 ) 2 S0 4 or FeCl^ NH 4 C1. The precipitate passes through the different stages of oxidation described in the preceding para- graph. In solutions containing double salts like those above mentioned, NH 4 OH produces no precipitate immediately, but oxidation by the air gradually takes place and there results a precipitate which is at first green, then black, and finally reddish brown. When these solutions or precipitates are treated with (NH 4 ) 2 S there is formed FeS, black. N^COg precipitates FeCO 3 , white, which is quickly oxidized by the air to a green compound and finally to Fe(OH) 3 . FeCO 3 is soluble in large excess ,of a concentrated solution of Na 2 CO 3 . I^S produces no precipitate in solutions containing dilute inorganic acids. From dilute neutral solutions a slight precipi- tate of FeS may be obtained by prolonged treatment with H 2 S. In the presence of a large amount of an alkali acetate there is a considerable precipitation of FeS, but this is not complete. (NH 4 ) 2 S precipitates .FeS, black ; attacked by hot HNO%, forming sul- phur and Fe(N0 3 ) 3 which dissolves. FeS is soluble in a large amount of cold dilute HC1. FeS on exposure to moist air is oxidized to FeS0 4 and finally to a basic ferric sulphate which is brown. HC1 produces no precipitate. H 2 S0 4 produces no precipitate. BaC0 3 shaken with a cold neutral or but slightly acid solution of FeC^ does not precipitate a compound of iron. BaC0 3 is most conveniently 56 AMMONIUM SULPHIDE GKOUP used for this purpose in a state of suspension in water. The reagent bottle should therefore be shaken before using. A solution of a ferrous salt always contains some ferric salt unless extraordinary precautions have been taken to prevent oxidation. For the first two of the following tests it is necessary to prepare a fresh solution of a ferrous salt. This may be done by adding dilute H 2 S0 4 and a few iron filings to a solution of ferrous sulphate and warming until the reac- tion is well started. This will quickly reduce to the ferrous condition any ferric salt that may be present. The operation may be carried out in a small Erlenmeyer flask, fitted with a cork through which passes a glass tube drawn out to small diameter at the outer end. It should be borne in mind that the mixture of gases in the flask is explosive. KCNS, potassium sulphocyanate, produces no coloration in solutions of ferrous salts. K 4 Fe(CN) 6 , potassium ferrocyanide, precipitates K 2 Fe"Fe(CN) 6 , potassium ferrous ferrocyanide. This compound is white, but is so easily oxidized that under ordinary conditions of preparation it has a light blue color, due to the formation of a small amount of Fe;"[Fe(CN) 8 ] s . K 3 Fe(CN) 6 , potassium ferricyanide, precipitates from neutral or acid solutions Fe 3 [Fe(CN) 6 ] 2 , dark blue; insoluble in dilute acids. HN0 3 . If a few drops of HNO% are added to a solution of a ferrous salt and the mixture is boiled, a ferric salt will be formed. Chlorine water oxidizes a solution of a ferrous salt to a ferric salt. The chlorine for this oxidation may be obtained by adding a small crystal of KC10 3 to a solution of a ferrous salt containing HCI and then boiling the mixture. Reactions of solutions of ferric salts. KOH or NaOH precipitates Fe(OH) 3 , reddish brown, gelatinous ; transformed by (NH 4 ) 2 S into FeS. NH 4 OH precipitates Fe(OH) 3 . The presence of non-volatile organic acids tends to prevent precipitation. Na 2 C0 3 precipitates a reddish brown basic carbonate which changes to Fe(OH) 3 when boiled with the reagent. The basic carbonate is soluble in a large excess of the concentrated reagent. IEON 5T H 2 S reduces solutions of ferric salts containing HC1 or H 2 S0 4 to the ferrous condition, sulphur being set free. (NH 4 ) 2 S precipitates FeS mixed with free sulphur. HC1 produces no precipitate. H 2 S0 4 produces no precipitate. BaC0 3 when shaken with a cold neutral or but slightly acid solution of a ferric salt precipitates Fe(OH) 3 or a basic salt. 1 KCNS when added in excess to a solution of a ferric salt forms Fe(CNS) 3 -9KCNS, a deep red salt 2 which is soluble in water and there- fore imparts its color to the solution. The color is most intense when the solution contains a small amount of a free inorganic acid. Solu- tions containing oxides of nitrogen and a free inorganic acid also give a red color when KCNS is added. Under certain conditions the forma- tion of Fe(CNS) 3 -9 KCNS and the consequent red coloration of the solution may not result. A ferric solution that contains an excess of an alkali acetate (a ferric acetate solution) will give no red color with potassium sulphocyanate until a considerable amount of an inorganic acid, such as hydrochloric acid, has been added. This is also true of ferric solutions containing phos- phoric, oxalic, tartaric, or boric acid. When ether is added to a solution containing ferric sulphocyanate and the mixture is shaken, the ferric sulphocyanate is taken up by the ether and imparts to the ether layer a red color. This fact may be utilized in testing for traces of iron so minute that the red coloration with KCNS is scarcely discernible. K 4 Fe(CN) 6 precipitates Fe'J'[Fe(CN) 6 ] 8 , dark blue ; not readily soluble in dilute inorganic acids. If the solutions are dilute, the liquid may assume a deep blue color without the formation of a precipitate. K 3 Fe(CN) 6 produces no precipitate but imparts a brown color to the solution. Most reducing agents easily reduce ferric to ferrous salts. 1 Dammer in his Handbuch der anorganischen Chemie states that the precipi- tate is ferric hydroxide. But Demarc.ay, the authority to whom he refers, says : " Iron is precipitated by barium carbonate always in the form of a basic salt," 2 Krliss and Moraht: Ber. d. chem. Ges., 22, 2061 (1889). 58 AMMONIUM SULPHIDE GROUP MANGANESE : Reactions of solutions of manganous salts. KOHor NaOH precipitates Mn(OH) 2 , white; oxidized quickly by the air, probably forming first manganous acid, MnO(OH) 2 , 1 which then reacts with Mn(OH) 2 to form manganous manganite, Mn 2 O 3 . These compounds are dark brown. NH 4 OH, in the absence of ammonium salts, precipitates one-half of the manganese as Mn(OH) 2 (see preceding paragraph), the other half remain- ing in solution as a double salt, e.g., MnCl 2 -2NH 4 Cl or MnS0 4 -(NH 4 ) 2 S0 4 . In solutions of such double salts, NH 4 OH produces no immediate pre- cipitate, but the solution soon begins to be oxidized by the oxygen of the air, and MnO(OH) 2 is precipitated. These solutions and precipitates are changed to MnS by (NH 4 ) 2 S. Na^Og precipitates MnCO 3 or basic carbonates, according to the conditions under which precipitation takes place; white, oxidizing in the air to MnO(OH) 2 . H 2 S produces no precipitate from solutions containing inorganic acids or a moderate amount of acetic acid. From a solution of Mn(C 2 H 3 O 2 ) 2 containing but little acetic acid, prolonged treatment with H 2 S may precipitate a small quantity of MnS, pink. (NH 4 ) 2 S precipitates MnS, pink; soluble in dilute inorganic acids or acetic acid. MnS turns brown upon oxidation in the air, with the formation of Mn 2 3 , MnS0 4 , and sulphur. Oxalates, tartrates, or citrates tend to prevent precipitation. By boiling the pink sulphide with a large excess of (NH 4 ) 2 S and NH 4 OH it may be changed into another sulphide, 2 which is green, more com- pact, less easily oxidized, and less easily soluble in acetic acid than the pink sulphide. HC1 produces no precipitate. H 2 S0 4 produces no precipitate. BaC0 3 when shaken with a cold neutral or but slightly acid solution of MnCL, does not precipitate a compound of manganese. Fusion with N^COg and KN0 3 oxidizes manganese compounds, 1 Tread well : Qualitative Analyse, p. 102. 2 Meineke : Ztschr. angew. Chem., 1888, p. 3. ZINC 59 in which the valence of manganese is less than six, to K 2 MnO 4 and Na 2 MnO 4 , green. Use a relatively small quantity of the dry substance to be tested and carry out the fusion in a platinum spoon. If the test is properly performed, the color is best seen at the edges of the fused mass. Pb0 2 , when boiled with dilute H 2 S0 4 and a small quantity of a manga- nese compound in which the valence of manganese is less than seven, oxi- dizes the latter to HMn0 4 which imparts a pink or purple color to the solution according to the amount of manganese present. The color is best seen after the PbS0 4 and the excess of Pb0 2 have been allowed to settle. If a chloride or HC1 is present in any considerable quantity, it should first be removed by adding concentrated H 2 S0 4 to the dry substance to be tested and heating until dense white fumes of S0 3 appear. ZINC : Reactions of solutions of zinc salts. KOH or NaOH precipitates Zn(OH) 2 , white, gelatinous ; soluble in an excess of either reagent, forming salts such as potassium zincate, Zn(OK) 2 . NH 4 OH produces in neutral solutions a partial precipitation of Zn(OH) 2 , white, gelatinous ; soluble in NH 4 OH and NH 4 C1, forming ZnCl 2 '4NH 3 . Zinc hydroxide is soluble in ammonium salts, forming double salts : with NH 4 C1, for example, there is formed ZnCl 2 -2NH 4 Cl. For this reason no precipitate is produced by the addition of NH 4 OH to a solution containing a relatively large amount of an ammonium salt. When any of these solutions or Zn(OH) 2 is treated with (NH 4 ) 2 S, ZnS is formed. Na^Og precipitates white basic carbonates; soluble in large excess of the concentrated reagent. H 2 S precipitates ZnS incompletely from neutral solutions of zinc salts of the inorganic acids. ZnS is white and, when freshly precipitated, is soluble in dilute inorganic acids. ZnS is only slightly soluble in acetic acid. The precipitation is complete if an alkali acetate (NaC 2 H 3 O 2 ) is present. 1 From alkaline solutions H 2 S precipitates ZnS completely. 1 For full discussion of this point see Introduction, page 4. 60 AMMONIUM SULPHIDE GliOUP (NH 4 ) 2 S precipitates ZnS. HC1 produces no precipitate. H 2 S0 4 produces no precipitate. BaC0 3 when shaken with a cold neutral or but slightly acid solution of ZnCLj does not precipitate a compound of zinc. I ALUMINUM : Reactions of solutions of aluminum salts. KOH or NaOH precipitates A1(OH) 3 , white, gelatinous ; A1(OH) 3 is easily soluble in an excess of either reagent, forming an alkali alumi- nate such as Al(ONa) 3 . No precipitate is formed when this solution is boiled alone, but if NH 4 C1 is added before boiling, A1(OH) 3 is pre- cipitated. A1(OH) 3 is easily soluble in dilute inorganic acids when freshly precipitated. NH 4 OH precipitates A1(OH) 3 , soluble with difficulty in NH 4 OH, probably forming A1(ONH 4 ) 3 . A1(OH) 3 is reprecipitated when this solution is gently warmed. vxNa 2 C0 3 precipitates A1(OH) 3 or basic carbonates. I^S produces no precipitate from neutral or acid solutions of aluminum salts. A1(OH) 3 will be precipitated by H 2 S from a solution of A1(OK) 3 , but this precipitate dissolves if more KOH is added. v/ (NH 4 ) 2 S precipitates A1(OH) 3 from aqueous solutions of aluminum salts. The hydroxide and not the sulphide is formed, because the sul- phide cannot exist in the presence of water. ^ HC1 produces no precipitate. v/H 2 S0 4 produces no precipitate. ^ BaC0 3 when shaken with a cold neutral or but slightly acid solution of A1C1 3 precipitates A1(OH) 3 or basic carbonates. CHROMIUM : Reactions of solutions of chromium salts. KOH or NaOH precipitates Cr(OH) 3 , grayish green or grayish lavender, gelatinous ; easily soluble in an excess of the reagent, forming Cr(OK) 3 or Cr(ONa) 3 , which imparts a green color to the solution. When this solution is boiled, Cr(OH) 3 is reprecipitated. The reprecipitation takes / CHKOMIUM 61 place more readily when the solution is dilute or an ammonium salt is present. Cr(OH) 3 is soluble in dilute inorganic acids. NH 4 OH precipitates Cr(OH) 3 ; soluble with difficulty in NH 4 OH in the presence of ammonium chloride, forming a red solution containing chiefly CrCl 3 '4 NHg. 1 When this solution is boiled, Cr(OH) 3 is reprecipitated. - Na^Og precipitates Cr(OH) 3 or basic carbonates; soluble in large excess of the concentrated reagent. H 2 S produces no precipitate in an acid solution of a chromium salt. ^(NH 4 ) 2 S precipitates Cr(OH) 3 . HC1 produces no precipitate. H 2 S0 4 produces no precipitate. BaC0 3 when shaken with a cold neutral or but slightly acid solution of CrCl 3 precipitates Cr(OH) 3 or a basic car- bonate. Fusion with KN0 3 and Na 2 C0 3 oxidizes chromium compounds, in which the valence of chromium is less than six, to K 2 CrO 4 and Na 2 CrO 4 , yellow. If the fused mass is dissolved in water and the solution acidified with acetic acid, the addition of lead ammonium acetate precipitates PbCrO 4 , yellow; soluble in NaOH, forming Pb(ONa) 2 . H 2 2 oxidizes a hot solution of Cr(OK) 3 or Cr(ONa) 3 to JLfrO^ or Na^rO^ When H 2 S0 4 is present H 2 2 oxidizes a chromate probably to perchromic acid, HCr0 4 , which imparts a blue color to the liquid. An excess of H 2 2 reduces the perchromic acid to chromium sulphate, the tendency to reduction increasing with the amount of free acid present. When ether is added to the above-mentioned blue solution of perchromic acid (?) and the mixture is shaken, the blue sub- stance is taken up by the ether, and imparts its color to the ether layer. 1 Dammer, Handbuch der anorganischen Chemie, III, p. 552. 62 AMMONIUM SULPHIDE GROUP METHOD OF ANALYSTS (NOTE. The portions of the text that are enclosed in heavy brackets deal with the analysis of the ammonium sulphide group when oxalates, tartrates, and phosphates are present. The study of this matter should be deferred until the student has famil- iarized himself with the procedure that is followed when the above-named salts are absent.) Unless oxalates, tartrates, and phosphates are known to be absent, they must be tested for before the analysis of this group is begun. If oxalates or tartrates are present, carefully evaporate the filtrate from the hydrogen sulphide group just to dry- ness. Grind the dry residue with five times its quantity of solid NH 4 NO 3 . In a large porcelain crucible fuse enough NH 4 NO 3 to cover the bottom and introduce the above mix- ture in very small portions. Maintain the contents of the crucible in gentle fusion during the addition and for some time thereafter. When cool, dissolve the contents of the cible in water, or HC1 if necessary, and treat the solu- ected in the next paragraph. If oxalates or tartrates are absent^ add NH 4 C1 to the filtrate from the hydrogen sulphide group or to the solution known to contain only members of the (NH 4 ) 2 S group, and divide into two portions. PORTION 1 To a small portion of the solution, freed from-H 2 S by boiling if the filtrate from the H 2 S group is used, add NH 4 OH till slightly alkaline, and note whether a precipitate forms. If it does, then add NH 4 OH quickly in large excess and observe whether the precipitate redissolves. Note the colors of both precipitate and solution and any changes in color that may occur during the operation, and record the inferences that may be drawn from these observations. ANALYSIS 63 PORTION 2 Place 5 cc. of this portion in a test tube, add NH 4 OH until the solution is very slightly alkaline, heat to boiling, and then add a few drops of (NH 4 ) 2 S. If no precipitate appears even after the addition of (NH 4 ) 2 S, analyze the remainder of Portion 2 as directed on page 75 unless members of succeeding groups are known to be absent. If a precipitate has appeared, discard the contents of the test tube and to the remainder of Portion 2 add NH 4 OH until the solution is very slightly alkaline. Heat to boiling. Add (NH 4 ) 2 S in slight excess, heat again to boiling, and allow the precipitate to settle. Filter and discard the nitrate if it is known that members of succeeding groups are absent ; other- wise treat the filtrate as directed on page 75. If this nitrate is dark brown in color, the presence of nickel is indicated. 1 Wash the precipitate immediately and thoroughly with hot water to which a few drops of NH 4 C1 and (NH 4 ) 2 S have been added, and without delay transfer the precipitate to an evap- orator and treat it with cold normal 2 HC1 for a few minutes. Stir the mixture vigorously to bring all parts of the precipitate into contact with the acid. If a black residue remains, the presence of nickel or cobalt is indicated. Filter and wash the residue immediately. (If the residue is light colored, not otherwise, it may be dissolved by warming with HC1.) 1 This filtrate should not have a brown color, and will not be so colored if the precipitation of the group has properly been performed. 2 See note, page 53. 64 AMMONIUM SULPHIDE GROUP ANALYSIS 65 2<5 So III- 66 AMMONIUM SULPHIDE GBOUP DISCUSSION If the filtrate from the hydrogen sulphide group contains oxalates or tartrates they must be removed, since in their presence some of the members of the ammonium sulphide group are precipitated only partially if at all. Moreover, if oxalates and at the same time members of the ammonium carbonate group are present, the latter will be precipitated as oxalates in the ammonium sulphide group and might thus escape detection. Fusion with ammonium nitrate oxidizes and destroys these and other organic compounds that might exert a disturbing influence. The separation of the members of the ammonium sulphide group from those of the following groups is based upon the insolubility in water of the hydroxides of aluminum and chro- mium and of the sulphides of the other members of the group. The hydroxides and sulphides of the alkali metals are soluble in water. The sulphides of the members of the ammonium carbonate group and of magnesium are not formed in the presence of water, but their hydroxides are produced by the action of ammonium hydroxide. These hydroxides, except that of magnesium, are soluble in water. The addition of ammonium chloride prevents the precipitation of magnesium hydroxide, there being formed the double salt MgCl 2 - 2 NH 4 C1, which is soluble in water and unaffected by ammonium hydroxide. The presence of ammonium chloride, moreover, prevents the formation of pseudo-solutions and causes the members of the ammonium sulphide group to be precipitated in a more compact form. It furthermore prevents the complete precipitation of borates of the ammonium carbonate group. The test with ammonium hydroxide often furnishes indica- tions of value and sometimes conclusive proof as to the presence or absence of certain members of the group. As ammonium sulphide is formed by the action of ammonium hydroxide on ANALYSIS 67 hydrogen sulphide, the latter must be expelled from the portion of the solution in which this test is made. In the precipitation of the group, ammonium hydroxide is added to neutralize any free acid which, if present, would decompose the ammonium sulphide. An excess of ammonium hydroxide is to be avoided, both on account of the solubility of the hydroxides of aluminum and chromium in that reagent, and also because its presence in large amount increases the solubility of nickel sulphide in ammonium sulphide, thus rendering nitra- tion slow and increasing the danger of oxidation. The group precipitate is filtered rapidly and immediately after precipitation, and is washed with water containing ammo- nium sulphide, in order to prevent the, oxidation of the sulphides of nickel and cobalt to the sulphates which would dissolve when the group precipitate is treated with dilute hydrochloric acid. Ammonium chloride is aocMi^ to the wash water to prevent the precipitate from dissolving in water and forming a pseudo-solution. The separation of nickel and cobalt from the other members of the group is based upon the fact that the sulphides of nickel and cobalt are but very slightly soluble in normal hydrochloric acid, while the sulphides of iron, zinc, and manganese and the hydroxides of aluminum and chromium are more readily dis- solved by hydrochloric acid of this strength. A black residue after this treatment indicates, but does not prove, the presence of nickel or cobalt, for some of the ferrous sulphide is at times left undissolved. Other members of this group may also remain with this residue even if a reasonable amount of hydrochloric acid has been used, for some of the sulphides and hydroxides that are dissolved by acids when freshly precipitated are much less readily dissolved some time after their formation. If, however, sufficient acid is used, enough of these elements will pass into the filtrate to be detected in the subsequent tests. 68 AMMONIUM SULPHIDE GROUP The separation of nickel from cobalt is based upon the facts, (1) that when their trihydroxides are boiled with ammonium hydroxide that of cobalt is unchanged, while that of nickel is reduced to Ni(OH) 2 ; and (2) that Ni(OH) 2 is dissolved by ammonium hydroxide and ammonium chloride, while Co(OH) 3 is insoluble. If in the oxidation to the trihydroxides any Co(OH) 2 is left unoxidized, this also will dissolve in ammonium hydroxide and ammonium chloride and will be precipitated from this solu- tion by hydrogen sulphide ; hence the necessity of fusing with the borax bead the precipitate obtained in the final test for nickel. After the removal of the sulphides of nickel and cobalt by filtration, hydrogen sulphide is expelled from the filtrate to pre- vent the formation of free sulphur in the subsequent oxida- tion, of ferrous to ferric salts. This oxidation is necessary (1) because the tests for ferric iron (KCXS, etc.) are more char- acteristic and delicate tnan those for ferrous iron, and (2) because iron is completely precipitated by barium carbonate only when it is in the form of a ferric salt. Ferrous salts in dilute hydrochloric acid solution are easily oxidized to ferric salts by boiling with a little HNO Z , and this oxidizing agent is used in the preparation of s the portion of the solution that is to be treated with baiftim carbonate. Since nitric acid may itself give a pink color with potassium sulphocyanate, a smaller portion of the dilute hydrochloric acid solution is oxidized by chlorine water instead of nitric acid, and this solution, after the removal of the excess of chlorine, is used in the final test for iron. The separation of iron, aluminum, and chromium from zinc and manganese is based upon the fact that from cold neutral solu- tions of the chlorides of the ammonium sulphide group, barium carbonate precipitates only the trivalent elements. Ammonium carbonate is first added to neutralize the greater part of the acid and thus save barium carbonate, the subsequent addition of which completes the neutralization. ANALYSIS 69 In the hydrogen peroxide tests for chromium the oxidation of a chromium salt in alkaline solution to a chromate presents no difficulty. The subsequent oxidation in acid solution to per- chromic acid (?) requires precise manipulation. The solution should contain very little free acid and the initial color change should be carefully observed, since the blue compound is easily reduced by hydrogen peroxide in the presence of free acid. The separation of aluminum from chromium is based upon the difference in behavior of solutions of Cr(ONa) 3 and of Al(ONa) 3 when boiled. The former is decomposed with the precipitation, of Cr(OH) 3 , while Al(ONa) 3 is unchanged and remains in solution. The sodium hydroxide used in this opera- tion is freshly prepared from sodmm carbonate and barium hydroxide, because the solvent action of sodium hydroxide on glass often introduces into a solution of that reagent com- pounds of aluminum and silicon which would interfere with the final test for aluminum. The separation of manganese from zinc is based upon the solubility of manganese sulphide in acetic acid and the relative insolubility of zinc sulphide in that reagent. The 9gpk^ioii of a manganous salt to permanganic acid by lead dioxide 1% the presence of sulphuric acid is a very delicate test. It may be performed with the original substance l if cer- tain precautions are observed. The presence of reducing agents interferes with the reaction. Hydrochloric acid is capable of reducing permanganic acid; and since this acid is set free from its salts by the action of sulphuric acid, the solution to be tested (manganous chloride in the systematic course of analysis) is boiled with concentrated sulphuric acid. Here, as in the hydrogen sulphide group, Division A, when decomposition of the sulphuric acid into sulphur trioxide and water takes place, it is assumed that a more volatile acid has been entirely expelled. 1 By original substance is meant the substance under examination before it has been subjected to chemical treatment, 70 AMMONIUM SULPHIDE GROUP Since manganous salts may be oxidized, it follows that they are themselves reducing agents ; they do, in fact, in acid solution reduce permanganic acid. It is therefore necessary to use in this test a portion of the manganous solution so small that the amount of lead dioxide taken will suffice to oxidize all of the manganese to permanganic acid. ^^ Analysis of the Ammonium Sulphide Group -when Phosphates are present The phosphates of the ammonium carbonate group and mag- nesium are soluble in dilute acids but are precipitated when such solutions are made neutral or alkaline. They are insoluble in water. It is evident, therefoi^^hat they can be present only when an acid solution is to be analyzed or when the original substance was insoluble in water but soluble in an acid. 1 In the systematic precipitation of the various groups it is not until the ammonium sulphide group is reached that the solution is made alkaline. The members of the ammonium carbonate group, if present, will therefore be precipitated as phosphates in the ammonium sulphide group, and provision n^ust there be made for their detection. No modification of the usual course of analysis is necessary until after the test for iron. The next step in the ordinary procedure is the addition of barium carbonate to the slightly acid solution. As this always introduces barium chloride into the filtrate, it is evident that a test for barium must be made before the addition ^ barium carbonate. After the presence or absence of barium has been determined, it is next necessary to separate phosphoric acid from manganese, zinc, and the members of the ammonium carbonate group. This is accomplished by taking advantage of the facts : (1) that phosphoric acid exhibits a greater tendency to combine with ferric iron than with the above-mentioned elements; and (2) that 1 This is strictly true only in the absence of ammonium salts. BAKIUM 71 when a solution containing ferric chloride and phosphoric acid is made neutral, ferric phosphate is precipitated. A quantity of ferric chloride equivalent to the amount of phosphoric acid present is therefore added to the solution, and the usual course of analysis proceeded with. When the solution is made neutral with barium carbonate all of the phosphoric acid is then precipitated with the hydroxides of chromium and aluminum as ferric phosphate, and the members of the ammonium carbonate group pass into the filtrate and are there tested for in the usual way, after the removal of zinc and manganese if the latter are present. AMMONIUM CARBONATE GROUP AND MAGNESIUM (The AlkalinejEarths) PRELIMINARY REACTIONS BARIUM : Reactions of solutions of barium salts. KOH or NaOH precipitates from concentrated solutions Ba(OH) 2 , white, voluminous ; insoluble in excess of the reagent, but soluble in water and therefore not precipitated from dilute solutions. NH 4 OH produces no precipitate. Na 2 C0 3 or (NH 4 ) 2 C0 3 precipitates from neutral or alkaline solutions BaC0 3 , white, flocculent at first. This precipitate becomes crystalline when gently wanned, the change taking place more quickly in dilute solu- * tions than in concentrated ones. BaC0 3 is very slightly soluble in NH 4 C1 ; soluble in HC1 or acetic acid, HC 2 H 3 2 , with effervescence. Barium carbonate is insoluble in excess of Na 2 CO 3 , but is somewhat soluble in water containing CO 2 or in a solution of a bicar- bonate, and therefore is not completely precipitated from acid solutions. H 2 S produces no precipitate. (NH 4 ) 2 S produces no precipitate. HC1 produces no precipitate. 72 AMMONIUM CARBONATE GROUP H 2 S0 4 or a solution of a sulphate, as K 2 S0 4 , precipitates BaS0 4 , white, finely divided l ; almost insoluble in water (i part in 800,000 parts of water) , acids, or alkalies. K 2 Cr0 4 precipitates from neutral or feebly acid solutions BaCr0 4 , yel- low; almost insoluble in water, slightly soluble in acetic acid, soluble in HC1. Na 2 HP0 4 or NaNH 4 HP0 4 precipitates from neutral or alkaline solutions Ba 3 (P0 4 ) 2 or BaHP0 4 , flocculent ; easily soluble in dilute HC1 or HN0 3 . (NH 4 ) 2 C 2 O 4 , ammonium oxalate, precipitates from concen- trated solutions BaC 2 O 4 , white ; soluble in HC1, HNO 3 , or hot acetic acid. STRONTIUM: Reactions of solutions of strontium salts. KOH or NaOH precipitates from concentrated solutions Sr(OH) 2 , resembling Ba(OH) 2 , but less soluble in water. NH 4 OH produces no precipitate. Na 2 C0 3 or (NH 4 ) 2 C0 3 precipitates SrC0 3 , resembling BaC0 3 . H^S produces no precipitate. (NH 4 ) 2 S produces no precipitate. HC1 produces no precipitate. H 2 S0 4 precipitates SrS0 4 . This substance resembles BaS0 4 but is more soluble in water (i part in about 7000 parts of water). SrS0 4 is more soluble in HC1 than it is in water ; therefore precipitation by H 2 S0 4 is not complete, and is less complete from a solution containing hydrochloric acid than from a neutral solution. Calcium sulphate, although but slightly soluble in water, is more soluble than strontium sulphate, and for this reason a solution of calcium sulphate will precipitate SrS0 4 from a concen- trated solution of a strontium salt. Precipitation is more complete when^ the mixture is warmed or when a concentrated solution of potassium sul- phate is used instead of the calcium sulphate. K2Cr0 4 does not precipitate SrCr0 4 from dilute solutions acidified with acetic acid. SrCr0 4 is more soluble than BaCr0 4 not only in acetic acid but also in water. Na 2 HP0 4 or NaNH 4 HP0 4 precipitates phosphates resembling those of barium in constitution and properties. (NH 4 ) 2 C 2 O 4 precipitates SrC 2 O 4 , white; soluble in HC1 or HNO 3 , difficultly soluble in acetic acid. - 1 See Introduction, page 15. CALCIUM 73 CALCIUM : Reactions of solutions of calcium salts. KOH or NaOH precipitates from sufficiently concentrated cal- cium solutions Ca(OH) 2 , similar to Sr(OH) 2 , but less soluble in water. NH 4 OH produces no precipitate. N^COgOr (NH 4 ) 2 C0 3 precipitates CaC0 3 which resembles BaC0 3 . Cal- cium carbonate is slightly soluble in an excess of a concentrated solution of sodium carbonate. H 2 S produces no precipitate. (NH 4 ) 2 S produces no precipitate. HC1 produces no precipitate. H 2 S0 4 or a soluble sulphate precipitates CaS0 4 only from concentrated solutions, and then but partially, on account of its solubility in water (i part in about 500 parts of water). K 2 Cr0 4 produces no precipitate in dilute solutions acidified with acetic acid, CaCr0 4 being readily soluble in water and also in acetic acid. Na 2 HP0 4 or NaNH 4 HP0 4 precipitates phosphates resembling those of barium and strontium. (NH 4 ) 2 C 2 4 precipitates CaC 2 4 , white, crystalline ; insoluble in water, ] soluble in HC1 or HN0 3 . Calcium oxalate is somewhat less soluble in acetic acid than are the oxalates of barium and strontium. MAGNESIUM: Reactions of solutions of magnesium salts. KOH or NaOH precipitates Mg(OH) 2 , white ; almost insoluble in water. NH 4 OH precipitates from neutral solutions containing no ammonium salts one-half of the magnesium as Mg(OH) 2 . The other half unites with the ammonium salt that is formed, producing double salts such as MgCl 2 -2NH 4 Cl. These double salts are soluble in water and are unaffected by NH 4 OH. In analytical practice there is usually present sufficient NH 4 C1 to unite with all of the magnesium, forming MgCl 2 -2NH 4 Cl, and therefore in the actual analysis ammonium hydroxide does not precipitate magnesium. Na 2 C0 3 or (NH 4 ) 2 C0 3 , in the absence of other ammonium salts, precipitates white basic carbonates ; soluble in large excess of the 74 AMMONIUM CARBONATE GROUP concentrated reagents. From solutions containing magnesium ammo- nium double sa/ts, magnesium is not precipitated by ammonium carbonate. H 2 S produces no precipitate. (NH 4 ) 2 S produces no precipitate. HC1 produces no precipitate. H 2 S0 4 produces no precipitate. Na 2 HP0 4 or NaNH 4 HP0 4 precipitates white flocculent phosphates similar to those of barium. From solutions of double salts, such as MgCl^ NH 4 C1, in the presence of NH 4 OH, an alkali phosphate pre- cipitates MgNH 4 P0 4 , white, crystalline; somewhat soluble in water, easily soluble in acids, insoluble in NH 4 OH. The crystalline condition of the precipitate is most apparent when precipitation takes place slowly from dilute solutions. Crystallization is hastened by stirring the solu- tion with a glass rod. If the solution is very dilute the precipitate will first appear where the rod has touched the inner surface of the test tube. ANALYSIS 75 METHOD or ANALYSIS The filtrate from the ammonium sulphide group may be colored by ammonium sulphide, see page 76, or by some of the members of the previous group soluble in that reagent or in NH 4 OH. If so, it should be boiled until the odor of H 2 S has disappeared and the solution is colorless, then concentrated to small bulk and filtered. This solution, or the original solution in case it is known that only members of this group are present, is then treated as follows : To the solution add NH 4 C1, NH 4 OH, and then (NH 4 ) 2 CO 8 . Warm the solu- tion. If a precipitate appears, warm gently until it becomes crystalline. (If no precipitate appears, test the solution for magnesium and the group of the alkali metals unless these are known to be absent. ) If a precipitate appears, filter. Precipitate : Barium, Strontium, Calcium Filtrate: Magne- Place the precipitate in a beaker and dissolve it in warm sium (and Alkali acetic acid, avoiding a large excess of the acid. To a small Metals) portion of the solution add K 2 Cr0 4 . (If no precipitate Divide into three appears, test the remainder of the solution for strontium portions. Reserve and calcium.) If a precipitate is formed, add K 2 CrO 4 to the larger portion the remainder of the solution, warm gently, and filter. for the detection of Precipitate : Filtrate : Strontium, Calcium the alkali metals, unless these are Barium Make alkaline with NH 4 OH, add known to be absent. Dissolve in HC1, warm the (NH 4 ) 2 CO 3 , and warm. (If no precipi- tate appears, calcium and strontium are To a small portion add NH 4 OH and a solution, and absent.) If a precipitate appears, filter, little NaNH 4 HP0 4 . add a few drops wash thoroughly, and dissolve on the Rub the inner sur- of H 2 S0 4 . A filter with the smallest possible quantity face of the test tube white precipi- tate proves the presence of ba- rium. of HC1. Evaporate just to dryness, dis- solve in a little water, filter if not clear, and evaporate to small bulk (about 2 cc.). Divide into two portions. with a glass rod. A crystalline precipi- tate proves the pres- ence of magnesium. (The precipi- Portion 1 Portion 2 If the precipitate is tate will appear yellow from the Strontium Calcium flocculent, add to an- other portion dilute K 2 CrO 4 in the Add a small Add K 2 S0 4 and H 2 SO 4 until acid, solution.) amount of a solu- heat to boiling. If and filter if a pre- tion of CaS0 4 , heat a precipitate is cipitate remains; to boiling, and if no' formed, filter, re- make the filtrate al- precipitate appears ject the precipitate, kaline with NH 4 OH, immediately, let and to the filtrate add (NH 4 ) 2 C 2 O 4 , stand for ten min- add NH 4 OH till warm, filter if a pre- utes. A fine white precipitate proves alkaline, then (NH 4 ) 2 C 2 O 4 , and cipitate appears, and test the filtrate for the presence of warm. A white magnesium with strontium. crystalline precipi- NaNH 4 HP0 4 . tate proves the presence of calcium. 76 AMMONIUM CARBONATE GROUP DISCUSSION [The filtrate from the preceding group contains ammonium sulphide. If this reagent has been freshly prepared, its presence will not interfere with the further analysis of the solution. But ammonium sulphide decomposes on long standing and there is formed a yellow polysulphide. When such a yellow ammonium sulphide has been used in precipitating the pre- ceding group and the filtrate from that group is boiled, sulphur is deposited and may interfere with the subsequent detection of the ammonium carbonate group. Therefore, if the filtrate from the ammonium sulphide group is yellow in color it is desirable, before proceeding with the analysis, to decompose the ammonium polysulphide by boiling. The precipitated sulphur should then be removed by filtration. Moreover, the filtrate from the ammonium sulphide group may contain nickel or chromium, the presence of nickel being due to the solubility of nickel sulphide in ammonium sulphide with the formation of a brown solution, while that of chromium is due to the solvent action of ammonium hydroxide and ammonium chloride upon chromium hydroxide, a red solution containing CrCl 3 -4 NH 3 being formed. The filtrate from the ammonium sulphide group may also con- tain aluminum because of the solubility of aluminum hydroxide in ammonium hydroxide. All of these solvents are volatile, and therefore when the solution is boiled down to small bulk they are removed, and any compound of nickel, chromium, or aluminum that may have been present in the filtrate from the ammonium sulphide group will be precipitated.] The members of the ammonium carbonate group, magnesium, and the alkali metals are not precipitated from solutions of their salts by hydrochloric acid, hydrogen sulphide, or ammonium sulphide. 1 1 The precipitation of the members of the ammonium carbonate group as oxalates, phosphates, etc., in the ammonium sulphide group has already been discussed (pages 66 and 70). ANALYSIS 77 The insolubility in water of the carbonates of the ammonium carbonate group is made the basis of the separation of these elements from the alkali metals. The separation of barium from strontium and calcium is based on the fact that barium chromate is but slightly soluble in acetic acid, while the chromates of strontium and calcium are easily soluble in that reagent. A large excess of acid will, however, dissolve sufficient barium chromate to interfere with the later test for strontium and should be avoided. The nitrate from the barium chromate precipitate contains acetic acid, potassium acetate, and potassium chromate, as well as the acetates of strontium and calcium if present. In order to obtain a solution containing only strontium and calcium, these elements are reprecipitated as carbonates, which are then changed to chlorides by treatment with hydrochloric acid. The fact that calcium sulphate is more soluble than strontium sulphate is made use of in the separation of calcium from strontium and in the test for strontium. Since both of these sulphates are somewhat soluble in water and still more so in hydrochloric acid, it is desirable that the solution should be concentrated and neutral. This condition is attained by evap- orating the solution to dryness and redissolving in but little water. From this solution of the chlorides, calcium sulphate will precipitate strontium sulphate. In order to remove the strontium from the portion of the solution of the chlorides which is to be tested for calcium, a strong solution of potassium sulphate is used. This precipitates the strontium almost completely and may also precipitate some of the calcium, but enough of the latter to give the calcium test will always pass into the nitrate because of the greater solubility of calcium sulphate. A comparison of the solubilities of those salts of barium, strontium, and calcium which are involved in their separation shows that the test for strontium assumes the previous removal 78 AMMONIUM CARBONATE GROUP of barium, and that the test for calcium assumes the previous removal of strontium. As has been seen, the separation of these elements by the above method is not altogether sharp and could not be used to detect traces of the members of this group. If used with care, however, it is sufficiently accurate for all ordi- nary purposes. Magnesium is distinguished from barium, strontium, and calcium by the greater solubility of its carbonate in ammonium chloride. If sufficient ammonium chloride is present to form a double salt, as MgCl 2 - 2 NH 4 C1, ammonium carbonate does not precipitate magnesium, and this element passes with the members of the group of the alkali metals into the nitrate. Sufficient ammonium chloride to form the double salt MgCl 2 -2NH 4 Cl is usually present in the nitrate from the preceding group. It should be borne in mind that the carbonates of barium, strontium, and calcium are slightly soluble in the presence of a large excess of ammonium chloride. The addition of sodium ammonium phosphate in the test for magnesium will produce a flocculent precipitate in case traces of barium, strontium, or calcium have passed through into the nitrate from the ammo- nium carbonate group. These elements may be removed almost completely, with the exception of traces of strontium, by taking advantage of the insolubility of the sulphates of barium and strontium and of the oxalates of strontium and calcium. The precipitate may also be flocculent (1) if any aluminum has been dissolved by the use of an excessive amount of ammonium hydroxide in the precipitation of the ammonium sulphide group and has not been removed by the treatment prescribed on page 75 since aluminum phosphate is insoluble in acetic acid, its removal from magnesium ammonium phosphate is easy or (2) if the quantity of magnesium is so great that the addition of the reagent produces an immediate precipitation, in which case the precipitate may be dissolved in an acid, the solution diluted, and the test for magnesium repeated. AMMONIA 79 AMMONIA, SODIUM, POTASSIUM (The Alkalies) PRELIMINARY REACTIONS AMMONIA: Reactions of solutions of ammonium salts. No precipitate is produced by NH 4 OH, Na^Og, H^S, (NH 4 ) 2 S, HC1, H^. KQH or NaOH when warmed with an ammonium salt in the solid form or in solution liberates ammonia gas. Moist, red litmus paper turns blue, and moist turmeric paper turns brown, when exposed to NH 3 . Co(N0 2 ) 3 '3NaN0 2 , sodium cobaltic nitrite, precipitates from acetic acid solutions Co(N0 2 ) 3 '3NH 4 N0 2 in the form of a yellow powder. When an ammonium salt is heated in a porcelain dish over the Bunsen flame it is decomposed into volatile constituents. The chloride is dissociated into NH 3 and HC1, which are volatile and reunite in the cool air above the dish, forming a white cloud. Other ammonium salts when heated lose their nitrogen either as free nitrogen, ammonia, or an oxide of nitrogen. SODIUM: Reactions of solutions of sodium salts. No precipitate is produced by KOH, NaOH, NH 4 OH, Na 2 C0 3 , H 2 S, (NH 4 ) 2 S, HC1, H 2 S0 4 . Co(N0 2 ) 3 '3NaN0 2 produces no precipitate in acetic acid solutions of sodium salts. Flame test. If a sodium salt is brought upon a platinum wire and introduced into the Bunsen flame, the flame is colored a brilliant yellow. Sodium salts are but slightly volatilized when heated in a porcelain dish over the Bunsen flame. POTASSIUM: Reactions of solutions of potassium salts. No precipitate is produced by KOH, NaOH, NH 4 OH, Na 2 C0 3 , , (NH 4 ) 2 S, HC1, H 2 S0 4 . 80 THE ALKALIES Co(N0 2 ) 3 -3 NaN0 2 when added to a concentrated solution of a potassium salt containing free acetic acid but no free inorganic acid, precipitates at once Co(N0 2 ) 3 '3KN0 2 in the form of a yellow powder. Precipitation takes place slowly from dilute solutions, but is hastened by gently warm- ing the mixture. If the solution is alkaline, it should be acidified with acetic acid before the addition of the sodium cobaltic nitrite : if it is acid, it should first be made neutral by the addition of sodium carbonate and should then be acidified with acetic acid. Flame test. If a potassium salt is brought upon a platinum wire and introduced into the Bunsen flame, the flame is colored violet. A very small quantity of sodium completely masks the color ; but if the flame is observed through a sufficiently thick layer of blue glass, the yellow sodium rays are absorbed, while the potassium rays pass through and may be recognized. Potassium salts are but slightly volatilized when heated in a porcelain dish over the Bunsen flame. DETECTION m 1 &' Ammonia. Place a small portion of the original substance, 1 whether it be in the solid form or in solution, in a small beaker and add NaOH. Cover the beaker with a watch glass on the underside of which is placed a moistened piece of turmeric paper or of red litmus paper. (If, in making this test, the student uses a paper which he himself has prepared by treating blue litmus paper with an acid, the paper should be thoroughly washed until it is free from acid.) Heat gently until the first bubble appears in the liquid, and then remove the beaker from the flame. If the turmeric paper is turned brown or the litmus paper blue, the presence of ammonia is proved. Sodium and potassium. Evaporate the filtrate from the ammo- nium carbonate group to dryness, and heat the solid residue in a porcelain dish directly over a Bunsen flame until white fumes are no longer given off. With a glass rod scrape into the 1 By original substance is meant the substance under examination before it has been subjected to chemical treatment. ANALYSIS 81 bottom of the evaporator any of the substance that may adhere to the sides of the dish, and heat again. Continue this heat- ing for ten minutes after white fumes have ceased to appear. Divide into two portions the dry residue thus obtained and test this substance for sodium and potassium in the manner described below (Portions 1 and 2). If the original substance is known to contain members of this group only and ammonia has been ,shown to be absent, the substance may be tested as directed below without previous heating. Portion i : Sodium (Potassium). Moisten a small portion of the residue with HC1, bring it upon a platinum wire, and introduce it into the Bunsen flame. A bright yellow flame proves the presence of sodium. A violet flame proves the presence of potassium. If sodium is present, observe the flame through a layer of blue glass thick enough to absorb the sodium rays. A violet flame proves the presence of potassium. Portion 2 : Potassium. Dissolve the remainder of the residu .5 in very little water. If the solution is alkaline, make slight iy acid with acetic acid. Add a few drops of sodium cobaltic nitrite, warm very gently, and allow to stand for a few minutes. A yellow precipitate proves the presence of potassium. DISCUSSION When testing the original substance with sodium hydroxide for ammonia, care should be taken that none of the sodium hydroxide comes in contact with either the turmeric paper or the red litmus paper. The flame test for sodium is so delicate and that element is so widely distributed that care must be exercised to distinguish the brilliant and permanent coloration due to a sodium com- pound in the mixture under analysis from the slight yellow 82 THE ALKALIES coloring of the flame due to the sodium chloride which is always present in the air of the room and in dust. If a large amount of sodium is present, it is not always easy to draw accurate conclusions from the flame test for potassium unless the blue glass is of exactly the proper shade. Unless such glass is used, more weight should be given to the test with sodium cobaltic nitrite. The filtrate from the ammonium carbonate group always contains ammonium salts because these have been introduced as reagents during the course of the analysis. These ammo- nium compounds will form with sodium cobaltic nitrite a pre- cipitate similar in appearance to that produced by potassium salts, and it is therefore necessary to remove them completely before testing for potassium. PART III THE ACIDS (Use solutions of the alkali metals in performing the preliminary reactions. In the directions for the detection of the acids reference will be made to solutions A, B, etc. The preparation of solutions thus designated is discussed on page 117.) 41 CHLORATES All chlorates are soluble in water. If a chlorate is introduced into a concentrated solution of sodium carbonate and that solu- tion is then boiled, transposition takes place and sodium chlorate is formed. When chlorates are highly heated they are decomposed, oxygen being set free and chlorides being formed. If a solution of a chlorate is acidified and is then colored blue by the addition of a drop of a solution of indigo, the subsequent addition of Na 2 SO 3 destroys the color. If a small crystal of a chlorate is placed in a dry test tube and a few drops of concentrated sulphuric acid are then added, a complicated reaction results and there is produced a greenish yellow gas, C1O 2 . This gas is very unstable and it explodes with considerable violence when the test tube is warmed over the Bunsen flame. This experiment should be carried on under the hood and the mouth of the tube should be directed away from the operator. DETECTION Acidify a small portion of solution A with H 2 SO 4 and test with indigo solution and Na 2 SO 3 . If the blue color is 83 84 THE ACIDS permanent, chlorates are absent. If the solution is bleached, they may be present, and a small portion of the original solid substance should be tested with concentrated H 2 SO 4 in the manner above described. DISCUSSION A number of substances other than chlorates will decolorize an indigo solution. The treatment of a solid substance with concentrated sul- phuric acid may give rise to reactions that furnish valuable information concerning the possible presence of substances other than chlorates. Some of these reactions are tabulated below. Product of Reaction Due to Recognized by C0 2 CO HCN S0 2 S0 2 H 2 S C10 2 a \ Br I N 2 5 HC1 HC 2 H 3 O 2 carbonate or oxalate oxalate or ferrocyanide cyanide, ferrocyanide, or ferri- cyanide sulphite, or strong reducing agent tartrate sulphide, or sulphite and strong reducing agent sulphide and oxidizing agent sulphite and reducing agent chlorate chloride and oxidizing agent chlorate and reducing agent bromide iodide nitrate chloride acetate lime water blue color of flame odor. Caution: This gas is very poisonous. odor odor of burnt sugar as well as of S0 2 ; liquid black- ened odor burns to S0 2 color; explosion color; odor color ; condenses to liquid color ; condenses to solid color odor odor CARBONATES 85 CARBONATES All normal carbonates except those of the alkalies are insoluble in water. The carbonates of arsenic, antimony, tin, chromium, and aluminum are rare or unknown. K 2 CO 3 is deliquescent. (NH 4 ) 2 CO 3 is volatile. Most carbonates are easily decomposed by hydrochloric acid with the evolution of C0 2 . If a drop of lime water in a loop tube 1 is exposed to C0 2 , it becomes turbid through the formation of CaC0 3 . DETECTION If carbonates have not already been detected in the analysis for the bases or in the preparation of solutions for the same, place a small portion of the original solid substance in a test tube, add a little HC1, and hold hi the upper part of the test tube a loop tube carrying a film of lime water. If the presence of carbon dioxide is not indicated, warm the contents of the test tube and, if the drop of lime water still remains clear, add a little HCI and again gently warm the liquid. DISCUSSION CO 2 in excess dissolves the precipitate that is at first produced in the film, probably forming an acid carbonate, CaH 2 (CO 3 ) 2 . The film should therefore be examined shortly after its exposure to the gas. 1 The loop tube is made from a piece of glass tubing 4 mm. in diameter by heating the tube about an inch from one end in the flame of the blast lamp, drawing it out quickly, and turning it back upon itself in the form of a shep- herd's crook, making a loop about 3 mm. in diameter. The tube is then cut off at the end of this crook. To fill the loop, hold the tube upright and pour upon the small encLthe reagent that is to be used'. This should be done in such a way as to force some of the liquid to rise in the main portion of the tube and to cause a film to form across the loop. 86 THE ACIDS Certain carbonates are not very readily attacked by cold dilute HC1. Among these are the carbonates of silver, lead, mercury (mercurous), copper, bismuth, and iron. These car- bonates are, however, attacked by warm dilute hydrochloric acid or by warm HCI. , Care must be taken as to the position of the loop in the test tube, especially if the tube is warmed or HCI is used, since hydrochloric acid carried up by effervescence or evolved as a gas dissolves the precipitate in the film or prevents its formation. A slight turbidity in the film is best observed against a dark background. SULPHIDES All normal sulphides are insoluble in water except those of the alkalies. By the action of HCI, hydrogen sulphide is readily evolved from the sulphides of the alkalies, alkaline earths, magnesium, zinc,Jmanganese, and iron; less readily from the sulphides of lead, msmuth, cadmium, antimony, tin, nickel, and cobalt; from other sulphides with difficulty or not at all. Most of the sulphides that are not readily attacked by HCI alone yield H 2 S when treated with zinc and hydrochloric acid. H 2 S blackens filter paper that has been moistened with lead ammonium acetate. By prolonged action of hot HNO% most sulphides are decom- posed, usually with the separation of sulphur and the formation of nitrates and sulphuric acid : PbS is partially oxidized to the sulphate, HgS is changed to Hg(NO 3 ) 2 -2 HgS, and the sul- phides of arsenic, antimony, and tin are changed to oxides or to H 3 AsO 4 , H 3 SbO 4 , and H 2 SnO 3 . Aqua regia decomposes the sulphides with the separation of sulphur and the formation of sulphuric acid. SULPHITES 87 When an insoluble sulphide is fused with a small piece of NaOH on a porcelain crucible cover, Na 2 S is formed. If this residue is dissolved in water and a drop of the solution is placed on a silver coin, a black stain of Ag 2 S appears. DETECTION If sulphides have not already been detected in the analysis for the bases, or in the preparation of solutions for the same, or in the tests for a chlorate or a carbonate, they may usually be detected as follows : add HC1 to a small portion of the original solid substance in a test tube, warm gently, and cover the mouth of the tube with a piece of filter paper that has been moistened with a drop of lead ammonium acetate solution. If the result of the test is negative and a colored residue remains hi the test tube, add a granule of zinc and repeat the test. DISCUSSION A few sulphides are not decomposed by hydrochloric acid even after the addition of zinc. Therefore, if the results of the above tests for a sulphide were negative and a colored residue still remains in the test tube, the residue should be separated by filtration, washed thoroughly, and dried. It is then fused with a small piece of NaOH on a porcelain crucible cover. The fused mass is allowed to cool, removed from the cover, placed upon a clean silver coin, and moistened with a drop of water. SULPHITES The sulphites of the alkalies are deliquescent and are the only sulphites that are easily soluble in water. All sulphites are transposed by boiling with Na 2 CO 3 . By the action of the air or of other oxidizing agents sulphites are oxidized to sulphates. 88 THE ACIDS All sulphites are decomposed by HC1 with the evolution of S0 2 . If a drop of a mixture of FeCl 3 and K 8 Fe(CN) 6 in a loop tube is exposed to SO 2 , the FeCl 3 is reduced to FeCl 2 which then, with the K 8 Fe(CN) 6 , forms Fe 3 [Fe(CN) 6 ] 2 which is blue. DETECTION If not already detected in the analysis for the bases or in the preparation of solutions for the same, or in the tests for a chlorate, a carbonate, or a sulphide, acidify a portion of solution A with HC1 and test the evolved gas as above directed. DISCUSSION FeCl 3 is also reduced by H 2 S. The evolution of this gas may be prevented as follows : if a sulphide is present, make a portion of solution A nearly neutral by the addition of HC1, then add a little HgCl 2 , and finally complete the acidification and test the evolved gas for SO 2 . CYANIDES The cyanides of the alkalies and alkaline earths, and mercuric cyanide, are soluble in water; most other cyanides are insoluble. All cyanides are transposed by boiling with Na 2 CO 3 . From a solution of a cyanide of an alkali metal, AgNO 3 precipitates AgCN, white ; insoluble in dilute HNO 3 ; soluble in KCN, forming AgCN-KCN ; soluble in NH 4 OH or (NH 4 ) 2 CO 3 , probably forming AgCN 2 NH 3 . AgCN is decomposed when heated to dull redness, metallic silver and cyanogen being formed. Hydrocyanic acid, HCN, a colorless and poisonous gas, is liberated when dilute sulphuric acid acts upon a cyanide of an CYANIDES 89 alkali. If a drop of (NH 4 ) 2 S X in a loop tube is exposed to this gas for a few minutes, NH 4 CNS is formed. After the excess of (NH 4 ) 2 S X has been expelled by evaporating the drop to dryness on a porcelain crucible cover, the addition of a drop of a dilute solution of FeCl 3 to the cooled residue produces the deep red Fe(CNS) 3 . When a solution of a cyanide of ag. alkali metal (KCN) is made strongly alkaline with KOH or NaOH, and a little FeSO 4 and a drop of FeCl 3 are then added, a precipitate is produced which consists of ferrous and ferric hydroxides. If the mixture is warmed, the .ferrous hydroxide is changed to Fe(CN) 2 which then combines with KCN to form K 4 Fe(CN) 6 . If now the mixture is acidified with HC1, the hydroxides of iron dissolve and the deep blue, insoluble ferric ferrocyanide appears. DETECTION If not already detected in the analysis for the bases, or in the preparation of solutions for the same, or in the test for a carbo- nate, add dilute H 2 SO 4 to a small portion of solution A and warm gently. Test the gas with (NH 4 ) 2 S X , etc., as described above. The loop tube should here be supported by thrusting its stem into a cork, and to relieve gas pressure in the tube the cork should have a shallow channel cut along its side. The film should be exposed to the action of the gas for ten minutes. If ferrocyanides and ferricyanides are absent, the ferric ferro- cyanide test for a cyanide should also be made, a small portion of solution A being used for the purpose. DISCUSSION In the sulphocyanate test the excess of (NH 4 ) 2 S X must be expelled by evaporation to prevent the formation of FeS when FeCl 3 is added. 90 THE ACIDS Unless the crucible cover is cooled before the addition of the drop of FeCl 3 , the latter will be evaporated to dryness by the hot cover and will leave a reddish residue that may mislead the student. PHOSPHATES The phosphates of the alkalies are soluble in water. Normal phosphates of the other metals are insoluble in water, but are readily soluble in dilute inorganic acids. Some phosphates are not readily transposed by boiling with Na 2 C0 3 . From a solution of an orthophosphate to which nitric acid has been added, ammonium molybdate, (NH 4 ) 2 Mo0 4 , precipitates ammonium phospho-molybdate, lemon yellow, pulverulent, adhering more or less to the walls of the test tube. The composition of the precipitate varies according to the conditions under which precipitation takes place. Pre- cipitation is best obtained by adding a small amount of the phosphate solution to a small test tube half full of ammonium molybdate and gently warming the mixture. Magnesium chloride that contains NH 4 C1 and NH 4 OH in excess precipitates MgNH 4 PO 4 from a solution of a phosphate. The precipitate is crystalline and does not appear immediately if the amount of phosphate present is small (see page 74). DETECTION (Unless phosphates are known to be absent, they should be tested for before the precipitation of the ammonium sulphide group in the analysis for the bases. If members of Division B of the hydrogen sulphide group are present, they must be removed by precipitation with H 2 S and nitration before the test for phosphates is made. A small por- tion of the nitrate is then evaporated to dryness and the residue thus obtained is used instead of the original substance in making the test.) SULPHATES 91 Place a small portion of the original solid substance in an evaporator, add HNO%, and evaporate to clryness. Add a little dilute HNO 3 , stir, and filter if the residue is not completely dis- solved. Add this solution to 5 cc. of (NH 4 ) 2 MoO 4 in a test tube, warm gently (not above 70), and if a precipitate does not appear, allow to stand for a few minutes. DISCUSSION The method of analysis of the ammonium sulphide group of the bases must be modified if phosphoric acid is present. It is therefore necessary to test for this acid before proceding with the analysis of that group. It is also necessary to remove members of Division B of the hydrogen sulphide group because arsenic acid gives a yellow precipitate with (NH 4 ) 2 MoO 4 . Evaporation with strong HNO 3 is necessary in order to transform salts of metaphosphoric acid, HPO 3 , and of pyro- phosphoric acid, H 4 P 2 O 7 , into orthophosphoric acid, H 3 PO 4 , which is the only acid of phosphorus that gives the test with (NH 4 ) 2 Mo0 4 . If the reagent solution of ammonium molybdate is too highly heated, a white precipitate of MoO 3 is produced. Ammonium molybdate gives a reddish brown precipitate with ferrocyanides. The evaporation to dryness with HNO%, if properly performed, will destroy ferrocyanides, or they may be precipitated by adding ZnSO 4 and BaCl 2 to a hot solution con- taining hydrochloric acid. The BaSO 4 is a heavy precipitate and carries down the zinc ferrocyanide. SULPHATES Most sulphates are readily soluble in water. The sulphates of silver, calcium, strontium, lead, and barium are slightly soluble in water, the solubility decreasing in the 92 THE ACIDS order given. Most basic sulphates are insoluble in water. Lead sulphate is soluble in ammonium acetate, forming lead ammonium acetate. If a sulphate is fused with a carbonate of an alkali metal, trans- position takes place and an alkali sulphate is formed; all sulphates except native BaSO 4 are transposed by boiling with Na 2 CO 3 . The sulphates of arsenic and antimony are rare and unstable. MnSO 4 is deliquescent. BaCl 2 precipitates BaS0 4 from a solution of a sulphate. The precipi- tate is white and is insoluble in HC1. DETECTION Make a portion of solution A strongly acid with dilute HC1, filter if not clear, reject the precipitate, and^to the warm 1 solu- tion add BaCl 2 in excess. DISCUSSION Dilute and not concentrated hydrochloric acid is used because the latter precipitates barium chloride from a solution of that salt of the concentration here employed (see list of reagents, CHROMATES Chromates of the alkalies and of magnesium and calcium are easily soluble in water ; strontium chromate is less soluble. All chromates are colored ; those most commonly met with are yellow or red. A water solution of a chromate is yellow, that of a dichromate reddish yellow. If a solution of a chromate is acidified a dichromate is formed, while if a solution of a dichro- mate is made alkaline a chromate is formed. Chromates are transposed by boiling with Na 2 CO 3 . Most reducing agents if added in excess to an acidified solution of a chromate reduce the latter to a dark green chromic salt, such as CrCl 3 , 1 See Introduction, page 15. CHROMATES 93 Cr 2 (S0 4 ) 3 , etc., according to the acid that was used in acidification, If the amount of acid is insufficient to form the chromium salt, a precipita- tion of Cr(OH) 3 may take place. A solution of a chromate will be reduced when treated as directed in a, 6, mixture assumes a deep red color. If the red, neutral solution is diluted and boiled, ferric hydroxide is precipitated and the supernatant liquid becomes colorless. 1 See page 66. IODIDES 99 DETECTION If an acetate has not already been detected in testing for chlorates, add a little concentrated H 2 SO 4 and a few drops of alcohol to the residue left on evaporating a portion of solu- tion C that has been rendered alkaline by the addition of sodium carbonate, or of solution A if it was found unnecessary to pre- pare solution C ; warm gently and note the odor. Confirm by the ferric acetate test. DISCUSSION Until the odor of ethyl acetate becomes familiar it is well to make a comparative test, using concentrated sulphuric acid and alcohol alone. In making the ferric acetate test it is desirable that the solu- tion should be rendered neutral before the ferric chloride is added. If the solution is acid, ferric acetate may not be formed ; if it is alkaline, ferric hydroxide will be precipitated. IODIDES Iodides are easily soluble in water with the exception of HgI 2 and the iodides of the hydrochloric acid group of metals. PbI 2 is very slightly soluble in cold water, but is readily soluble in hot water. All iodides with the exception of the insoluble iodides are easily transposed by boiling with Na 2 CO 3 . The latter are easily transposed upon fusion with dry Na 2 CO 3 . AgN0 3 precipitates Agl from a solution of an iodide acidified with HN0 3 . The precipitate is yellow, and is insoluble in cold NH 4 OH or (NH 4 ) 2 C0 3 . Agl is not decomposed when heated to dull redness; it is decomposed by zinc and sulphuric acid, forming zinc sul- phate, hydriodic acid, and metallic silver. If the reaction 100 THE ACIDS is so vigorous that the acid becomes hot, iodine may be liberated. Silver iodide is soluble in a solution of potassium cyanide. Oxidizing agents, such as K 2 Cr0 4 or chlorine water, liberate iodine from an acidified solution of an iodide. If carbon bisulphide is shaken with a solution containing free iodine, the latter is extracted and imparts a violet color to the CS 2 . Concentrated H 2 SO 4 or phenol-sulphonic acid liberates iodine from an iodide. DETECTION Cool a portion of solution B and acidify with HNO 3 . (Use solution A if it has been unnecessary to prepare solution B.) Add a few drops of K 2 CrO 4 and 2 cc. of CS 2 . Shake vigor- ously and note the color of the CS 2 . If there is no change, add a few more drops of K 2 CrO 4 and HNO 3 and shake again. DISCUSSION If in the preparation of solutions for the analysis of the bases a residue that is insoluble in water and acids is obtained, an iodide should be tested for as directed on page 113. BROMIDES Bromides are easily soluble in water with the exception of the bromides of the hydrochloric acid group of metals. All bromides except the insoluble bromides are readily trans- posed by boiling with Na 2 CO 3 . The insoluble bromides are easily transposed upon fusion with dry Na 2 CO 3 . AgN0 3 precipitates AgBr from a solution of a bromide acidified with HN0 3 . This precipitate is light yellow, but darkens when exposed to the light. It is soluble with difficulty in cold NH 4 OH ; insoluble in cold (NH 4 ) 2 C0 3 . AgBr is not decomposed when heated to dull redness; it is decomposed by zinc and sulphuric acid, forming zinc sulphate, FERKOCYANIDES 101 hydrobromic acid, and metallic silver. Silver bromide is solu- ble in a solution of potassium cyanide. HBr is less easily oxidized than Hi ; K 2 Cr0 4 , for example, does not set bromine free from a solution of a bromide slightly acidified with HN0 3 ; chlorine water does so, however. If CS 2 is shaken with a solution con- taining free bromine, the latter is extracted and imparts a yellow or red color to the CS 2 according to the amount of bromine present. Concentrated H 2 SO 4 or phenol-sulphonic acid liberates bro- mine from a bromide. DETECTION If iodides have been found to be absent, add to the contents of the test tube used in the test for an iodide a few drops of chlorine water and shake. If the CS 2 is not changed in color, add a few drops more of chlorine water and shake again. If an iodide has not been completely decomposed by K 2 CrO 4 , iodine will be liberated when chlorine water is added. There- fore, if iodides have been found to be present, filter through a wet filter the contents of the test tube used in the test for an iodide. With the filtrate repeat the test for an iodide until the iodide is completely decomposed, and the last filtrate when shaken with CS 2 shows no trace of iodine. Then test for bromides with chlorine water as directed in the preceding paragraph. DISCUSSION If in the preparation of solutions for the analysis of the bases a residue that is insoluble in water and acids is obtained, a bromide should be tested for as directed on page 113. FERROCYANIDES Ferrocyanides of the alkalies and alkaline earths are soluble in water ; most other f errocyanides are insoluble in water and in cold acids. The ferrocyanides of the alkalies are more insoluble in alcohol than the corresponding ferricyanides. 102 THE ACIDS Ferrocyanides are transposed by boiling with Na 9 CO 3 or NaOH. When potassium ferrocyanide, K 4 Fe(CN) 6 , is boiled with con- centrated H 2 SO 4 it is decomposed with the formation of CO, but HCN is formed if the acid is dilute. When heated to dull redness Ag 4 Fe(CN) 6 is decomposed with the formation of metallic silver. Ferrocyanides in acid solution are easily oxidized to ferri- cyanides by the action of oxidizing agents. When FeSO 4 is added to an acid solution of potassium ferro- cyanide there is formed potassium ferrous ferrocyanide. This is a whit& precipitate, but it is so easily oxidized that under ordinary conditions of preparation it has a light blue color due to the formation of a small amount of ferric ferrocyanide, Fe 4 [Fe(CN) 6 ] 8 . From an acid or neutral solution of a ferrocyanide FeCl 3 pre- cipitates Fe 4 [Fe(CN) 6 ] 3 , dark blue. DETECTION To a small portion of solution B add a few drops of FeCl 3 . A dark blue solution or precipitate indicates a ferrocyanide. If it has been unnecessary to prepare solution B, use a portion of solution A acidified with H^SO^ 1 DISCUSSION It should be borne in mind that a ferricyanide gives a deep blue precipitate with ferrous salts. If, therefore, a mixture contains a ferricyanide together with a substance that is capable of reducing ferric salts to the ferrous condition, a deep blue precipitate will be produced even in the absence of ferrocyanides. 1 For the detection of the bases when an insoluble ferrocyanide or ferri- cyanide is present see Fresenius' Qualitative Analysis, Wells's translation, page 529 (1897). FERKICYANIDES 103 FERRICYANIDES Ferricyanides of the alkalies and alkaline earths are .soluble in water ; most other f erricyanides are insoluble in w^r and in cold acids. Ferricyanides of the alkalies are rnj#f% soluble in alcohol than the corresponding ferrocyanides. Ferricyanides are transposed by boUing with Na 2 CO 3 or NaOH. When potassium ferricyanide, K 3 Fe(CN) 6 , is boiled with concentrated H 2 SO 4 it is decomposed with the formation of CO, but HCN is formed if the acid is dilute. When heated to redness Ag 3 Fe(CN) 6 is decomposed with the formation of metallic silver. Ferricyanides in alkaline solution are easily reduced to ferro- cyanides by reducing agents. From an acid solution of a ferricyanide FeSO 4 precipitates ferrous ferricyanide, Fe 3 [Fe(CN) 6 ] 2 , dark blue. FeCl 3 colors a solution of a ferricyanide dark brown or yel- low according to the degree of dilution. DETECTION Add a freshly prepared solution 1 of FeSO 4 to a portion of solution B. A dark blue solution or precipitate indicates a ferricyanide. (If it has been found unnecessary to prepare solution B, use a portion of solution A acidified with H 2 SO 4 .) 2 DISCUSSION If a ferrocyanide is present, make the solution to be tested neutral, concentrate to one-third its volume, add a relatively large quantity of alcohol, and allow the mixture to stand for half an hour. Filter and test the filtrate as directed in the preceding paragraph. 1 See page 56. 2 See note, page 102. 104 THE ACIDS CHLORIDES All chlorides are soluble in water, except the chlorides of the hydrochloric acid group of metals arid a few basic chlorides, such as BiOCl. Lead chloride is somewhat soluble in cold water and is easily soluble in hot water. The chlorides of mercury, silver, lead, cadmium^ cobalt, barium, strontium, potassium, sodium, and ammonium are not deliquescent; with the exception of a few basic chlorides, most other chlorides are deliquescent. All chlorides are readily transposed by boiling with Na 2 CO 3 , with the exception of the insoluble chlorides. The latter are easily transposed by fusion with dry Na 2 CO 3 . AgN0 3 precipitates AgCl immediately from a solution of a chloride acidified with nitric acid. This precipitate is curdy, white, changing to lavender and finally to black on exposure to the light ; soluble in NH 4 OH or (NH 4 ) 2 C0 3 , probably forming AgCl-2NH 3 . When this ammoniacal solution is acidified with nitric acid AgCl is precipitated. Silver chloride is not decomposed when heated to dull red- ness ; it is decomposed by zinc and sulphuric acid, forming zinc sulphate, hydrochloric acid, and metallic silver. Silver chloride is soluble in a solution of potassium cyanide. DETECTION (If a white precipitate was formed in the test for the silver nitrate group of acids, and iodides, bromides, cyanides, and ferrocyanides have been found to be absent, a chloride is conclusively proved to be present, and no further test is necessary.) To a portion of solution B add HNO 3 , and then AgNO 3 , filter, and/, wash with water several times by decantation. (If it ha^-been found unnecessary to prepare solution B, use a portion of solution A that has been strongly acidified with HNO 3 .) CHLORIDES 105 a. If iodides are present and bromides are absent, digest the precipitate for a few minutes with NH 4 OH without warming filter, and acidify the filtrate with HNO 3 . A white precipitate proves the presence of a chloride. b. If a bromide is present, whether accompanied by an iodide or not, digest the precipitate for a few minutes with a solution 1 of (NH 4 ) 2 CO 3 without warming. Filter and acidify the filtrate with HNO 3 . A white precipitate proves the pres- ence of a chloride. c. If a cyanide, ferrocyanide, or ferricyanide is present, dry the precipitate in a porcelain crucible and heat to dull redness. When cool add a piece of zinc and a little H 2 SO 4 , and allow the reduction to proceed for at least half an hour, adding more zinc and sulphuric acid if necessary. Filter and to the filtrate add a few drops of HNO 3 , and then AgNO 3 . Filter and test the precipitate as directed above in a or b. DISCUSSION If in the preparation of solutions for the analysis of the bases a residue that is insoluble in water and acids is obtained, a chloride should be tested for as directed on page 113. When the treatment described under c is carried out, and iodides and bromides have been found to be absent, the produc- tion of a precipitate on the addition of AgNO 3 proves that a chloride is present. \ The separation of iodides, bromides, and chlorides by the dif- |erent solubilities of their silver salts is not altogether sharp, if the digestion takes place in the cold. In a and 6, there- fore, something more than a faint turbidity should be produced upon acidification of the filtrate before the presence of a chloride is considered tp be conclusively proved. J The shelf reagent contains free NH 4 OH which must be changed to the carbonate by passing a rapid current of carbon dioxide through the solution for ten minutes before using the reagent for the above test. 106 THE ACIDS A more delicate method 1 for the detection of iodides, bromides, and chlorides in the presence of one another consists in first oxidizing the iodide with ferric sulphate and removing the iodine by boiling, then oxidizing the bromide with potassium permanganate and removing the bromine by boiling, and finally testing for a chloride in the residual solution. NITRATES All nitrates are soluble in water, except a few basic nitrates, such as BiONOg. The nitrates of silver, lead, barium, strontium, potassium, sodium, and ammonium are not deliquescent; with the excep- tion of a few basic nitrates, most other nitrates are deliquescent. All nitrates are transposed by boiling with Na 2 CO 3 , the insoluble basic nitrates less readily than the others. When a little concentrated HN0 3 is added to a solution of FeS0 4 the ferrous salt is oxidized, and NO is formed. This last-named substance unites with the unchanged ferrous sulphate to form a brown unstable compound (FeS0 4 ) 2 NO. In testing for a nitrate, concentrated nitric acid results from the action of strong sulphuric acid upon th? nitrate-s^jFhe test is performed as described below. When a few drops of a solution of a nitrate are evaporated to dryness on a porcelain crucible lid and the cool residue is treated with a few drops of phenol-sulphonic acid and then gently warmed, picric acid (?) is formed. If NH 4 OH is now addf. t# the cooled solution,^ deep yellow ammonium picrate is formed. Jr BKECT ION i Ferrous sulphate test. Place a small portion of solution C in a test tubej adttW c$i of concentrated H 2 SO 4 to set free nitric acid .from any nitrate that may be present, and cool the test tube. Hold 'the test tube in a slanting position, and pour in carefully s^e^ral cubic centimeters of FeSO 4 and gently tap the fsolut to dryness on a crucible lid and test with j -V" >honicionC in the manner described above. (If soluti i it sho-cid be made alkaline by the addition of sodi .ate befuld being evaporated to dryness. Use solutio L was founcf unnecessary to prepare solution C.) DISCUSSION Ferrous sulphate test. The contents of the test tube is cooled after the addition of the concentrated sulphuric acid to the solu- tion to be tested because the brown substance, (FeSO 4 ) 2 NO, is easily decomposed by heat. The ferrous sulphate test can be used only when iodides, bromides, ferrocyanides, ferricyanides, chromates, permanganates, and chlorates are not present in the substance under examination, or have been removed. If these substances are present, they interfere as follows : Iodides and bromides are oxidized by the concentrated sul- phuric acid with the liberation of iodine and bromine. These elements may be removed by adding silver acetate or silver sulphate to the solution and filtering off the precipitated silver salts. The high cost of the reagents, however, makes it pref- erable to use the phenol-sulphonic acid test when iodides or bromides are present. Ferrocyanides and ferricyanides form blue precipitates with the ferrous sulphate. Before the test is made these compounds may be removed by adding sulphuric acid to the solution to be tested until the 'latter is slightly acid, and then adding ferric chloride and ferrous sulphate. The solution is then heated to boiling, barium chloride is added, the solution is filtered, and the nitrate is tested for a nitrate. The barium chloride is 108 THE ACIDS added to form barium sulphate, a heavy precipitate which will carry down the ferrocyanides and ferricyanides. Chromates are reduced by ferrous sulphate to chromium sul- phate, which will appear as a green layer at the place where the brown ring should form, and will therefore obscure the test for a nitrate. Chromates are reduced, and chromium is removed, in the preparation of solution C. Permanganates interfere because of their color. These also are removed in the preparation of solution C. Chlorates interfere because of the violent reaction that takes place when they are treated with concentrated sulphuric acid. These also are reduced in the preparation of solution C. Phenol-sulphonic acid test. If ferrocyanides, ferricyanides, chromates, permanganates, and chlorates are not present in the original substance or have been removed as directed above, this test may be used even when an iodide or bromide is present. PAET IV SYSTEMATIC ANALYSIS OF A SOLID SUBSTANCE The analyst should first carefully observe the physical prop- erties of the substance, noting whether or not it is crystalline, homogeneous, deliquescent, or efflorescent, and whether or not it possesses the characteristics of a metal. The color of the substance and, if it is clearly a mixture, the colors of the differ- ent ingredients should be observed by spreading the substance upon a sheet of white paper and breaking up the particles by gentle pressure with a spatula. Whether the substance uno^er examination contains a ferrous or a ferric compound, an arsenious or an arsenic compound, etc., must be ascertained by special tests before the material has been subjected to the action of either an oxidizing or a reducing agent. I. PREPARATION OF SOLUTIONS FOR THE DETECTION OF THE BASES A. THE SUBSTANCE is NOT A METAL In the analysis of a solid that may be a mixture of several substances the attempt is first made to separate these by treating the solid successively with various solvents. The usual order in which the solvents are applied is : (1) hot water, (2) hydrochloric acid, (3) nitric acid, and (4) aqua regia. That portion of the substance which is soluble in hot water is first entirely removed by that solvent, then that portion of the residue which is soluble in hydrochloric acid is removed, etc. 109 110 ANALYSIS OF A SOLID SUBSTANCE It is desirable to bring about as complete separation as is possible by means of these solvents. The residue that is left undissolved after treatment with any solvent is washed thor- oughly with water, and the first washings are added to the filtrate. If there is doubt as to whether any of the substance has been dissolved by a solvent, a little of the filtrate is evaporated to dryness in a porcelain dish. If no residue remains, the solution may be discarded. (A very slight, impalpable, brown residue is usually due to dust and may be disregarded.) If solution takes place but slowly, as is the case when certain oxides are treated with acids, it is well to determine when the solvent action is practically complete. This is done by treating the washed residue a second time with the same solvent, filter- ing, and evaporating a small portion of the filtrate to dryness. If an appreciable amount of residue remains, the treatment with the solvent should be repeated. Before the substance is treated with any of the solvents it should be thoroughly mixed, and about a gram of it should be finely pulverized in a small mortar unless grinding produces an explosion. (This should be ascertained in the beginning by grinding a very small portion of the substance in a mortar.) WATER SOLUTION If any of the substance has dissolved in water, test the solu- tion with litmus paper. a. If it is neutral or acid, proceed to the analysis for the bases (page 21). b. If it is alkaline, make slightly acid with HNO q . If there is formed a precipitate that is soluble in HNO 3 (test a small portion), dissolve it by the further addition of that acid. If the precipitate does not dissolve, filter, wash it with water, and treat it as if it were a solid substance presented for analysis and found PBEPABATION OF SOLUTIONS 111 to be insoluble in water. The solution acidified with HNO 3 , or the nitrate from the precipitate, is analyzed for the bases (page 21). HYDROCHLORIC ACID SOLUTION The residue that may remain after extracting all of that por- tion of the substance which is soluble in water is treated with dilute HC1, the acid being heated to boiling if that is necessary to effect solution. If the residue seems to have undergone no change, add HCI and boil until the solvent has no further action upon the residue. This treatment may give indications as to the presence of certain substances, e.g. : CO 2 from a carbonate, or from an oxalate and an oxidizing agent, HCN from a cyanide, SO 2 from a sulphite, H 2 S from a sulphide, or from a sulphite and a strong redu- cing agent, Sulphur from a sulphide and an oxidizing agent, or from a sulphite and a reducing agent, Chlorine from an oxidizing agent, Bromine from a bromide and an oxidizing agent, Iodine from an iodide and an oxidizing agent, Oxides of nitrogen from a nitrate and a reducing agent. If the treatment with hydrochloric acid seems to form a chloride of one of the members of the hydrochloric acid group of metals, 1 1 When hot HCI is used as a solvent, the substance is sometimes transformed entirely into white needle-like crystals. These should be removed and dissolved in hot water and a small portion of the solution then tested for lead. If lead is found to be present, it is well to remove it at this point as completely as possible. This may be done by evaporating the HCI solution to very small bulk, cooling it, and filtering off the crystals of lead chloride. The operation should be repeated seVeral times, and the filtrate finally obtained should be treated as directed on page 38. The product of the different crystallizations should be united, dis- solved in water, and the water solution tested for the bases, especially for mercuric mercury. 112 ANALYSIS OF A SOLID SUBSTANCE discard the mixture, and treat a fresh portion of the original substance with the solvents in the following order: (1) hot water, (2) nitric acid, (3) hydrochloric acid, etc. The hydrochloric acid solution can contain no member of the hydrochloric acid group of metals excepting lead unless concen- trated acid was used. Concentrated hydrochloric acid, however, dissolves silver chloride to some extent and any of this substance that may be present in solution should be precipitated as com- pletely as possible by diluting the solution with three times its volume of water. After the precipitate has been removed by filtration and tested for silver, the analysis should be proceeded with as directed on page 38. If the hydrochloric acid group of metals is absent, treat the solution as directed on page 38. NITRIC ACID SOLUTION Analyze the solution as directed on page 21, first diluting it with water in case HNO% has been used. NOTE. If there is present a sulphide that is not readily acted on by hydrochloric acid, it will often cause the separation of free sulphur in the preparation of the nitric acid or aqua regia solution. AQUA REGIA SOLUTION Boil to expel chlorine, and precipitate the hydrochloric acid group of metals as completely as possible by diluting the solu- tion with two or three times its volume of water ; then proceed with the filtrate as directed on page 38. RESIDUE INSOLUBLE IN WATER AND ACIDS This may contain sulphur, AgCl, AgBr, Agl, PbSO 4 , BaSO 4 (SrSO 4 , CaSO 4 ), certain oxides, SiO 2 , silicates, and some salts of hydroferrocyanic and hydroferricyanic acids. PREPARATION OF SOLUTIONS 113 Sulphur. If this substance is set free from a compound of the element, it should be separated by filtration and dried. It may then be recognized either by burning it to SO 2 or by con- verting it into sodium sulphide and causing this to act upon metallic silver (see page 87). Hal ides of silver; sulphates of lead, barium (strontium, calcium); stannic oxide. Treat the insoluble residue with a concentrated solution of ammonium acetate containing a little free acetic acid, boiling the mixture vigorously for a few minutes. Filter if complete solution does not result, and test the filtrate for lead and sulphuric acid. If an insoluble residue still remains after treatment with ammonium acetate solution, wash it thoroughly with hot water until the wash water is not darkened by the addi- tion of ammonium sulphide. Divide the residue into two portions. Place one portion in a porcelain crucible, add metallic zinc and dilute sulphuric acid, and allow the action to continue for at least fifteen minutes. The halides of silver will be reduced by this treatment, the silver separating in metallic form and the halogen acid being set free. Filter and test the residue for silver, and the filtrate for HI, HBr, and HC1. 1 Warm the other portion of the residue insoluble in ammo- nium acetate with a solution of potassium cyanide. This treat- ment will dissolve the halides of silver. Filter. (Do not precipitate the halides of silver from the filtrate by adding an excess of an acid, because HCN, a very poisonous gas, would be set free.) 1 It must be borne in mind that when silver bromide and silver iodide are boiled with aqua regia they are changed partly or completely to silver chloride. If, therefore, insoluble halides of silver are found in the insoluble residue, a fresh portion of the original substance should be extracted first with water and then with cold dilute nitric acid. The residue thus obtained should then be treated with zinc and sulphuric acid and tested for the halogen acids as above described. 114 ANALYSIS OF A SOLID SUBSTANCE A residue that now remains after these treatments of the insoluble residue with ammonium acetate and potassium cyanide may contain sulphates of the alkaline earths and stannic oxide. There may still be present traces of lead sulphate and the halides of silver. These last-mentioned substances must be entirely removed before proceeding with the analysis. To ascertain whether this removal has been complete, place a very small portion of the residue upon a porcelain crucible lid and treat it with a drop of ammonium sulphide. If it is darkened, extract the remainder of the residue once more with ammonium acetate and test a particle of the residue again with ammonium sulphide. If it is still blackened, treat the residue with potas- sium cyanide and continue these alternate treatments until ammonium sulphide has no action upon the residue. Now dry the residue and pulverize it in a mortar. Mix one-half with six times its bulk of a mixture of equal parts of Na 2 CO 3 and K 3 CO 3 . Transfer this mixture to a platinum crucible and fuse it over a blast lamp for twenty minutes, or until the mass is in quiet fusion. The sulphates of barium, strontium, and calcium are by this treatment converted into carbonates, and soluble sulphates of potassium and sodium are formed. The fused mass is allowed to cool and is then extracted with hot water, the solution filtered, and the filtrate tested for a sulphate. The portion remaining upon the filter is thoroughly washed with hot water, and is then extracted with hydrochloric acid and the solution tested for the alkaline earths. The other half of the dry pulverized residue is mixed with six times its bulk of a mixture of equal parts of anhydrous sodium carbonate and sulphur. This mixture is placed in a small porcelain crucible, the crucible is covered, and it is heated for some time over a low flame, until the excess of sulphur has been distilled off and burned. The crucible is then allowed to cool and the substance is treated with warm water. The solu- tion is then filtered, aim the filtrate, which contains sodium PREPARATION OF SOLUTIONS 115 sulpliostannate, Na 2 SnS 3 , if tin was present in the insoluble resi- due, is acidulated with hydrochloric acid. This will precipitate stannic sulphide, which should be identified by further tests. Silicates. Salts of silicic acids are decomposed when treated with concentrated hydrofluoric acid, the silicon uniting with fluorine to form silicon tetrafluoride, SiF 4 , which is a gas. This gas is decomposed 'by water with the formation of insoluble silicic acid, H 4 SiO 4 , and soluble fluosilicic acid, H 2 SiF 6 : 3 SiF 4 + 4 H 2 = H 4 Si0 4 + 2 H 2 SiF 6 . If a silicate is present in the substance under examination, there will appear in the water minute white particles of silicic acid; and if the water is then carefully evaporated to dryness on a piece of clean platinum foil, a white residue will remain. If this is then highly heated with the Bunsen burner, the silicic acid will be changed to white silicon dioxide, SiO 2 , which will not be volatilized by the heating. To ascertain by the aid of the above-described reactions whether a substance contains silica or a silicate, proceed as follows : Place about 1 cc. of concentrated sulphuric acid in a clean, dry platinum crucible and add carefully about one-fourth of a gram of the substance to be tested. Warm the contents of the crucible gently and allow it to stand until effervescence ceases. Allow the contents of the crucible to cool and add 1 cc. of pure hydrofluoric acid. Cover the crucible with its platinum cover in such a manner that an opening about 3 mm. wide is left on one side, and hold at this opening a platinum wire loop containing a drop of distilled water. Warm the contents of the crucible gently with a very low Bunsen flame. Observe whether white particles of silicic acid appear in the drop of water, and then carefully evaporate the drop to dryness on a piece of clean platinum foil or on the crucible cover. If there remains a white residue that is not volatilized upon high heat- ing, silica was present in the original substance. 116 ANALYSIS OF A SOLID SUBSTANCE Some of the salts of silicic acid are decomposed by strong acids, others are not. When a silicate is broken down by an acid the bases in the silicate unite with the acid to form salts which usually are soluble in water, while the silicic acid is set free either as a white gelatinous substance or as a gritty powder. Silicates that are not attacked by strong acids may be decomposed by fusing the finely powdered substance with four times its weight of a mixture of equal parts of sodium carbonate and potassium carbonate, allowing the mass to cool, and then treating it with an excess of hot hydrochloric acid. The silicic acid set free by either of these treatments is not entirely insoluble in water. Since its presence in solution might interfere with tests for some of the bases, it should be completely removed before the analysis for the bases is under- taken. This is done by placing the hydrochloric acid solution in an evaporating dish and evaporating it to dryness on a water bath, continuing the heating until the mass is dry and fumes' of hydrochloric acid cease to be given off. The dry residue is now moistened with HGI and is warmed for a few moments. Boiling water is then added, the whole is heated for a short time, the insoluble SiO 2 is allowed to settle and is removed by nitration. The nitrate is then analyzed for the bases in the usual manner. The insoluble residue may contain substances other than silica. This is ascertained by tearing off the point of the filter containing the precipitate, dipping it into a saturated solution of ammonium nitrate, placing it in a platinum crucible, and heating it over the flame until the paper is entirely consumed. Hydrofluoric acid is then added to the residue ; the crucible is placed in the hood and is gently heated until the liquid is driven off. Two or three repetitions of this treatment will completely remove all of the silica, and the residue that still remains should be examined as directed on page 112. If it is desired to detect the alkalies that are present in the silicate, this may be done by decomposing the substance by means PREPARATION OF SOLUTIONS 117 of ammonium chloride and calcium carbonate 1 ; but if only a qualitative analysis of the material is desired, the alkalies may be detected with greater ease and accuracy by means of the spectroscope. Ferrocyanides and Ferricyanides. For the detection of the bases in insoluble ferrocyanides and ferricyanides, the student is referred to Fresenius' Qualitative Analysis, Wells's transla- tion, page 529 (1897). B. THE SUBSTANCE is A METAL OR AN ALLOY Boil 1 gram of the substance with 20 cc. HNO% until red fumes are no longer abundantly given off and the action ceases. Dilute, filter, and analyze the filtrate as directed on page 21. Wash the residue thoroughly and boil with HCI. If a residue still remains, fuse it with a mixture of Na 2 CO 3 and sulphur as directed on page 114. The metals considered in this manual are acted upon by HNO% and are changed by that reagent into soluble compounds, with the exception of tin and antimony, which are changed to oxides. The oxide of tin is insoluble in HNO& and the oxide of antimony but slightly so. II. PREPARATION OF SOLUTIONS FOR THE DETECTION OF THE ACIDS In the analysis of a solution for the bases the different groups of metals are precipitated by the successive addition of various reagents. This mode of procedure is not followed in the analy- sis for the acids. It is true that the acids are classified into groups according to the behavior of solutions of their salts towards certain reagents, but these reagents are used solely for 1 Method of J. Lawrence Smith, Crookes' Select Methods in Chemical Analy- sis, page 26 (1894). 118 ANALYSIS OF A SOLID SUBSTANCE the purpose of determining in one operation whether any mem- bers of a group are present in the solution. If the addition of such a group reagent to the solution shows that all the members of this group are absent, no further tests need be made in this solution for the acids of this group. If, however, the group reagent shows that some members of the group are present, then special tests for the individual members of this group must be made. These tests for the different acids are performed with sepa- rate portions of the substance under examination or of solutions prepared from it. Such a method of procedure renders it possible that the test for one acid may be interfered with by the presence of other acids if the various acids are not successively removed, as is the case in the analysis for the bases. It is impossible to prepare a single solution with which tests for all of the acids may be performed. If the substance under examination is completely soluble in water or in some one acid, this solution may be often used in the tests for the detection of all of the acids except the solvent and those acids that are decomposed by it. It is usually preferable, however, to remove the bases that are present, and for this reason the original sub- stance is boiled with a solution of sodium carbonate. Most salts of the metals are transposed by this treatment, the bases, with the exception of alkalies, being converted into insoluble carbonates, and the acids with which the bases were originally united being changed to soluble sodium salts. But certain salts are not transposed by this treatment, and for this reason it is necessary to test for some acids either in a solution prepared in a different manner or in the original substance. It is apparent that the original substance must be used in testing for carbon- ates, and it may also conveniently be employed in testing for those acids whose salts liberate gases when they are treated with dilute acids. PREPARATION OF SOLUTIONS 119 In the analysis for the acids the student should not proceed blindly to the preparation of the sodium carbonate solution, but should carefully consider the results of his analysis for the bases and should draw conclusions as to what acids may be present. The intelligent student should then be able to decide whether or not it is possible to detect the acids without preparing the sodium carbonate solution. SOLUTION A Pulverize about a gram of the well-mixed substance in a mortar, unless grinding produces an explosion. Treat this with 15 cc. of a saturated solution of Na 2 CO 3 and boil for at least fifteen minutes, unless complete solution is effected in a shorter time. Add water from time to time to replace that which evaporates. Finally add 10 cc. of water and filter if a residue remains, rejecting the residue. Mark the filtrate " Solution A." If solution A is colored purple by a permanganate, it may be decolorized by adding crystallized oxalic acid to the boiling solu- tion. Filter and use the filtrate where solution A is prescribed. SOLUTION B If the acidification of solution A produces a yellow precipi- tate, make a portion of solution A acid with dilute H 2 SO 4 , filter, reject the precipitate, and boil the filtrate until H 2 S has been entirely expelled. Mark this " Solution B." SOLUTION C If a chlorate, chromate, or permanganate is present, place some Na 2 SO 3 in a porcelain dish, introduce a small portion of the original substance, and add a little dilute HC1. Warm gently toward the end of the reaction. After the reduction has been completed make sure that all of the sulphite has been decomposed and that SO 2 has Jbeen expelled from the 120 ANALYSIS OF A SOLID SUBSTANCE solution. Filter if a residue remains, and reject the residue. If the nitrate is colored, add powdered Na 2 CO 3 until effer- vescence ceases, boil for a few minutes, dilute with water, and filter. Mark this " Solution C." PRELIMINARY TESTS WITH SOLUTION A 1. Barium Chloride Group. Acids that are precipitated from neutral solutions by barium chloride : (H 2 CO 3 ), (H 2 SO 3 ),H 2 SO 4 , H 2 CrO 4 , H 3 PO 4 , H 3 BO 3 , H 2 C 2 O 4 , H 2 C 4 H 4 O 6 , H 3 AsO 3 , H 3 AsO 4 . Place a small portion of solution A in a flask and add HC1, drop by drop, until effervescence ceases and the solution is but slightly acid. Heat the solution to boiling, filter if it is not clear, cool the filtrate, and then add a dilute solution of NH 4 OH, drop by drop, until the liquid is slightly alkaline. Then add BaCl 2 and CaCl 2 . A precipitate appears : A member of the BaCl 2 group of the acids is present. No precipitate appears : a. If no residue was left in the preparation of solution A with Na 2 CO 3 , members of the BaCl 2 group (except H 2 CO 3 and H 2 SO 3 ) are not contained in the substance under exami- nation. . b. If a residue was left in the preparation of solution A with Na 2 CO 3 , members of the BaCl 2 group (except H 2 CO 3 and H 2 SO 3 ) are not present in solution A ; these may, however, be present in the original substance, since certain salts are not easily transposed by Na 2 CO 3 . CaCl 2 is used in addition to BaCl 2 , since certain calcium salts of the acids of this group, such as the oxalate and the tartrate, are less soluble than the corresponding barium salts. If boric acid is present in but small quantity, it may not be precipitated by BaCl 2 and CaCl 2 . PKEPAKATION OF SOLUTIONS 121 The presence of a large quantity of ammonium salts may prevent the complete precipitation of some of the members of this group. 2. Silver Niirate Group. Acids that are precipitated fromjiitric acid solution by silver nitrate : (H 2 S), HI, HBr, (HCN),H 4 Fe(CN) 6 , H 3 Fe(CN) 6 , HC1. To a portion of solution B add 5 cc. of HNO 3 and then AgNO 3 in excess. If there is no precipitate, members of the silver nitrate group of acids (except H 2 S and HCN) are absent. If it was found unnecessary to prepare solution B, use solution A , in this test, making it acid with HNO 3 , and warming it, to expel H 2 S if a sulphide is present, before the AgNO 3 is added. If a precipitate is formed, note its color ; filter, place in a test tube the filtrate, or the solution in which AgNO 3 failed to produce a precipitate, and add a dilute solution of NH 4 OH, holding the tube in a slanting position. If a precipitate forms where the two layers of liquid meet, its color may indicate the presence of certain members of the BaCl 2 group of acids. Silver salts are white with the following exceptions: phos- j)hate, arsenite, iojlide, yellow ; bromide, f errocyanide, pale yellow ; arsenate, light brown ; chromate, dark red ; ferricyanide, orange ; sulphide, black. It must be remembered that in the preparation of solution B, or in the special preparation of solution A for this test, H 2 S and HCN have been expelled. If but a faint turbidity is produced on the addition of AgNO 3 , it is usually due to a trace of a chloride. In such a case the other members of this group need not be tested for. The presence of a large quantity of ammonium salts may prevent the precipitation of traces of the members of the silver nitrate group of acids. 122 ANALYSIS OF A SOLID SUBSTANCE Silver oxalate and silver chromate may be precipitated by silver nitrate in the test for the silver nitrate group of acids. These precipitates will dissolve, however, when more nitric acid is added and the mixture is warmed. After these preliminary tests have been performed, special tests should be made for such acids as have not here been shown to be absent. III. INTERPRETATION OF RESULTS All of the data, both physical and chemical, that have been secured during the examination of the substance are now to be utilized in drawing conclusions as to its composition. The fol- lowing examples are introduced to illustrate the method of reasoning that may be followed in the interpretation of the results of the analysis. 1. If but one base and but one acid have been found, as, for example, cadmium and carbonic acid, the chemical evidence proves that the substance contains cadmium carbonate ; but it does not prove that the substance is cadmium carbonate alone, for there may also be present metallic cadmium, cadmium oxide, or cadmium hydroxide. If metallic cadmium is present, the silver-white color of the metal will be observable. If cadmium oxide is present, its yellow color will render it apparent. If, however, the substance is to all appearances a homogeneous white powder, we may conclude that it is either cadmium car- bonate alone or a mixture of cadmium carbonate and cadmium hydroxide. To decide which is the case, a quantitative analysis would be necessary, since we have no simple means of detecting the presence of hydroxyl groups. 2. The successive treatment of the substance with different solvents often throws light upon its composition. Suppose, for example, that zinc, sodium, and sulphuric acid have been found in the analysis of a white mixture. If both INTERPRETATION OF RESULTS 123 zinc and sodium were found in the water solution, we may conclude that the substance is a mixture of zinc sulphate and sodium sulphate ; but if sodium was the only base in the water solution, and if the zinc was found only in the acid solution, it is evident that the zinc is present in the form either of the oxide or of the hydroxide. Suppose, again, that zinc and cadmium, and sulphuric and carbonic acids, have been found in the analysis of a white mix- ture. If the cadmium was found in the water solution, and the zinc in the acid solution, the substance evidently contains cad- mium sulphate and zinc carbonate (oxide, hydroxide) ; but if the zinc was found in the water solution and the cadmium in the acid solution, the mixture contains zinc sulphate and cad- mium carbonate (hydroxide). If zinc and cadmium were found both in the water solution and in the acid solution, the mixture consists of zinc sulphate, cadmium sulphate, zinc carbonate, and cadmium carbonate (zinc oxide, zinc hydroxide, cadmium hydroxide). 3. In some cases it is difficult to determine how the acids and bases are combined. If, for example, a mixture contains two such salts as sodium sulphate and ammonium chloride, the qualitative analysis will fail to show whether these compounds are present or whether the mixture contains ammonium sulphate and sodium chloride, unless the chemist is able to pick out crystals of the different substances and to test them separately for both base and acid. In a case of this nature it is customary to report the acids and bases as combined to form such salts as are commonly met with in analytical practice. 4. If a substance contains compounds that interact when an attempt is made to dissolve the mixture, the problem becomes more complex. For example, when water is added to a mixture of sodium sulphate and barium chloride, these substances inter- act to form sodium chloride and barium sulphate. In such a case it is sometimes possible to ascertain the composition of the 124 ANALYSIS OF A SOLID SUBSTANCE original mixture by mechanically separating the ingredients and analyzing each. If, however, the substance is in the form of a finely ground powder, it is difficult to do this and it is con- sequently almost impossible to ascertain by ordinary means how the bases and acids were combined in the original substance, unless a product of the interaction differs in appearance from the original mixture. APPENDIX LIST OF APPARATUS The student should find the following set of apparatus in his desk. Any deficiency should be reported at the first laboratory practice to the instructor; otherwise it will not be made good. At the end of each term and at the end of his course the student must clean all apparatus, obtain from the storeroom anything lacking from the set, and then submit his desk for inspection. Glass : Beaker : 50 cc., one Bottles (glass-stoppered) : 60 cc., for AgN0 8 , one 250 cc., for alcohol, one Bottle (small, cork-stoppered vial), containing 0.5 gram AgN0 8 in crystals Delivery tube (drawn out at one end) : one Flasks (round, flat bottomed) : 100 cc. for hydrogen generator (with rubber stopper, funnel tube, delivery tube, rubber connec- tion, and platinum foil), one 250 cc. for wash bottle (with rubber stopper and two glass tubes), one 60 cc., one Flasks (Erlenmeyer, conical): 60 cc., two 100 cc., two 300 cc., for H 2 S precipitation (with one-hole rubber stopper, bent tube, and rubber tubing, as described on page 38) : one 125 126 APPENDIX Funnels : 60 mm., three Loop tube (with supporting cork) : (see page 85), one Plate : 10 x 10 cm., one Reduction tubes (of difficultly fusible glass, with bulb on end, for fusion of compounds of arsenic, etc.) : three Rods : 10 cm., two Test tubes : 15 cm., twelve 10 cm., six Watch glass : 55 mm. diameter, one Platinum wire : one Porcelain : Crucible (30 mm. diameter, with cover) : one Evaporator (70 mm. diameter) : one Mortar and pestle : one Bunsen burner (with 60 cm. of rubber tubing) : one Forceps : one Horn spatula (10 cm. long) : one Iron stand, with two rings : one Iron wire gauze : one Iron wire triangle : one Sponge : one Test tube brush : one Test tube rack : one Filters : 9 cm. diameter, one hundred 7 cm. diameter, fifty Litmus paper and Turmeric paper : Safety matches : (no other kind of match should be used in the laboratory) "I To be obtained from the storeroom on special Platinum spoon : , , , , , > check only when needed, and to be returned "Dl/i+^mim fii/;KlA ! <* To be obtained on order at the storeroom Platinum crucible : J immediately after use REAGENTS IN SOLUTION 127 REAGENTS IN SOLUTION It is often necessary in qualitative analysis to use solutions of acids and alkalies of certain definite strength. It is convenient to make up these solutions in such a way that a given volume of the one is exactly neutralized by the same volume of the other, or by some simple multiple of that volume. By a normal solution of an acid is meant a solution containing in one liter (a) the molecular weight of the acid in grams if the acid contains one hydrogen atom replaceable by a metal ; (b) the molecu- lar weight of the acid in grams divided by two if the acid con- tains two atoms of hydrogen thus replaceable, etc. Thus : one liter of a normal solution of HC1 contains 36.18 grams of hydrochloric acid ; of sulphuric acid, ^ grams of the acid, or 48.67 grams. A A normal solution of an alkali is one made up of such strength that any volume of the solution is exactly neutralized by the same volume of a normal solution of any acid. The word normal is represented by the letter N; 2 N means that the solution is twice normal strength ; 3 N that it is three times normal strength ; while N N means one-half normal strength; means one-third normal Jj O strength, etc. The further explanation of normal solutions belongs to the sub- ject of quantitative analysis. Acids : Acetic, HC 2 H 3 2 2 N Hydrochloric, HC1, dilute 2 N Hydrochloric, HCI 6 N Hydrochloric, HCI, concentrated, 13 N specific gravity 1.20 Nitric, HN0 8 , dilute 2 N Nitric, HNO Z 6 N Nitric, HNO 8 , concentrated, 16 N specific gravity 1.42 Volume of acid to be diluted to one liter 138.5 cc. of 80% acid 154.3 cc. cone, acid 462.9 cc. cone, acid 126.3 cc. cone, acid 378.8 cc. cone, acid 128 APPENDIX TTXT/-V r Nitric, HN0 3 , fuming, specific grav- ity 1.60 Sulphuric, 7/ 2 $0 4 Sulphuric, H 2 S0 4 , concentrated, specific gravity 1.84 Hydrofluoric, HF, fuming, chemi- cally pure, 40%. Tartaric, H 2 C 4 H 4 6 Phenol-sulphonic Volume of acid to be diluted to owe Kter 6 N 166.03 cc. cone, acid 36 (see page 130) (see below) Grams of substance in one liter O y so i ution Alkalies : Ammonium hydroxide, NH 4 OH 4 266 cc. concentrated NH 4 OH (sp. gr. = 0.90) diluted to 1 liter with water Potassium hydroxide, KOH. The pure potassium hydroxide used for analyt- ical purposes contains about 20% of water. Hence the amount necessary for a twice-normal solution is 2 N = (55.7 x 2) x | = 139.25 Sodium hydroxide, NaOH. The pure sodium hydroxide used for analyt- ical purposes contains about 10% of water. Hence the amount neces- sary for a solution four times nor- mal is 4 N = (39.76X4)X Y- = 176.70 Salts in solution and other reagents in liquid form: Ammonium acetate, NH 4 C 2 H 3 2 300 Ammonium carbonate, (NH 4 ) 2 C0 3 (see below) Ammonium chloride, NH 4 C1 100 Ammonium molybdate, (NH 4 ) 2 Mo0 4 (see below) Ammonium oxalate, (NH 4 ) 2 C 2 4 H 2 70 Ammonium polysulphide, (NH 4 ) 2 S X (see below) Ammonium sulphide, (NH 4 ) 2 S (see below) Barium carbonate, BaC0 3 (in suspension in water) REAGENTS IN SOLUTION 129 Grams of substance in one liter of solution Barium chloride, BaCl 2 2 H 2 60 Bromine water (see below) Calcium hydroxide, Ca(OH) 2 (lime water), saturated solution Calcium sulphate, CaS0 4 , saturated solution Carbon bisulphide, CS 2 Chlorine water (see below) Ethyl alcohol, C 2 H 5 OH Ferric chloride, FeCl 3 6 H 2 90 Ferrous sulphate, FeS0 4 7 H 2 (see below) Hydrogen peroxide, H 2 2 , 3% solution Indigo (see below) Lead ammonium acetate, Pb(C 2 H 3 2 ) 2 + NH 4 C 2 H 3 2 Magnesium chloride, MgCl 2 6 H 2 25 Mercuric chloride, HgCl 2 30 Methyl alcohol, CH 3 OH Potassium chromate, K 2 CrO 4 96 Potassium cyanide, KCN 30 Potassium ferricyanide, K 3 Fe(CN) 6 30 Potassium ferrocyanide, K 4 Fe(CN) 6 3 H 2 50 Potassium iodide, KI 20 Potassium sulphate, K 2 S0 4 85 Potassium sulphocyanate, KCNS 50 Silver nitrate, AgN0 3 42.5 Sodium acetate, NaC 2 H 3 2 135 Sodium ammonium phosphate, NaNH 4 HP0 4 4 H 2 70 Sodium carbonate, Na 2 CO 3 10 H 2 200 Sodium cobaltic nitrite, Co(N0 2 ) 3 3 NaN0 2 (see below) Stannous chloride, SnCl 2 , concentrated (see below) (used in the Bettendorff test for arsenic) Stannous chloride, SnCl 2 , dilute (see below) Tartaric acid, H 2 C 4 H 4 6 75 Zinc sulphate, ZnS0 4 7 H 2 140 130 APPENDIX REAGENTS IN SOLID FORM Aluminum foil Ammonium nitrate, NH 4 N0 3 Ammonium sulphite, (NH 4 ) 2 S0 3 H 2 O Barium hydroxide, Ba(OH) 2 8 H 2 O Borax, Na 2 B 4 7 . 10 H 2 Calcium carbonate, CaC0 3 (marble) Copper foil Cotton, absorbent Ferrous sulphate, FeS0 4 7 H 2 Iron filings Iron foil Lead acetate, Pb(C 2 H 3 2 ) 2 . 3 H 2 Lead dioxide, Pb0 2 Oxalic acid, H 2 C 2 4 - 2 H 2 Potassium chlorate, KC10 3 Potassium cyanide, KCN Silver nitrate, AgN0 3 Sodium ammonium phosphate, NaNH 4 HPO 4 4 H 2 Sodium carbonate, Na 2 C0 3 Sodium carbonate and potassium nitrate, Na 2 C0 3 -f- KN0 3 (for fusions) Sodium and potassium carbonates, Na-jCOg + K 2 C0 3 (for fusions) Sodium hydroxide, NaOH Sodium sulphite, Na 2 S0 3 -7H 2 Sulphur, powdered Tartaric acid, H 2 C 4 H 4 O 6 Tin foil t Zinc foil Zinc granulated OTHER SOLUTIONS 131 OTHER SOLUTIONS USED IN PRELIMINARY Aluminum sulphate, A1 2 (S0 4 ) 3 - 18H 2 O Ammonium nitrate, NH 4 NO S Antimony chloride, SbCl 3 . 25 grams of the substance, 250 cc. of concen- trated HC1, and sufficient water to make 1 liter of solution Arsenic pentoxide, As 2 5 Arsenic trioxide, As 2 3 . 33 grams of the substance, 50 cc. of concentrated HC1, and sufficient water to make 1 liter of solution Bismuth nitrate, Bi(N0 3 ) 3 5 H 2 0. (The solution should contain some free nitric acid) Bismuth chloride, BiCl 3 . (The solution should contain some free hydro- chloric acid) Boric acid, H 3 B0 3 , saturated solution Cadmium chloride, CdCl 2 2 H 2 Calcium chloride, CaCl 2 6 H 2 Chromium sulphate, Cr 2 (S0 4 ) 3 Cobalt nitrate, Co(N0 8 ) 2 . 6 H 2 O Copper chloride, CuCl 2 2 H 2 O Copper sulphate, CuS0 4 5 H 2 Lead acetate, Pb(C 2 H 3 2 ) 2 3 H 2 O Lead nitrate, Pb(N0 3 ) 2 Manganese sulphate, MnS0 4 7 H 2 Mercurous nitrate, HgN0 3 H 2 O Nickel nitrate, Ni(N0 3 ) 2 6 H 2 O Potassium bichromate, K 2 Cr 2 7 Potassium bromide, KBr Sodium phosphate, secondary, Na 2 HP0 4 .12H 2 Strontium chloride, SrCl 2 6 H 2 EXPERIMENTS Grams of substance in one liter of solution 25 20 38 40 26 about 40 25 25 30 35 20 30 90 40 35 70 35 50 30 120 30 132 APPENDIX PREPARATION OF SPECIAL REAGENTS Ammonium carbonate. Dissolve 200 grams of the salt in a mixture of 80 cc. of NH 4 OH (sp. gr. 0.90) and 500 cc. of water, and after solution is complete, dilute with water to 1 liter. Ammonium molybdate. Dilute 100 cc. of NH 4 OH (sp. gr. 0.90) with 150 cc. of water and dissolve in this solution 50 grams of molybdic acid. Dilute 250 cc. of concentrated nitric acid (sp. gr. 1.42) with 500 cc. of water and pour this into the first solution, adding it slowly and stirring constantly. Allow to stand in a warm place for forty-eight hours and decant the clear supernatant liquid for use. Ammonium sulphide. Pass hydrogen sulphide through 750 cc. of NH 4 OH (sp. gr. 0.90) until the solution is saturated with the gas. Then add to the solution 500 cc. of ammonium hydroxide (sp. gr. 0.90) and 750 cc. of water. Ammonium polysulphide. Dissolve a little sulphur in some of the ammonium sulphide solution just described. Bromine solution. Dissolve 50 grams of potassium bromide in 500 cc. of water and add 10 cc. of bromine. Shake until the bromine is dissolved. Chlorine water. Saturate water with chlorine gas. The solution decomposes less readily if kept in a dark place or in a bottle of dark- colored glass. Chromic acid cleaning mixture. Dissolve about 40 grams of pow- dered commercial K 2 Cr 2 7 in about 150 cc. of warm water. Cool the solution and pour it slowly and with constant stirring into about 230 cc. of concentrated H 2 S0 4 . Keep in a 500 cc. wide-mouth glass bottle. The crystals of chromic acid which settle on standing con- stitute the active constituent of the cleaning mixture, and therefore the bottle should be shaken before the solution is introduced into the vessel that is to be cleaned. A portion of it that has been used for cleaning may be poured back into the bottle, provided it has not been diluted with water. Ferrous sulphate. Dissolve 140 grams of the crystallized salt in about 800 cc. of water to which 5 cc. of concentrated H 2 S0 4 has been added. Place in the solution some pieces of metallic iron (clean tacks) and dilute it to 1 liter. PBEPABATION OF SPECIAL KEAGEOTS 133 Indigo solution. Place five parts of fuming sulphuric acid in a beaker that is immersed in cold water and then add slowly and with constant stirring one part of finely pulverized indigo. Cover the beaker, allow the mixture to stand for forty-eight hours, and then pour it into twenty times its volume of water ; stir thoroughly and filter. Phenol-sulphonic acid. Dissolve 24 grams of phenol (white crys- tals are preferable) in a mixture of 148 cc. of concentrated H 2 S0 4 and 12 cc. of water. The solution slowly turns dark if exposed to the light. Sodium cobaltic nitrite. Dissolve 100 grams of sodium nitrite in 300 cc. of water, add acetic acid to slight acid reaction, and then add 10 grams of cobalt nitrate. Allow the solution to stand for several hours and filter it if not clear. The solution decomposes slowly, and it is therefore advisable to prepare only small quantities at a time. Stannous chloride, concentrated (to be used in the Bettendorff test for arsenic). Heat an excess of granulated tin in hot hydrochloric acid (sp. gr. 1.20) until the solution is saturated. Platinum scrap may be added for the purpose of hastening the action of the acid on the tin. Dilute this solution with four times its volume of water. Stannous chloride, dilute. Dilute the preceding solution with an equal volume of water. Place this solution, together with a few pieces of granulated tin, in a bottle provided with a two-hole rubber stopper. Through one opening of the stopper insert a siphon tube provided near its outer extremity with a glass stopcock. Through the other opening in the stopper insert a glass tube and connect its outer end with a carbon dioxide generator in such a manner that when stannous chloride is drawn off through the siphon, carbon dioxide and not air enters to take its place. 134 APPENDIX TABLE OF ATOMIC WEIGHTS OF THE ELEMENTS (F. W. CLARKE) 1902 Aluminum 26.9 Antimony 119.5 Argon 39.6 Arsenic 74.45 Barium 136.4 Bismuth 206.5 Boron 10.9 Bromine 79.35 Cadmium 111.55 Cesium 131.9 Calcium 39.8 Carbon 11.9 Cerium 138.0 Chlorine 35.18 Chromium 51.7 Cobalt 58.55 Columbium 93.0 Copper 63.1 Erbium 164.7 Fluorine 18.9 Gadolinium 155.2 Gallium 69.5 Germanium 71.9 Glucinum 9.0 Gold 195.7 Helium 3.93 Hydrogen 1.000 Indium ...... 113.1 Iodine ....... 125.89 Iridium 191.7 Iron 55.5 Lanthanum 137.6 Lead ....... 205.36 Lithium 6.97 Magnesium 24.1 Manganese 54.6 Mercury 198.50 H = Molybdenum ..... 95.3 Neodymium ..... 142.5 Nickel ....... 58.25 Nitrogen ...... 13.93 Osmium ....... 189.6 Oxygen ....... 15.88 Palladium ...... 106.2 Phosphorus ...... 30.75 Platinum ...... 193.4 Potassium ...... 38.82 Praseodymium . . . .139.4 Rhodium ...... 102.2 Rubidium ..... .84.75 Ruthenium ...... 100.9 Samarium ...... 149.2 Scandium ...... 43.8 Selenium . . . ... 78.6 Silicon ....... 28.2 Silver ... . . . . 107.11 Sodium ....... 22.88 Strontium ...... 86.95 Sulphur ....... 31.83 Tantalum ...... 181.5 Tellurium ...... 126.1 Terbium ...... 158.8 Thallium ...... 202.61 Thorium ...... 230.8 Thulium ...... 169.4 Tin ........ 118.1 Titanium . ... . . . 47.8 Tungsten ...... 182.6 Uranium ...... 237.8 Vanadium ' ...... 51.0 Ytterbium ...... 171.9 Yttrium ....... 88.3 Zinc ........ 64.9 Zirconium . 89.7 INDEX Abbreviations of titles of journals, 136. Acetates, detection of, 99. Acetates, reactions of, 84, 98. Acids, preparation of solutions for detection of the, 117. Acids, the, 83. Alkalies, detection of the, 80. Alkalies, the, 79. Alkali metals, detection of, 75. Alkaline earths, 71. Aluminum, detection of, 64. Aluminum, reactions of salts of, 60. Ammonia, 79. Ammonia, detection of, 80. ^ Ammonium carbonate group, 71. Ammonium carbonate group, analysis of, 74. Ammonium salts, reactions of, 79. Ammonium sulphide group, 63. Ammonium sulphide group, analysis of, 62, 64. Ammonium sulphide group, analysis of, when a phosphate is present, 64, 70. Ammonium sulphide group, analysis of, when oxalates and tartrates are present, 62, 64. Antimony, behavior of, in Bettendorff tests for arsenic, 36, 47. Antimony, behavior of, in Gatehouse test for arsenic, 36. Antimony, Bettendorff test for, 36, 47. Antimony, detection of, 44, 47, 49. Antimony, Gutzeit test for, 36, 47. Antimony, reactions of compounds of, 34. Apparatus, list of, 125. Appendix, 125. Arsenic, Bettendorff tests for, 32, 34, 46. Arsenic, detection of, 44, 45, 49. Arsenic, Gatehouse test for, 32, 34, 46. Arsenic, Gutzeit test for, 31, 45. Arsenic, reactions of compounds of, 30. Arsenic, Reinsch test for, 32, 34, 46. Atomic weights of the elements, 134. B Barium, detection of, 75. Barium, reactions of salts of, 71. Bases, preparation of solutions for detection of the, 109. Bases, the, 18. Bettendorff test for antimony, 36, 47. Bettendorff tests for arsenic, 32, 34, 46. Bettendorff tests for arsenic, behavior of antimony in, 36, 47. Bismuth, detection of, 27. Bismuth, reactions of salts of, 26. Borates, detection of, 94. Borates, reactions of, 94. Bromides, detection of, 101. Bromides, reactions of, 84, 100. 139 140 INDEX Cadmium, detection of, 27. Cadmium, reactions of salts of, 25. Calcium, detection of,J75._ Calcium, reactions of salts of, 73. Carbonates, detection of, 85. Carbonates, reactions of, 84, 85. Chlorates, detection of, 83. Chlorates, reactions of, 83. Chlorides, detection of, 104. Chlorides, reactions of, 84, 104. Chromates, detection of, 93. Chromates, reactions of, 92. Chromium, detection of, 64, 69. Chromium, reactions of salts of, 60. Cobalt, detection of, 64. Cobalt, reactions of salts of, 54. Colloids, 12. Conversion factors, 138. Copper, detection of, 27, 29, 43. Copper, reactions of salts of, 24. Cyanides, detection of, 89. Cyanides, reactions of, 84, 88. Detection of the acids, preparation of solutions for the, 117. Detection of the bases, preparation of solutions for the, 109. E Earths, the alkaline, 71. Equations, 7. Equations, balancing of, 8. Equations involving oxidation, 8. Evaporation, 17. F Ferricyanides, detection of, 103. Ferricyanides, detection of the bases in insoluble, 117. Ferricyanides, reactions of, 84, 103. Ferrocyanicles, detection of, 102. Ferrocyanides, detection of the bases in insoluble, 117. Ferrocyanides, reactions of, 84, 101. Filtration, 14. Gatehouse test for arsenic, 32, 34, 46. Gatehouse test for arsenic, behavior of antimony in, 36. Gutzeit test for antimony, 36, 47. Gutzeit test for arsenic, 31, 45. H Hydrochloric acid group, 18. Hydrochloric acid group, analysis of, 21. Hydrogen generator, 30. Hydrogen sulphide group, 23. Hydrogen sulphide group, analysis of Division A of, 27. Hydrogen sulphide group, analysis of Division B of, 44, 48. Hydrogen sulphide group, Division A of, 23. Hydrogen sulphide group, Division B of, 30. Hydrogen sulphide group, precipita- tion of, 38. Hydrogen sulphide group, separation of Divisions A and B of, 39. Insoluble substance, analysis of, 112. Interpretation of results, 122. Introduction, 1. Iodides, detection of, 100. Iodides, reactions of, 84, 99. Iron, detection of, 64. Iron, reactions of salts of, 55, 56. INDEX 141 Journals, abbreviations of titles of, 136. Lead, detection of, 21, 27. Lead, reactions of salts of, 19. List of apparatus, 125. List of reagents, 127. Loop tube, 85. Potassium, reactions of salts of, 79. Precipitates, action of excess of reagent upon, 13. Precipitate, solution of a, 16. Precipitates, solubility of, 11. Precipitates, washing of, 16. Precipitation, 11. Precipitation, conditions affecting the completeness of, 11. Pseudo-solutions, 12. Magnesium, detection of, 75. Magnesium, reactions of salts of, 73. Manganese, detection of, 64, 69. Manganese, reactions of salts of, 58. Mercury, detection of, 21, 27. Mercury, reactions of salts of, 18, 23. Metric measures, table of, 137. N Nickel, detection of, 64. Nickel, reactions of salts of, 53. Nitrates, detection of, 106. Nitrates, reactions of, 84, 106. Organic matter, destruction of, 62, 66. Organic matter, detection of, 97. Oxalates, detection of, 96. Oxalates, reactions of, 84, 96. Oxidation and reduction, 8. Periodic System, 135. Phosphates, detection of, 90. Phosphates, reactions of, 90. Potassium, 79. Potassium, detection of, 80. Qualitative analysis, definition of, -1. Quantitative analysis, definition of, 1 . Reactions, 3. Reactions, conditions affecting, 5. Reactions, reversible, 4. Reagents, list of, 127. Reinsch test for arsenic, 32, 34, 46. Results, interpretation of, 122. Reversible reactions, 4. Silicates, 115. Silver, detection of, 21, 22. Silver, reactions of salts of, 18. Sodium, 79. Sodium, detection of, 80. Sodium, reactions of salts of, 79. Solid substance, systematic analysis of, 109. Solution of a precipitate^ 16. Solutions for the detection of the acids, preparation of, 117. Solutions for the detection of the bases, preparation of, 109. Strontium, detection of, 75. Strontium, reactions of salts of, 72. Sulphates, detection of, 92. Sulphates, reactions of, 91. 142 INDEX Sulphides, detection of, 87. Tartrates, reactions of, 84, 97. Sulphides, reactions of, 84, 85. Tin, detection of, 44, 47^ 49. Sulphites, detection of, 88. Tin, reactions of compounds of, 36. Sulphites, reactions of, 84, 87. Systematic analysis of a solid sub- ^ stance, 109. Washing of precipitates, 16. Table of metric measures, 137. Zinc, detection of, 64. Tartrates, detection of, 98. Zinc, reactions of salts of, 59. LABORATORY RECORD (Remove this sheet and paste it in the front of the Laboratory Notebook) The results of an experiment should be recorded when the experiment is performed. PRELIMINARY EXPERIMENTS Enter on the left-hand page of the notebook, in the form of an equation, every reaction that can thus be expressed. Enter on the right-hand page a description of the character, solubility, and color of the precipitate, and any change in color that the solution may undergo. ANALYSIS OF A KNOWN MIXTURE Enter on the left-hand page the successive chemical changes undergone by each member of the group in the course of the analysis. The right-hand page should be kept as in Analysis of an Unknown Mixture (see below). EXAMPLE (Left-Hand Page) Pb(N0 3 ) 2 ] HgN0 3 [ + AgN0 3 HC1 PbCl 2 HgCl AgCl HgCl + Hot Water NH 4 OH AgCl 2 NH 3 + HN0 3 PbCl 2 + K 2 Cr0 4 PbCr0 4 PbCl 2 NaOH KI HgNH 2 Cl Agd-2NH, AgCl PbCr0 4 Pb (ONa) 2 PbI 2 ANALYSIS OF AN UNKNOWN MIXTURE Keep a record of your work as illustrated below, entering on the right-hand page your observations together with statements as to what the observed phe- nomena indicate or prove concerning the presence or absence of the members of the group. EXAMPLE Left-Hand Page Right-Hand Page Unknown sol. + HC1 >Ppt. 1 + Sol. 1. Ppt. 1 + hot water Ppt. 2 + NH 4 OH Sol. 3 + HNO 3 Sol. 2 + K 2 CrO 4 Ppt. 5 + NaOH Sol. 2 + KI .Ppt. 2 + Sol. 2. Ppt. 3 + Sol. 3. Ppt. 4. Ppt. 5. Sol. 4. .Ppt. 6. White ppt. indicates HC1 group. White res. indicates Ag or Hg, or both. Black res. proves Hg. White ppt. proves Ag. Yellow ppt. indicates Pb. Soluble ; confirms Pb. Yellow ppt. soluble in hot water, and crystallizing in shining plates on cooling, proves Pb. 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