8061 'tZWiM 'A 'N ' THE ELECTRO-DEPOSITION OF BRASS FROM CYANIDE SOLUTIONS BY EARL GROVER STURDEVANT THE ELECTRO-DEPOSITION OF BRASS FROM CYANIDE SOLUTIONS By EARL GROVER STURDEVANT A THESIS Submitted to the Faculty of the Graduate School of the University of Michigan in partial fulfillment of the re- quirements for the degree of Doctor of Philosophy. 1920 ACKNOWLEDGMENT The writer wishes to express his sincere appre- ciation for the kind advice and instruction during ttee progress of this work, and for the criticism C 6f t fhe thesis rendered by Dr. Alfred L. Ferguson, /3.t r w f nose suggestion and under whose supervision this investigation was carried out. To Professor S. Lawrence Bigelow, for his most valuable criticism of this thesis, sincere thanks are also due. TABLE OF CONTENTS. I. Introduction 5 II. Theoretical 6 III. Materials and Method of Manipulation 10 IV. Metal Ratio 11 V. Metal Content 16 VI. Current Density 17 VII. Temperature 19 VIII. Sodium Cyanide 20. Acid Substances 23 Alkaline Substances 27 Neutral Substances 31 XII. Brass as Anodes 33 XIII. Single Potential of Brass 35 XIV. Summary 38 458724 THE ELECTRO-DEPOSITION OF BRASS FROM CYANIDE SOLUTIONS. Introduction. A survey of the literature on the electro-deposition of brass shows that brass has been obtained from cyanide solutions which vary decidedly in ratio of copper to zinc, metal content, cyanide content, alkalinity, current density, and temperature. This fact is clearly evident from the table compiled by C. W. Bennett (Trans. Amer. Electrochem. Soc. (1913), 23, 251), which is a summary of brass plating solutions that have been recommended. Calculations from the figures given in this table show that the composition of these solutions, expressed in grams per liter, varies from 076 to 47.77 g. of copper; 1.59 to 23.38 g. of zinc; 0.00 to 165.00 g. of potassium cyanide; 0.00 to 180.00 g. of alkaline sub- stances ; and 0.00 to 90.00 g. of slightly acid substances. The extreme variation in metal ratio is shown by solutions 1 and 12 in this table. In solution 1 the ratio is 10 parts of copper to 80 parts of zinc; in solution 12 the ratio is 10 parts of copper to 1 part of zinc. The electro-deposition of brass has been investigated by F. Spitzer (Zeitsch. Electrochem. (1905), 11, 172), S. Field (Trans. Faraday Soc. (1909), 9, 172), A. W. Davison (Jour. Phys. Chem. (1914), 18, 488), and A. Honig (Zeitsch. Electrochem. (1916), 22, 286). They worked under decidedly different experimental conditions as well as with solutions of widely different composi- tion. These different experimental conditions may, to some ex- tent at least, account for the poor agreement of their results. The lack of conformity is readily apparent from Table I. TABLE I. Grams Copper per Liter Grams Zinc per Liter Ratio of Copper to Zinc Percent Copper Deposited Investigator 6.35 6.53 0.97 H-^ 73.0 Spitzer 317 9.85 0.32 70.0 ' 34.25 22.00 1.56 96.2 Field 34.25 31.00 1.10 93.5 " 17.12 15.50 1.10 86.6 " 6.80 22.00 0.37 11 47.8 " 50.80 7.10 7.15 71.5 Davison 6 EAR!, GROVER STURDEVANT. This table shows the influence of metal content and metal ratio on the composition of the deposit. It is seen that the copper varies from 3.17 to 50.80 g. per liter; the zinc from 6.53 to 31.00 g. per liter; and the total metal content from 12.88 to 65.25 g. per liter. The slight influence which the metal content has on the composition of the deposit is shown by the fact that Spitzer obtained a deposit of 70 percent copper from a solution which contained 3.17 g. of copper, while Davison obtained a deposit of only 71.5 percent copper from a solution which contained 50.80 g. of copper. Field found that by diluting a solution which gave a deposit of 93.5 percent copper with an equal volume of water, the percentage of copper in the deposit was reduced to 86.6. From this it is evident that factors other than the absolute metal content and the ratio of copper to zinc in the solution influence the composition of the deposit to a marked degree. A similar analysis of the results obtained by them on the in- fluence of cyanide content, alkalinity, current density, and tem- perature makes clearly evident the fact that our present knowledge of the brass plating solution is decidedly indefinite and incomplete. The lack of specific data regarding the influence of each of these factors, together with the need for such data, owing to the exten- sive use of the brass plating solution commercially, makes it appear that a thorough investigation of the influence of each of these factors is highly desirable. Each factor in some way affects the metal ion concentration of the solution. All the effects produced by changes in metal ratio, metal content, cyanide content, alkalinity, current density, and temperature must ultimately result from changes in metal ion concentration. Since the influence of all of these factors is the result of a change in metal ion concentration, it seems that a study of metal ion concentrations is the most logical method of attacking this problem. Such a study is most successfully made by potential measurements. For this reason, therefore, the in- fluence of each of these factors upon the single potentials of copper and zinc in their cyanide solutions was determined. Theoretical. A difference of potential exists between an ionizable substance and a solution of its ions. The magnitude of this potential in the case of a metal is given by the well-known equation THE ELECTRO-DEPOSITION OF BRASS. NQ p In this equation is the potential in volts which a metal of solu- tion tension P and valence N shows against a solution of one of its salts in which the osmotic pressure of the metal ions is repre- sented by p. The absolute temperature is represented by T, Faraday's constant by Q, and the gas constant by R. E is the equilibrium potential which must be exceeded before metal ions will deposit from such a solution. Metals which become posi- tively charged in a solution, normal with respect to their ions, will be known as electro-positive in this discussion. Those which assume a negative charge will be known as electro-negative. In the electro-deposition of a metal from an aqueous solution of its ions it is necessary to keep in mind that hydrogen ions are also present. If the applied potential exceeds a certain value hydrogen will be deposited. The magnitude of this potential is given by the equation P f i < where e h is the hydrogen over-voltage for the particular electrode, P' is the solution potential of hydrogen and p' is the osmotic pres- sure of hydrogen ions in the solution. The other letters in the equation have the same significance as in equation (1). Accord- ing to equation ( 1 ) the potential necessary to separate the metal is RT P E = - log, - . NQ p It is evident that if 3 is greater than metal only will be de- posited, if i is equal to hydrogen and the metal will separate simultaneously, and if x is less than hydrogen alone will be deposited. This reasoning applies equally well for any other positive ion in the solution. From this it follows that, from a solution in which there are a number of metallic ions,. only that metal is deposited which is the most electro-positive. For example, in an ordinary solution containing copper and zinc ions, copper alone is deposited, since copper is nearly a volt more positive than zinc. 8 EARL GROVER STURDEVANT. In order to separate two metals simultaneously, as pointed out above, the single potentials which these metals show in the solu- tion must be nearly equal. Since the solution potential of a metal has a definite characteristic value, it follows from equation (1) that the potential which a metal shows in a particular solution must depend upon the osmotic pressure of its ions in that solu- tion. If in the above solution of copper and zinc sulphate, the concentration of the copper ions be made continuously smaller, the copper becomes less electro-positive and thus approaches the value of zinc in the same solution. If this process could be con- tinued long enough zinc and copper would deposit simultaneously. The practical impossibility of accomplishing this by dilution is indicated by a calculation from equation (1) which shows that the copper ion concentration in such a solution is only about 1 x 10- 38 N. The decrease in metal ion concentration, by dilution, decreases also the conductivity. Both the low metal content and the low conductivity thus produced are undesirable. When metal content is low there is no ready source for the renewal of the ions re- moved from about the cathode, and they are practically all removed by the passage of only a small quantity of electricity. When the conductivity is low a greater amount of energy is con- sumed in the deposition of a given amount of metal. In order, therefore, to decrease the ion concentration of a solution without decidedly decreasing the conductivity, and at the same time to pro- vide a ready source of ion supply, substances are used which form complex ions containing the metal. For example, the addition of sodium cyanide to a solution of copper sulphate forms the com- plex salt Na 2 Cu(CN) 3 , which dissociates until the following equilibrium is established Na 2 Cu(CN)i ^2Na + + 'Cu(CN)^2Na r -f Cu+ + 3CN. Equilibrium is established with an extremely small concentration of copper ions. Zinc forms a similar complex salt Na 2 Zn(CN) 4 and a similar equilibrium is established ** 2Na r + ZnCN 2Na + + Zn ++ + 4CN. In case copper or zinc are removed from such solutions, new ions, formed by dissociation, immediately replace them. The THE ELECTRO-DEPOSITION OF BRASS. 9 copper complex compound is much more stable than the zinc, hence, by the addition of sodium cyanide to a solution of copper and zinc sulphates, the copper ion concentration is decreased de- cidedly more than the zinc (Spitzer, loc. cit.). This means that the single potentials of zinc and copper approach each other on the addition of sodium cyanide. By the addition of large quan- tities of sodium cyanide the potentials become equal, or copper may even become more electro-negative than zinc. (Spitzer, loc. cit. S. P. Thompson, Chem. News (1887), 55, 300.) The cop- per ion concentration in such a solution is about 10~ 38 N. From this it appears possible to prepare a solution of the com- plex cyanide compounds of copper and zinc in which these metals show the same single potentials. During the process of elec- trolysis of such a solution the single potentials of copper and zinc are different from the equilibrium value. This results from the change in ion concentration immediately surrounding the elec- trodes. The actual ion concentration in the vicinity of the elec- trodes depends upon the current density, temperature, rate of stirring, and rate of ionization of the complex compounds. Other conditions remaining the same, it is evident that an increase in current density results in an increase in the metal ion concen- tration at the anode and a decrease at the cathode. Increase in temperature, by increasing diffusion, counteracts to some extent these changes in ion concentration. Stirring and ionization also tend to maintain a uniform ion concentration. The value of electrolytically deposited brass is determined chiefly by its color and this depends largely upon its composition. A brass of 65 percent copper and 35 percent zinc has, under favorable conditions, a satisfactory, bright yellow appearance. In order that the composition of the solution which deposits brass of this composition may remain unaltered the metals must dis- solve from the anode in this ratio and the efficiency of solution must be the same as that of deposition. These are the conditions which must be fulfilled by an efficient brass plating solution and the maintenance of these conditions constitutes the chief problem of brass plating. The ratio in which copper and zinc dissolve or deposit is determined by the ratio of their ion concentrations in the solution immediately surrounding the electrode. The prob- lem, then, consists in establishing this ratio and in maintaining 10 KARL GROVKR STURDEVANT. it by the proper regulation of factors which influence ion con- centration. Materials and Method of Manipulation. The cells used were 600 c.c. beakers. There were two anodes and one cathode in each cell. One anode was a sheet of cast zinc 50 by 50 by 2 mm. and the other was a sheet of electrolytic cop- per of the same dimensions. These were supported parallel to each other and about 8 cm. apart. The cathode was a platinum sheet 50 mm. square, supported midway between the anodes. Between each anode and the positive terminal of the battery were placed a coulometer, a variable resistance, and an ammeter. A return wire from the cathode to the negative terminal of the battery completed the circuit. The solution was vigorously stirred by a glass rod provided with a paddle about 3 cm. long. The rod was rotated about 1200 r. p. m. The potential measurements were made by the compensation method with a small Leeds and Northrup potentiometer graduated to 0.5 millivolt. A suitable galvanometer was used as zero in- strument. As a standard comparison potential, a cadmium cell with a potential of 1.01845 at 20 C. was used. Normal calomel electrodes were used as reference electrodes. To avoid including in the potential measurements some of the potential fall due to the current in the solution, the ends of the calomel electrodes were placed as near the anodes as possible and on the side opposite to the cathode. The end of the calomel electrode against which the cathode potential was measured was placed near the edge of the cathode for the same reason. Only the highest grade C. P. materials were used in this work. Analysis showed that they did not require further purification. Two cells were set up as above described, and all determina- tions were run in duplicate. Only one set of results for each determination, however, is recorded in the tables. At the begin- ning of each determination the anodes and cathodes were weighed and placed in the cells. The calomel electrodes were placed in position and a set of potential readings taken before the current was turned on. The current density was 0.15 ampere per sq. dm. at each anode and 0.30 ampere at the cathode. The current was THE ELECTRO-DEPOSITION OF BRASS. II allowed to flow for two hours, giving a cathode deposit of about 0.6 g. With the current flowing, three sets of potential readings were taken, one immediately after the current was turned on, one at the middle of the determination, and one just before the current was turned off. Immediately after the current was turned off another set of potential readings was taken. All these read- ings are recorded in the tables. At the end of the determination the anodes and cathode were immediately removed from the solution, washed with tap water, with distilled water, and finally with alcohol. They were then allowed to dry at room temperature before weighing. From the loss in weight of the anodes and the quantity of electricity as shown by the coulometers, the efficiencies of anode corrosion were calculated. The deposit was dissolved in dilute nitric acid and the copper determined electrolytically. Zinc in the deposit was obtained by difference. From these data the percentage of copper in the de- posit and the cathode efficiency were calculated. The efficiency of corrosion or deposition is obtained by dividing the weight of material dissolved or deposited by the weight of material equiva- lent to the amount of electricity shown by the coulometers. Unless otherwise stated, the temperature was 25 C. 1. A closer regulation of the temperature was not considered neces- sary. Metal Ratio. It has already been pointed out that there are many factors which influence the metal ion concentration of the brass plating solution and the nature of the deposit. These factors were indi- vidually studied and the exact influence of each determined. Metal ratio was the first of these factors to receive special atten- tion. It seemed logical to begin with a solution containing the two metals in about the same ratio. The solution first used contained 4.5 grams of copper and 6.7 grams of zinc per liter in the form of complex cyanides. (This is the metal content of solution 4 in Bennett's (loc. cit.) table.) No other constituents were pres- ent. The solution was electrolyzed for twenty hours and the changes which resulted in it observed. These observations are- recorded in Table II. 12 EARL GROVER STURDEVANT. The many disadvantages of this solution are readily apparent. The results show at first a high anode solution due to the cor- rosive action of the excess cyanide. The excess cyanide also pro- duces a high cathode polarization which causes the deposition of large quantities of hydrogen and consequently a low cathode efficiency. As electrolysis is continued the excess cyanide is re- moved ; this, as is to be expected, increases the cathode efficiency and decreases the zinc anode efficiency. The potentials of both copper and zinc decrease. In cyanide solutions the solution pres- sure of these metals is greater than the osmotic pressure of their ions, hence a decrease in the potentials indicates an increase in TABU; II. Efficiencies Potential Measurements Per- Grams Row Anodes ' cent of of Free Copper Cyan- r-jtVi i i* 1 De- ide ner Total Hrs. Run With Current Without Current Character of Deposit posit Liter Copper Zinc Copper Zinc Copper Zinc 1 108.0 87.2 6.9 45.5 7.4 2 0.951 1.231 1.073 1.272 Dull Red 2 107.2 100.4 75.8 58.9 5.4 4 0.906 1.115 1.059 1.248 3 107.2 129.3 91.2 57.4 4.5 6 0.822 1.100 0.987 1.241 Uniform Yellow 4 109.3 96.7 96.9 39.9 3.6 8 0.834 1.129 0.992 1 1.228 5 114.4 94.1 100.2 36.5 3.3 10 0.814 1.149 0.983 1.231 Purple 6 107.5 116.2 98.7 43.7 3.5 12 0.679 -0.531 0.826 0.891 Dark Brown 7 107.6 57.1 95.3 48.3 3.5 14 0.660 -0.595 0864; 0.940 i " 8 106.5 49.2 96.4 44.7 3.0 16 0.655 -0.575 0.870 0.795 9 108.8 55.8 97.0 45.8 2.9 18 0.654 -0.573 0.817 0.705 10 108.8 60.3 97.1 47.7 2.6 20 0.606 -0.676 0.804 0.665 metal ion concentration. The change in ion concentration is more pronounced in the case of zinc than of copper ; so much more, in fact, that the copper becomes more electro-negative than zinc. The change appears to be most pronounced at a definite stage in the electrolysis. At this stage a white precipitate of zinc cyanide appears on the zinc anode and the efficiency of this anode sud- denly decreases. This change in relative ion concentration is probably responsible for the fluctuation in the percentage of copper in the deposit. The percentage of copper in all the deposits obtained here was lower than that required for a good brass. This low percentage of copper indicated that the copper content of the solution was too THIv ELECTRO-DEPOSITION OF BRASS. low. It seemed, therefore, that by adding copper, a solution might be obtained which would give a deposit of suitable copper content in the presence of sufficient free cyanide to produce efficient anode corrosion. The addition of copper, as the complex cyanide, to the electrolyzed solution, showed that the percentage of copper in the deposit could be increased in this way. The results of such additions are given in Table III. A solution of the composition of that used in Table II was elec- trolyzed nearly to the stage at which the decided change in ion concentration occurred. It then gave the values in row 1 of Table III. To this solution copper sulphate, with the equivalent sodium cyanide, was added in sufficient quantity to double the original III. Row Efficiencies Per- cent of Copper in De- posit Grams of Cop- per per Liter of Sol. Potential Measurements Character of Deposit Anodes Cath. With Current Without Current Copper Zinc Copper Zinc Copper Zinc 1 2 3 4 5 103.8 104.7 108.1 109.1 109.7 95.5 106.5 102.8 100.3 104.7 96.7 80.1 76.5 99.0 98.7 36.7 59.1 62.4 66.1 80.7 446 8.92 13.37 17.83 23.40 0.640 1.015 0.750 0.967 0.861 0.918 0.869 0.994 0.854 ! 0.991 i 0.865 0.964 0.926 0934 0.902 1.189 1.130 1.125 1.009 1.027 Dull yellow " Bright " Dull " Bright ' copper content. The effect this had on the solution is indicated in row 2. Further additions of copper were made and the results obtained are given in rows three, four and five. The increase in copper concentration is' accompanied by the following changes: (1) the percentage of copper in the deposit increases, and as might be expected the nature of the deposit becomes more satis- factory ; (2) the potential of zinc decreases, indicating an increase in zinc ion concentration; (3) the potential of copper increases, indicating a decrease in the copper ion concentration. This de- crease in copper ion concentration is not exceptional, since in all electrolytes the ion concentration becomes less with increase in concentration of the electrolyte beyond a certain point. This phenomenon in the case of copper has also been observed by Spitzer (loc. cit.). Copper tends to form complex compounds GROVER STURDEVANT. containing more cyanide than Na 2 Cu(CN) 3 ; and since the copper complex is more stable than the zinc, the latter will tend to be decomposed in a solution which contains both. This will allow more zinc cyanide to dissociate and hence increase the zinc ion concentration. From the table it appears that, with a solution which contained 23.4 g. of copper and 6.7 g. of zinc, a deposit of the desired copper content can be obtained with approximately 100 percent anode and cathode efficiencies. Accordingly, a solution which contained 24.0 g. of copper and 6.7 g. of zinc was prepared, but it was found that this solution, when boiled for a half hour, gave a deposit of only 60,0 percent copper. All solutions prepared at TABUS IV. Efficiencies Potential Measurements Per- Grams Row Anodes cent of Copper in De- of Cop- per pr Liter With Current Without Current Character of Deposit A A A A Copper \_ain. Zinc ; posit of Sol. Cath. Cath. Copper 1 Zinc Copper Zinc 1 105.5 117.4 97.6 60.1 24.46 0.900 0.878 1.444 0.974 1.076 0.973 DullYellow 2 107.2 104.4 96.4 67.7 28.46 0.874 0.942 1.463 0.963 1.085 0.987 3 104.0 90.8 100.0 75.2 32.46 0.892 0.922 1.388 0.940 1.030 0.960 " Red 4 102.8 96.3 I 98.8 80.9 37.00 0.875 0.942 1.319 0.936 1.037 0.987 room temperature had a characteristic red-brown color which was removed by boiling and also by continued electrolysis. From these results it appeared that there was a close connection between the removal of the red-brown color and the decrease in the per- centage of copper in the deposit, and that the changes within the solution, which are responsible for the decrease in the percentage of copper in the deposit, may be brought about by boiling the solution as well as by continued electrolysis. The potentials ob- tained in the boiled solution corresponded to those obtained in an unboiled solution of lower copper content. This is in agreement with the results of Honig (loc. cit.) who found that the separa- tion potential of copper from its cyanide solution was increased to a marked extent by boiling the solution. The exact nature of the change produced is unknown. THE ELECTRO-DEPOSITION OF BRASS. 15 These results indicated that in order to obtain the desired per- centage of copper in the deposit still greater concentrations of copper were necessary. Four solutions, containing respectively 24.5, 28.5, 32.5 and 37.0 g. of copper, were prepared and boiled TABLE V. Efficiencies N un> Potential Measurements Row Per- cent of Copper her of Hours Character of Anodes With Current Without Current r* *u in De- Elec- A A ueposu Copper Zinc Lath. posit tro- lyzed Cath. Anodes Cath. Copper Zinc Copper Zinc 1 107.0 85.2 93.6 64.0 2 0.891 0.976 1.420 0.955 1.064 1.000 Dull light 0.897 0.960 0.979 1.080 yellow 0.903 i 0.980 2 105.6 87.9 98.4 64.9 6 0.883 0.950 1.381 0.956 1.039 0.996 Dull yellow 0.883 0.950 0.967 1.051 0.884 0.948 3 105.6 103.3 98.5 65.9 10 0.858 0.980 1.370 0.941 1.045 0.987 " 0.864 0.983 0.958 1.032 0.867 0.986 4 103.9 92.2 99.0 68.4 18 0.873 0.956 1.365 0.947 1.030 0.981 " 0.869 0.939 0.955 1.054 0.867 0948 5 104.1 93.4 98.3 68.2 34 0.875 0.954 1.346 0.951 1.075 0.983 " 0.875 0.983 0.957 1.108 0.879 0.978 6 104.4 96.2 98.1 63.3 54 0.887 1.035 1.368 0.963 1.085 0.997 " 0.890 0.985 0.973 1.119 0.887 0.959 7 105.0 107.9 103 : 3 50.8 72 0.853 0900 1.371 0.935 1.090 1.023 Bright yellow 0.841 0.869 0.950 1.107 0.837 0.891 8 104.1 106.0 ; 104.2 50.3 74 0.847 0.805 ' 1.358 0.933 1.100 0.994 0.836 0.807 0.931 1.107 0.820 0.803 9 103.2 108.8 j 103.5 42.1 88 0.755 0.377 1.394 0.860 1.201 1.195 " silver 763 0.384 0.891 1.215 color 0770 0.389 1 previous to electrolysis. All these solutions contained 6.7 g. of zinc per liter. These solutions on electrolysis gave the results recorded in Table IV. The data here given show that an increase in the ratio of copper to zinc in the solution increases the per- centage of copper in the deposit A solution containing 28.5 g. of copper per liter is necessary under these conditions to give a deposit containing 65 percent of copper. 1 6 EARL GROVER STURDEVANT. To determine whether or not a solution of this composition was suitable for use in the remainder of this investigation, two samples were electrolyzed for a period of eighty-eight hours. At various stages in the electrolysis determinations were made of the cathode and anode efficiencies and of the percentage of copper in the deposit. The usual potential readings were taken. The results are recorded in Table V. These results show that a solution of this composition gives satisfactory anode and cathode efficiencies for a period of about fifty hours. The high copper anode efficiencies may be accounted for by the solvent action of the cyanide on copper. The zinc anode efficiencies recorded are not as reliable as those for copper, because in nearly all cases copper deposited on the zinc anode and decreased the accuracy of the results. The constancy of the ion concentrations during this period is indicated by the constancy of the potential measurements. Satisfactory bright yellow brass deposits are obtained during the first fifty hours. Later the deposits become slightly dull and at the end of eighty-eight hours they have a pink appearance. It is noticed that even in this solution the percentage of copper in the deposit finally decreases. Since, however, this solution can be relied upon to give constant results for a period of about fifty hours, it is satisfactory for use in the remainder of this work. Metal Content. The influence of a decrease in metal content of the electrolyte was next determined. Two dilutions were made ; one by adding an equal volume of water to the original solution, and the other by adding an equal volume of water to the diluted solution. The usual potential measurements and efficiency determinations were made and the data recorded in Table VI. The potential measurements show that as the solution is diluted there is practically no change in zinc ion concentration, but an increase in copper ion concentration. The increase in the degree of ionization of the copper salt on dilution must increase the cop- per ion concentration more than dilution decreases it. The in- crease in copper ion concentration would normally increase the percentage of copper in the deposit, but the increase in cathode polarization shows that the metal ions are removed by electro- THE ELECTRO-DEPOSITION OF BRASS. 17 deposition more rapidly than they are furnished by dissociation in these dilute solutions. This results in the deposition of rela- tively greater amounts of the more electro-negative metal, zinc. In the second dilution, however, this effect is overcome by the more rapid increase in copper ion concentration. The increase in cathode polarization is accompanied by the usual decrease in cathode efficiency. During the electrolysis of the dilute solutions white deposits of metal cyanides form on the anodes. This indicates that the TABLE VI. Efficiencies Potential Measurements Per- cent of centra- With Current Without Current Character Row Anodes Cath. Copper in De- posit tion, Origi- nal nf Dpnrxsit Anodes Anodes Cath. Cath. Copper Zinc Copper Zinc Copper Zinc 1 1032 103 9 97 1 666 0.872 1.007 1.485 0946 1046 1 022 null rpH 0.867 1.000 1.495 0.976 1.052 0.869 1.006 1.500 2 108.0 108.3 92.2 59.5 y* 0.844 0.983 1.555 0.937 1.135 0.996 " erav 0.831 0.963 1.564 0.950 1.080 0.830 0.930 1.550 3 68.2 73.8 81.3 61.0 % 0.795 0.915 1.619 0.900 1.100 0.980 -0.535 0.760 ' 1.625 0.921 1.140 0.550 0.690 ! 1.621 cyanide concentration is too low, and accounts for the low anode efficiencies. The results obtained here are due not only to the decrease in metal content but also to the decrease in the cyanide concentration. It is clearly evident from these results that the original solution is the most satisfactory. Current Density. The original solution, when electrolyzed at different current densities, gave the results recorded in Table VII. The data show that both the cathode efficiency (Curve 1, Plate I) and the percentage of copper in the deposit (Curve 3, Plate I) uniformly decrease and that the cathode polarization (Curve 2, i8 EARI, GROVER STURDEVANT. Plate I) uniformly increases with increase in current density. The potential measurements show that the ion concentrations re- main practically unchanged. The decrease in cathode efficiency and percentage of copper in the deposit are to be expected, since E7ff.f 0.7 fa m <2b m m A\n?. urve 5 W Eff- ^urve 1 m- PLATE I. Influence of Current Density. 1. Change in cathode efficiency with increase in current density. 2. Potential of copper anode with current flowing. 3. Potential of cathode with current flowing. an increased cathode polarization produces the deposition of rela- tively larger quantities of zinc and hydrogen. Neither a maxi- mum percentage of copper nor a minimum cathode efficiency, such as was obtained by Spitzer (loc. cit.), is here observed. This is probably due to the difference between the metal content of this solution and that used by Spitzer. THE: ELECTRO-DEPOSITION OF BRASS. The character of the deposit becomes less satisfactory with in- crease in current density beyond 0.3 ampere per sq. dm. of cathode surface. For this reason a current density of 0.3 ampere is used in the remainder of this work. Temperature. The work of other investigators shows that an increase in tem- perature increases the percentage of copper in the deposit and the TABLE VII. Efficiencies Per- Cur- Potential Measurements Row cent of Conner Den- With Current Without Current , Character of Cath. in n ^' sity Amp./ Anodes Anodes Cop per Zinc dm. 2 r Copper Zinc Cath. Copper Zinc Cath. ; 1 104 L6 91.4 100.5 78.6 0.2 0.907 0.950 1.328 0.978 1.070 0.991 Dull red 0893 0.943 0.966 1.042 0.894 0.937 2 102 !.9 89.2 98.2 64.8 0.3 0.852 0.941 1.440 0.966 1.091 0.989 Dull red, not 0.853 0.937 0.959 1.064 uniform 0.849 0.941 3 10= 5.8 97.7 97.0 53.8 0.5 0.862 0.968 1.500 0.983 1.130 1.009 Dull gray 0.858 0.949 0.985 1.082 ! 0.859 0.938 4 102.0 1122 95.7 52.9 0.7 0.858 0942 1.560 0.984 1.100 1.004 " 0.858 0.954 0.985 1.056 0.864 0.961 i 5 100.9 101.6 92.4 52.1 1.0 0.795 0.540 1.651 0.986 1.097 0.999 " 0.791 0.744 0.979 1.093 I 0.787 0.880 cathode efficiency. Since the compositions of the solutions used by them were much different from the one here used, and since no potential measurements were made, it was considered desirable to obtain a series of measurements at different temperatures. The results are given in Table VIII. It is observed that with increase in temperature there is a slight gradual increase in the potentials of copper and zinc in the solu- tion (Curves 1 and 2, Plate II). This is probably accounted for by the more rapid diffusion of the dissolved material away from 20 GROVER STURDEVANT. the electrodes at higher temperatures. The pronounced decrease in the cathode polarization with increase in temperature (Curve 3, Plate II) probably results from the rapid diffusion of metal ions to the cathode. Accompanying the decrease in cathode polarization is the usual increase in the percentage of copper in the deposit (Curve 4, Plate II). These results are in good agreement with those obtained by Field (loc. cit.) and others. E M.'F urve tt- -3$ -80 PLATE II. Influence of change in Temperature. 1. Potential of copper without current. 2. Potential of zinc without current. 3. Potential of cathode with current flowing. 4. Change in the percentage of copper in the deposit. Sodium Cyanide. The results obtained when sodium cyanide was added to the solution are given in Table IX. A study of the literature gives the impression that an increase in free cyanide increases the anode efficiency. These results, however, fail to show this effect. In fact, the efficiency of the zinc anode tends to decrease with increase in free cyanide. As the concentration of the free cyanide increases, the potentials of the anodes and the cathode increase (Curves 1, 2 and 3, Plate III) ; this indicates a decrease in metal ion concentration. This THE ELECTRO-DEPOSITION OF BRASS. 21 decrease in metal ion concentration makes necessary an increase in cathode potential in order to deposit the metals. The increase in cathode potential causes the deposition of relatively larger quan- tities of hydrogen and results in a decrease in cathode efficiency. The percentage of copper in the deposit shows a peculiar change. There is first a decided decrease, then a gradual increase as the concentration of free cyanide increases (Curve 4, Plate III). TABLE VIII. Efficiencies Potential Measurements Row Per- cent of Copper in De- nnsit Tem- pera- ture Cent. Character of Deposit Anodes Cath. With Current Without Current Anodes Anodes Copper Zinc " Cath. ' Cath Copper Zinc Copper Zinc 1 103.1 88.0 98.1 64.2 26 0.865 0.943 ' 1.456 0.960 1.054 0.992 Dull red 0.868 0.960 0.957 1.043 0.868 0.933 2 105.4 93.2 100.2 80.9 40 0.927 0.983 1.371 0.976 1.083 1.023 Bright red- 0.928 0.960 0.982 1.053 yellow 0.936 0.962 3 104.8 72.0 100.7 83.9 50 0.958 0.991 1.327 1.000 1.070 1.037 Bright red- 0.965 0.997 1.007 1.056 yellow 0.972 1.007 4 104.6 83.7 99.2 85.2 60 1.008 .984 1.253 1.026 1.056 1.040 Bright red- 1.006 1.007 1.031 1.056 yellow 1.009 1.009 5 103.8 57.7 99.6 90.2 70 1.041 1.045 1-200 1.046 1 078 1.062 Bright red- 1.039 1.044 1.055 1.078 yellow 1.038 1.040 This could not be accounted for by observed changes in potential and was at first thought to be due to error in analysis. Two other sets of determinations, the data for which are not given, showed a similar behavior. It thus appears that there are two concentra- tions of free cyanide at which a brass containing 65 percent of copper (a satisfactory composition) can be obtained from this solution. Since the lower cyanide concentration gives a much better cathode efficiency and an equally good anode efficiency, it is the one used in the remainder of this work. 22 KARL GROVER STURDEVANT. CM "N G ^ + + tc '^ O 5 a t a N K d "^ i !|^ > .^4- THE ELECTRO-DEPOSITION OE BRASS. 23 Acid Substances. The greater part of the copper and zinc in their cyanide solu- tions is present as Na 2 Cu(CN) 3 and Na 2 Zn(CN) 4 (F. Kunchert, Zeitsch. anorg. Chem. (1904), 41, 337). A study of the various possible equilibria that may exist in such solutions shows that acid and alkaline substances may influence the extent to which E.M.F i / /) // s*3 ^^ > X.' I.IU 1 f) ft X"' ^ ^' 1 *l/U f] Q/] / !5 U.JU 5 1 1 s a 2 5 Gr. NoCN/L. PI.ATE III. Influence of Sodium Cyanide. 1. Potential of copper anode without current. 2. Potential of zinc anode without current. 3. Potential of cathode with current. 4. Percentage of copper in the deposit. the indicated reactions take place and hence the metal ion con- centration in these solutions. (See preceding page.) It is apparent that the addition of alkaline substances forces reactions (I a 2) and (II a 2) to the left; this increases the con- centration of molecular sodium cyanide. Reactions (I a 1) and (II a 1) are then forced to the right, increasing the concentration 24 EARL GROVER STURDEVANT. of sodium and cyanide ions. This increase in sodium and cyanide ions from the sodium cyanide causes a decrease in the concen- tration of copper and zinc ions. The addition of acid substances forces reactions (I a 2) and (II a 2) to the right with the formation of slightly dissociated hydrocyanic acid, thus increasing the cyanide ion concentration IX. Efficiencies Potential Measurements Per- Grams Row Anodes Cath. cent of of Free Copper Cyan- in De- ide per posit Liter With Current Without Current Charac- ter of Deposit Anodes Anodes Copper Zinc j Copper Zinc Lath. Copper Zinc Cath. 1 103.2 103.9 97.1 66.6 7.0 0.850 0.956 1.439 0.921 1.052 0.989 Dull red 0.870 0.965 1.435 0.955 1.040 0.880 0.981 1.436 2 103.6 87.5 94.7 60.4 12.0 0.879 1.024 1.539 0.990 1.125 1.035 " gray 0.882 1.009 1.542 1.002 1.055 0.883 1.006 1.546 3 103.7 74.1 79.6 62.3 17.0 0.943 1.060 1.595 1.057 1.169 1.095 0.926 1.067 1.590 1.062 1.142 0.929 1.060 1.595 4 102.7 86.3 51.3 64.8 22.0 0.971 1.111 1.650 1.126 1.189 1.146 " " 0.960 1.105 1.642 1.111 1.183 0.950 1.104 1.644 5 103.3 86.5 38.2 65.5 27.0 1.065 1.195 1 696 1.200 1.245 1.174 " " 1.051 1.166 1.707 1.164 1.220 1.013 1.130 1.712 produced from- the sodium cyanide. This permits a greater dis- sociation of the copper and zinc complexes and results in an in- crease in the concentration of copper and zinc ions. It should be mentioned here that, since the stability of the copper complex is greater than that of the zinc (Kunchert, loc. cit.) the effects in the two cases are not of the same magnitude. As a result of these considerations all the substances other than sodium cyanide which have been added to brass plating solutions are arbitrarily classified as acid, neutral, or alkaline. It is believed that their influence can be explained by their action as acid, neu- THE ELECTRO-DEPOSITION OF BRASS. 25 tral, or alkaline substances upon the above equilibria existing in brass plating solutions. A set of experiments was carried out with each of the three acid substances, ammonium chloride, sodium hydrogen sulphite, and boric acid. Ammonium chloride and sodium hydrogen sul- TABLE X. Sodium Hydrogen Sulphite. Efficiencies Potential Measurements Per- cent of Copper in De- posit Grams NaHSO 3 per Liter Character of Deposit Row Anodes Cath. With Current Without Current Anodes Anodes r* it /"* *l Copper Zinc Copper Zinc Latn. Copper Zinc Lath. 1 106.9 106.8 95.2 62.6 0.0 0.864 0.970 1.390 0.952 1.038 0.984 Dull Red 0.876 0.967 0.964 1.049 0.880 0.951 2 101.6 111.7 97.8 72.3 5.0 0.865 0.907 1.358 0.942 .985 0.963 0.868 0.917 0.945 1.004 0.873 0.912 3 103.7 124.2 99.1 77.0 10.0 0.858 0.883 1.307 0.914 0.981 0.940 < 0.866 0.890 0.926 0.996 0.867 0.880 4 105.9 148.3 99.0 80.6 15.0 0.841 0.860 1.220 0.887 0.924 0.920 0.844 0.863 0.901 0.901 0.849 0.853 5 109.4 128.3 99.9 85.7 20.0 0.828 0.821 1.144 0.878 0.940 0.899 0.828 0.485 0.883 0.910 0.830 0.153 6 111.5 112.0 97.1 96.9 30.0 0.797 0.723 1.082 0.837 0.899 0.875 " " 0.770 0.723 0.843 0.901 0.470 0.723 phite were selected because they are the most commonly used acid substances. Boric acid was used because its action can be only that of a weak acid. The results obtained by the addition of variable amounts of each of these substances are recorded in Tables X, XI and XII. The general influence of the three substances is the same. Sodium hydrogen sulphite produces the greatest effect and ammo- nium chloride the least. Since sodium hydrogen sulphite is only 26 EARL GROVER STURDEVANT. slightly acid, the effect observed here seems greater than was to be expected. This point is given further consideration following the discussion of alkaline substances. The potentials in all cases show a regular decrease (Curves 1 and 2, Plates IV and V), which indicates an increase in metal ion concentration. This in- TABLE XL Ammonium Chloride. Efficiencies Potential Measurements Per- r Row Anodes Cath. cent of Ggtns Copper * "poSft- filer With Current Anodes rvtii Without Current Anodes n~4.i. Character of Deposit Copper Zinc Copper Zinc Copper Zinc 1 103.1 87.6 97.3 65.6 0.0 0.892 0.968 1.445 0.962 1.062 0994 Dull red 0.895 0.965 0.973 1.039 0.891 0.966 i 2 103.1 58.8 98.2 66.3 3.0 0.880 0.950 1.421 0.967 1.052 0993 0.882 0.950 0.970 1.038 0.887 0.940 3 103.1 82.9 97.5 68.9 5.0 0.898 0.962 1.436 0.959 1.046 0.992 0.904 0.958 0.971 1.034 0.903 0.930 4 102.6 81.9 98.0 68.7 7.0 0.893 0.975 1.423 0.968 1.034 0.996 " " 0.899 0.969 0.973 1.016 0.903 0.935 5 92.2 72.4 97.8 71.9 11.0 0.909 0.903 1.400 0.956 1.011 0.993 " " 0.904 0.909 0.965 1.021 0.900 0.934 6 100.9 57.0 97.6 72.5 13.0 0.903 0.915 1.395 0.962 0.998 0.998 " " 0.903 0.930 0.970 1.021 0.902 0.921 crease in metal ion concentration is to be expected from the above conclusions regarding the influence of acid substances upon the equilibria which exist in the solution. The decrease in cathode polarization (Curve 3, Plates IV and V) is accompanied by the customary increase in the percentage of copper in the deposit (Curve 4, Plates IV and V). It is concluded from these results that no advantage is to be gained by the addition of any of these substances to a solution of the composition here used. A solution THE ELECTRO-DEPOSITION OF BRASS. 27 of lower copper content, which normally gives a deposit too low in copper, may be made to give a deposit of the desired copper content by the addition of acid substances. TABLE XII. Boric Acid. Efficiencies Per- Grams Potential Measurements _r ~ cent of OI Boric With Current Without Current Character Row Anodes in De* Acid per Anodes Anodes of Deposit posit T . [ Cath. Cath. Copper Zinc Copper Zinc Copper Zinc 1 103.4 94.6 97.8 65.7 00 0.831 0.918 1.467 0.895 1.072 0.997 Dull red 0.854 0.925 0.925 1.063 0.857 0.975 2 j 102.2 111.3 98.4 67.4 2.0 0.846 0.940 1.468 0.924 1.014 0.981 0.849 0.929 0.943 1.038 0.850 0.926 3 99.8 115.9 99.2 67.7 40 0.856 0.920 1.460 0.930 1.028 0.973 a . 0.856 0.916 0.943 1.023 0.857 0.920 4 99.2 117.8 99.2 71.2 8.0 0.838 0.924 1.458 0.917 1.017 0.959 0.838 0.916 0.926 1.000 0.841 0.910 5 100.6 111.8 99.4 71.4 15.0 ^ 0.830 0.884 1.432 0.906 0.994 0.950 0.830 0.875 0.921 1.000 0.833 0.882 6 101.2 115.9 98.8 73.7 20.0 0.827 0.892 1.412 0.886 0.977 0.938 1 0.835 0.885 0.908 0.993 1 0.841 0.881 7 98.3 110.3 100.6 78.6 25.0 0.845 0.829 1.354 0.892 0.981 0.931 0.845 0.832 0.910 0.975 0.845 0.825 Alkaline Substances. The alkaline substances first used were ammonium hydroxide and sodium carbonate. The results produced by the addition of variable amounts of each of these substances to the original solu- tion are recorded in Tables XIII and XIV. The increase in potential of both copper and zinc (Curves 1 and 2, Plates VI and VII) shows that the metal ion concentra- 28 EARL GROWER STURDEVANT. tions decrease with increase, in the alkalinity of the solution. This decrease in metal ion concentration produces an increase in cathode polarization (Curve 3, Plates VI and VII) and thus a decrease in the percentage of copper in the deposit (Curve 4, TABLE XIII. Ammonium Hydroxide. Efficiencies Potential Measurements i Per- Eauiva- Row Anodes cent of Copper Cath. in De ' lent of NH 4 OH per With Current Without Current Character of Deposit Anodes Anodes posit Copper Zinc [ Copper Zinc Copper Zinc 1 103.8 95.5 ! 99.3 63.9 0.0 0.830 .975 1.425 0.907 1.090 1.005 Dull red 0.846 .972 1.435 0.935 1.028 0.842 .992 2 103.5 93.4 98.4 58.0 0.15 0.918 1.019 1.460 0.968 1.127 1.021 Bright yellow 0.919 1.011 1.475 1-005 1.109 0.916 1.007 3 103.3 95.4 99.6 55.3 0.30 0.894 1.045 1.468 0.970 1.097 0.986 0.910 1.040 1.475 1.008 1.121 0.916 1.056 4 103.2 95.8 98.9 54.6 0.45 0.950 0.994 1.481 1.020 1.170 l.oil 0.945 1.018 1.485 1.031 1.135 0.951 1.005 5 101.9 128.9 99.4 53.3 0.60 0.936 1.020 1.504 1.021 1.140 1003 0.927 1.001 1.500 1.028 1.198 0.924 0.965 6 102.2 95.7 99.3 54.6 0.75 0.951 1.055 1.484 1.028 1.160 0.991 0.951 1.023 1.483 1.032 1:172 0.949 1.014 7 104.6 101.3 99.3 53.0 1.05 0.979 1.084 1.499 1.059 1.219 1.056 0.975 1.090 1.474 1.047 1.215 0.948 1.041 Plates VI and VII). As is to be expected, the influence of these substances is opposite to that of acid substances. Ammonium hydroxide has a greater effect than sodium carbonate, but the effect in both cases compared with that of acid substances is small. These substances have a pronounced effect on the nature of the deposit. A bright-yellow deposit is obtained in both cases. As the percentage of copper in the deposit decreases the brass changes THE ELECTRO-DEPOSITION OF BRASS. 29 from a dull red to a bright yellow at about 58 percent of copper, and remains bright yellow throughout the determinations. From these results it appears desirable to add a small quantity of an alkaline substance to the solution because of the favorable in- fluence it has on the nature of the deposit. It was pointed out above that sodium hydrogen sulphite pro- duces a greater effect than is to be expected from its acidity. This E.M.F. /~ /^ /* 'lurve ^ ^N^ 1 * N ^ ^ / ' ^ / \ / # / / \ \ III frf^ \ \ ^ \ ^ -) 0.8 0. 65 0. & 0. ^-^ . ^ 0. J &.!? )gen Sulp PLATE IV. Influence of Sodium- Hydrc hite. 1. Potential of copper anode without current. 2. Potential of zinc anode without current. 3. Potential of cathode with current. 4. Percentage of copper in the deposit. suggested that it might have some influence other than that of an acid substance. It was thought possible that it might exert a reducing effect on the solution and thus influence the metal ion concentration. In order to obtain more information on this point two series of experiments were performed ; one, by the addition of variable amounts of sodium hydrogen sulphate, and the other, by the addition of variable amounts of sodium sulphite. Sodium sulphite was used to obtain the influence of the sulphite ion in 3 KARL GROVER STURDEVANT. the absence of an acid substance. Sodium hydrogen sulphate was used to obtain the influence of the hydrogen ion in the absence of the sulphite ion. The results obtained by the addition of these substances to the original solution are given in Tables XV and XVI. TABLE: XIV. Sodium Carbonate. Row Efficiencies Per- cent of Copper in De- posit Equiva- lent of Na 2 C0 3 per Liter Potential Measurements Character of Deposit Anodes Cath. With Current Without Current Anodes Cath. Anodes Cath. Copper Zinc Copper Zinc Copper Zinc 1 103.0 89.2 98.2 64.8 0.0 0.852 0.941 1.440 0.966 1.091 0.989 Dull red 0.853 0.937 0.959 1.064 0.849 0.931 2 102.5 88.4 98.2 57.0 0.094 0.857 0.930 1.440 0.953 1.036 0.999 " reddish 0.869 0.998 0.969 1.079 yellow 0.878 0.988 3 102.8 97.6 99.4 57.62 0.188 0.866 0.993 1.430 0.966 1.066 0.998 " yellow 0.874 0.957 0.971 1.079 0.873 0.961 4 103.2 94.8 99.4 58.2 0.282 0.873 0.952 1.439 0.968 1.088 0.998 ! Bright " 0.869 0.958 0.972 1.107 0:869 0.962 5 102.0 94.7 99.3 5&0 0.376 0.869 0.981 1.430 0.984 1.094 1.000 " 0.866 0.972 0.973 1.133 0.864 0.976 6 101.5 89.6 98.8 56.8 0.564 0.861 0.995 1.451 0.996 1.177 1.007 " 0.855 0.979 0.979 1.099 0.855 0.987 7 101.6 91.7 98.7 55.6 0.752 0.861 1.015 1.454 1.001 1.154 1.004 ; " 0.856 1.015 0.998 1.125 0.853 0.997 The changes produced by sodium sulphite are slight. The only observed effect is that of an extremely weak base. This indicates that the sulphite ion has no specific influence. Sodium hydrogen sulphate has approximately the same effect as sodium hydrogen sulphite. One would expect sodium hydrogen sulphate to have the greater effect, since in a pure solution of sodium hydrogen sulphate the hydrogen ion concentration is distinctly greater than THE ELECTRO-DEPOSITION OF BRASS. 31 in. a pure solution of sodium hydrogen sulphite. Even though the influence of sodium hydrogen sulphite is more pronounced than can be accounted for by the concentration of hydrogen ions in a pure solution, nevertheless it appears to have no influence other than that of an acid substance. In the preparation of the original solution, however, a quantity of sodium hydrogen sulphite equivalent to the copper is beneficial in that it prevents the loss of ^ ^- L - *~ <1~*u Anodes Cath. Copperi Zinc Copper V^dLU, Zinc Copper Zinc 1 102.3 95.5 91.5 61.4 0.0 0.860 0.941 1.503 0.906 1.035 0.991 Dull red- 0.854 0.947 1.502 0.932 1.042 yellow 0.851 0.950 1.501 2 99.4 98.5 96.1 61.7 5.0 0.837 0.944 1.500 0.911 1.038 0.983 Slightly dull 0.840 0.948 1.493 0.929 1.042 yellow 0.840 0.915 1.491 3 1 102.7 93.0 97.3 63.8 15.0 0.831 0.935 1.478 0.893 1.042 0.980 0.844 0.933 1.497 0.935 1.048- 0.845 0.900 1.493 4 102.3 107.3 97.6 65.3 25.0 0.848 0.931 1.481 0.904 1.052 0.976 0.847 0.950 1.479 0.937 1.029 0.838 0.940, 1.479 5 102.6 103.5 98.9 66.2 35.0 0.840 0.934 1.453 0.906 1 048 0.979 0845 0.939 1.454 0.938 1.047 0.846 0.943 1.455 * 6 99.8 118.6 99.1 65.3 45.0 0.825 0.924 1.475 0.908 1.025 0.972 0.826 0.925 1.476 0.930 1.035 0.828 0.925 1.466 other words, brasses of the composition here used dissolve uni- formly as such. The efficiency of corrosion is in all cases about the same as that of copper. Single Potential of Brass. In all the tables in which the single potentials of copper and zinc are given it is observed that the potentials of zinc are approxi- mately 0.1 volt or more greater than those of copper. -According 36 KARL GROVKR STURDKVANT. to the theory of electro-deposition it is impossible for two metals to be deposited simultaneously when their potentials differ by this amount. Some other highly influential factor, which has not yet been considered, must consequently be active here. It is a well-known fact that strongly electro-negative metals can, under certain conditions, be deposited from solutions at potentials much lower than their equilibrium potentials. For ex- ample, sodium may be deposited from an aqueous solution of its salts, provided mercury be used as cathode. This is explained Lflf. ^ * j fCu _* _ - -" Curve 4 7fl 7,T I 7\. ^~~^~ ' 4 a. -^ - -I jO //, ^^ r hU 0. * 2 4 0. >*. 6 0. 1 8 Eq U IV. /Kfa C OT.IL. PLATE VII. Influence of Sodium Carbonate. 1. Potential of copper anode without current. 2. Potential of zinc anode without current. 3. Potential of cathode with current. 4. Percentage of copper in the deposit. by assuming that sodium alloys with the mercury, and the result- ing alloy shows a low electrolytic solution pressure for sodium. It appears that some similar action must take place in the deposi- tion of brass from cyanide solutions. It is known that zinc and copper do form alloys. The poten- tials which such alloys show depend upon their nature and com- position. A. J. Allmand (Principles of Applied Electrochemistry, p. 136) pointed out that the potentials of those alloys which are simply solid solutions of one metal in another vary continuously with composition from the potential of one metal to that of the THE; ELECTRO-DEPOSITION OF BRASS. 37 other. In case the two metals form a compound, it shows a char- acteristic potential which may lie between the potentials of the two metals or may even exceed the potentials of either. If the alloy consists of a mixture of two or more constituents the poten- tial which the alloy shows is that of the more electro-negative constituent. TABLE XVIII. Percentage of Copper Efficiency of Corrosion 62.3 104.8 66.2 105.2 70.7 103.8 76.0 103.4 81.4 103.2 85.0 104.0 It seems entirely possible that a thin film of copper is first de- posited, and this greatly decreases the potential necessary for the deposition of zinc. This depolarization may be so great that the simultaneous deposition of the two metals takes place from solu- tions in which their single potentials are not the same. The two metals thus deposited form an alloy, the potential and other prop- erties of which depend upon its nature and composition. In order to obtain some information on the potentials of brasses formed in this way, ten samples of brass, the composition of which ranged from 37.6 to 82.0 percent of copper, were prepared by varying the acidity of the stock solution. The potentials of these brasses were then measured in the original solution and the data recorded in Table XIX. TABLE XIX. Percent of Copper Potential 37.6 1.005 56.7 0.925 62.3 0.928 66.0 0.917 69.5 0.978 72.2 0.979 79.1 0.984 82.0 0.990 Pure Copper 0.914 Pure Zinc 1.080 It is seen from this table that the potential decreases to a mini- mum with increase in copper to 66.0 percent and then increases. Since a brass deposit which contains 66.0 percent of copper is the 38 EARI, GROVER STURDEVANT. most satisfactory, there may be some relation between this low potential and the nature of the deposit. The potentials of these brasses lie between the single potentials of copper and zinc in the same solution and most of them are much nearer the potential of copper than the potential of zinc. These results lead one to believe that the alloys formed by electro-deposition are solid solu- tions of a compound of copper and zinc in copper. Further ex- periments of this kind would doubtless afford valuable informa- tion regarding the exact nature of copper-zinc alloys. Summary. As a result of this work the following statements may be made : (1). Increase in the ratio of copper to zinc in the solution increases the percentage of copper in the deposit. A solution in which the ratio of copper to zinc is 4.2 gives a deposit of about 65 percent copper (ratio 1.9). (2) Solutions of high metal content are more satisfactory than dilute solutions. A solution containing thirty-five grams of metal per liter, in the above ratio, gives satisfactory deposits. (3) Increase in temperature decreases cathode polarization and consequently increases the percentage of copper in the deposit. (4) Increase in current density produces a gradual decrease in the percentage of copper in the deposit. At current densities greater than 0.3 ampere per sq. dm., the deposit becomes granular, non-adherent, and dull in color. (5) Increase in free cyanide does not increase anode efficiency, but does decrease cathode efficiency. Its influence on the percentage of copper in the deposit is variable. (6) Slightly acid substances increase the percentage of copper in the deposit. A weak acid may be used in place of any of the acid substances that have been recommended. (7) Slightly alkaline substances decrease the percentage of copper in the deposit. The presence of slightly alkaline substances is beneficial in that it improves the appearance of the deposit. (8) Neutral substances have no influence on the deportment of the cyanide brass plating solution. THE ELECTRO-DEPOSITION OF BRASS. 39 (9) Brasses which vary in composition from 62.3 to 85.0 per- cent of copper dissolve as such anodically. The efficiency of cor- rosion is about the same as that of copper. (10) Decided depolarization of zinc by copper takes place and makes possible the deposition of brass from solutions in which the potentials of the two metals are not equal. (11) Electro-deposited brasses which vary in composition from 37.6 to 82.0 percent copper give nearly the same potentials in a plating solution. These potentials are nearer to that of copper than to that of zinc. UNIVERSITY OF CALIFORNIA LIBRARY BERKELEY Return to desk from which borrowed. This book is DUE on the last date stamped below. MAY REC'D LD NOV 24 REC'D LD MW 25'6A -1 LD 21-100m-9,'47(A5702sl6)476 4587^4 UNIVERSITY OF CALIFORNIA LIBRARY '