THE PEPTIZATION OF CUPROUS OXIDE AND THE ELECTRODEPOSITION FROM AND DECOLORIZATION OF AMMONIACAL COPPER SOLUTIONS B Y E. A . VUILLEUMIER
When copper is plated from an ammoniacal cupric solution the deposit obtained is relatively dense, smooth and adherent. The author observed, however, some years ago, that when the electrolyte had been in contact with metallic copper for a few hours prior to the electrolysis there was a striking change in the nature of the deposit to be obtained. I t then consisted of a dull red, thick, moss-like, coarse, non-adherent mass, which under a low power microscope was seen to consist of relatively large, bright copper crystals. It was also observed that upon the addition of concentrated ammonia or ammonium sulphate (but not sodium sulphate), or by thorough oxidation by aeration of the partially reduced electrolyte good results, Le., dense, smooth, adherent deposits were again obtainable. The present investigation indicates that a cuprous oxide cathode film is responsible for the unsatisfactory deposit; that an electrolyte from which an unsatisfactory deposit is obtained is one which does not peptize cuprous oxide. It was found that cuprous oxide is readily peptized by concentrated ammonia water, or by ammonium sulphate in the presence of diluted ammonia water. Furthermore it was found that an ammoniacal cupric solution could be decolorized by metallic copper only upon adding concentrated ammonia or ammonium sulphate. Concentrated ammonia water or ammonium sulphate in the presence of diluted ammonia water evidently prevents the coating of metallic copper by a film of cuprous oxide, or peptizes the film if formed. Such a film would explain the incomplete reduction of the cupric solution, and, forming on the cathode, would, where broken through, give rise to a mass of relatively large, non-adherent crystals. Procedure A stock solution was prepared by dissolving 2 5 grams of the pentahydrate of cupric sulphate and 60 C.C.of concentrated ammonia water in sufficient water to make a liter of solution. A I O O C.C. portion of this solution was electrolyzed for 20 minutes a t a current density of 6 milliamperes per square centimeter, using copper electrodes. A dense, bright, adherent, relatively smooth, and therefore good, deposit was obtained. The electrolysis was discontinued, but the electrodes were allowed to remain in the bath over night. The electrodes were then replaced by fresh ones. The solution was again electrolyzed, and after 20 minutes a coarse, non-adherent mass was observed. A good, fresh electrolyte was also converted into a bad one by allowing a strip of copper to stand in it for three hours, or by adding a centi-
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gram of cuprous oxide. The addition of two grams of ammonium sulphate, or of an excess of concentrated ammonia, to the unsatisfactory bath resulted in a good deposit. Several attempts were made to decolorize the slightly ammoniacal stock electrolyte by means of copper in the absence of air. The solution, however, remained dark blue indefinitely. But upon adding concentrated ammonia or ammonium sulphate to the solution it was promptly decolorized by the metallic copper. If a film of cuprous oxide is responsible for the bad deposits, and for making it impossible to decolorize the stock solution by means of copper, it was to be expected that cuprous oxide would be peptized by concentrated ammonia water, or by ammonium sulphate, but not by diluted ammonia. It was found that the oxide was readily dissolved in concentrated ammonia water. Diluted ammonia water, or ammonium sulphate separately, were found to be without effect. But when ammonium sulphate was added to a slightly ammoniacal suspension of cuprous oxide the liquid rapidly became clear and colorless. Summary I . By electrolyzing a cupric ammonia complex a relatively dense, smooth, adherent, Le., good deposit is obtained. 2 . If metallic copper or cuprous oxide is dissolved in the ammoniacal cupric solution the deposit obtained consists of a thick, coarse, non-adherent mass of relatively large copper crystals. 3. The addition of concentrated ammonia water, or of ammonium sulphate to this now unsatisfactory electrolyte results in a good deposit upon further electrolysis. 4. A cuprous oxide film on the cathode is apparently responsible for the unsatisfactory deposit. Where this film is broken there grow loose crystals of copper. 5 . An electrolyte from which an unsatisfactory deposit is obtained is one which does not peptize cuprous oxide. 6. Cuprous oxide is peptized by concentrated ammonia, or by ammonium sulphate in the presence of a small amount of ammonia. 7. The cuprous oxide film formed on copper added to a slightly ammoniacal cupric solution prevents the decolorization of the solution. The addition of concentrated ammonia or of ammonium sulphate peptizes the oxide, and the solution is decolorized. Acknowledgment The author desires to express his sincere thanks to Professor Bancroft for his interest in this problem, and to Professor Dennis for making available the facilities of the Cornel1 laboratory. The preliminary observations described in the paper were verified by Mr. C. C. Bowman. Dzckinson College, Carlzsle, Pa.
