October, 19501 NOKWITZ: DIRECT ELECTROLYSIS OF COPPER Direct Electrolysis of Copper from Silver Chloride Filtrates BY GEORGE NORWITZ SYNoPsIs-Copper can be deposited directly from silver chloride filtrates if an excess of hydrogen peroxide is present. The procedure saves 3 hours in the analysis of alloys containing silver and copper, as it avoids the necessity for evaporating the sulphuric acid until it fumes to remove the chloride ion. 551 IN the analysis of alloys containing silver and copper, it is the usual practice to dissolve the sample in nitric acid, determine silver as silver chloride, and determine copper electrolytically in the silver chloride filtrate. Before depositing the copper, however, it is necessary to evaporate the sulphuric acid until it fumes1 to remove the hydrochloric acid, since hydro- chloric acid interferes with the electrolytic determination of copper.3 This evaporation takes about 3 hours, and obviously increases the chances of error.The author has found that by adding an excess of hydrogen peroxide to the silver chloride filtrate, the copper can be deposited directly in the presence of the hydrochloric acid. Hydrazine sulphate has been used for the electrolytic determination of copper in the presence of hydrochloric acid.2 Attempts by the author, however, to use this reagent for the electrolytic determination of copper in silver solders failed. This failure can be traced to the fact that hydrazine sulphate cannot be used in the presence of more than very small amounts of nitric acid.2 EXPERIMENTAL A silver nitrate solution was prepared by dissolving 15.7 g.of silver nitrate in water and diluting to 1 litre. Ten millilitres of this solution contained approximately 0.1 g. of silver. A standard copper nitrate solution was prepared as follows: 5.000 g. of pure electrolytic copper were dissolved in 40 ml. of diluted nitric acid (1 + 1) by warming on a steam bath. The oxides of nitrogen were boiled off and the solution was cooled and diluted to 500 ml. in a volumetric flask. Varying amounts of silver nitrate solution and standard copper nitrate solution were measured into 300-ml. beakers. Measuring cylinders were used to measure the silver nitrate solution, and pipettes for the copper nitrate solution. The solutions were diluted to 150 to 175 ml. with water, and were treated with 3 ml.of 70 per cent. nitric acid. The silver was precipitated as the chloride according to the A.S.T.M. method,l and the solutions filtered through Gooch filters. Four millilitres of 70 per cent. nitric acid and 30 ml. of 3 per cent. hydrogen peroxide were added to the filtrates, and the copper was electrolysed at 2 amperes for 1 hour with platinum gauze cathodes, 60mm. in height and 50mm. in diameter, and platinum spiral anodes. The results obtained for copper, as can be seen from Table I, were excellent. The deposits were in- variably bright and adherent. The above experiment was repeated on two samples without the addition of the hydrogen peroxide. The copper plated out was so dark that the electrolysis was discontinued after 10 minutes for fear of injuring the platinum.During the electrolysis the solutions were stirred. TABLE I RESULTS FOR COPPER IN SYNTHETIC MIXTURES OF COPPER AND SILVER Silver present, g. 0-25 0.25 0.50 0.50 0-75 0.75 0.10 0.10 Copper present, g. 0.7500 0.7500 0.5000 0-5000 0.2500 0.2500 0*1000 0~1000 Copper found, €5 0-7499 0.7496 0.4996 0.5001 0.2498 0.2498 0.0997 0.1001552 NOKWITZ : DIRECT ELECTXOLYSIS OF COPPER [Vol. 75 A typical silver solder was analysed by the R.S.T.M. procedure,l and found to contain 15.47 per cent. of copper, 49.90 per cent. of silver, 17.97 per cent. of cadmium and 16.60 per cent. of zinc. This sample was analysed for copper four times by the following modification of the method described here. A l-g. sample was dissolved in 20 ml. of diluted nitric acid (I + 4), and the silver precipitated as the chloride according t o the A.S.T.M.procedure.1 The silver chloride filtrate was treated with 4 ml. of ‘70 per cent. nitric acid and 30 ml. of 3 per cent. hydrogen peroxide, and electrolysed for copper. The results obtained for copper are shown in Table 11. The method shows high accuracy and precision. TABLE I1 RESULTS FOR COPPER IN A TYPICAL SILVER SOLDER Copper present, Copper found, % 73 15.47 16-52 15.47 15.50 15.48 Average 15.49 + 0.02 To find out how much hydrochloric acid could be present during the electrolysis the following experiment was performed. Six 25-ml. portions of standard copper solution were pipetted into 300-ml. beakers, and 100 ml. of water and 7 ml. of 70 per cent. nitric acid were added to each portion.Amounts of 36 per cent. hydrochloric acid ranging from 0 to 5 drops were added, followed by 30 ml. of 3 per cent. hydrogen peroxide. The solutions were diluted to 200 ml. and electrolysed for copper. The results obtained for copper, as shown in Table 111, were satisfactory, but the deposits obtained in the presence of 4 to 5 drops of hydrochloric acid were slightly discoloured. No attempt was made to use more than 5 drops of hydrochloric acid. TABLE I11 RESULTS FOR COPPER USING VARIOUS AMOUNTS OF HYDROCHLORIC ACID Hydrochloric acid present, drops . . 0 1 2 3 i 5 Copper found, g . . . .. .. . . 0.2498 0.2501 0.2496 0.2495 0.2499 0.2496 Two platinum anodes that had been used throughout the experiments with solutions containing hydrogen peroxide were weighed before and after the series of electrolyses. One was found to have lost 0.3 mg. after eight runs and the other 0.2 mg. after ten runs. The average loss was 0.028 mg. per run. This is approximately the usual loss obtained in electro- lysing copper from nitric acid solutions which do not contain hydrochloric acid. REFERENCES 1. “A.S.T.M. Methods of Chemical Analysis of Metals,” American Society for the Testing of Materials, 2. Diehl, H., “Electrochemical Analysis with G.raded Cathode Potential Control,” G. Frederick 3. Kolthoff, I. M., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” Macmillan & Philadelphia, Pa., 1946, p. 253. Smith Chemical Co., Columbus, Ohio, 1948, pp. 36, 39. Co., New York, 1943, p. 422. 3353 RIDGE AVENUE PHILADELPHIA 32, PA. Afiril, 1950