The Electrodeposition of Silver from Solutions of Silver Nitrate in the Presence of Addition Agents

Abstract

The electrolytic deposition of silver from a neutral solution of silver nitrate gives a deposit which is rough, non-adherent and of large-grained crystalline form. In industry, whenever a smooth, firm, uniform, silver deposit is desired, the complex silver cyanide bath is almost always used. The film of deposited silver is flat white in appearance, but assumes its characteristic metallic luster when it is burnished. Although the cyanide bath gives satisfactory silver deposits, it would be desirable to find a substitute, because the cyanides are such violent poisons. It has been found by various investigators (1) (2) (3) (4) (5), that the presence of many impurities or agents, in the electrolyte, have an effect upon the form of the deposit. The theory of addition agent action has been discussed in the literature (6) (7). On the whole, the progress along this line has been largely empirical, as illustrated by the addition to plating baths of substances whose functions are not understood or predictable, although they may lead to desired results. Briefly, if any substance is added to the electrolyte, the ions migrate to the electrodes and if not discharged, they accumulate at the cathode and will thus alter the environment from which the simple ions are being discharged. Other effects which are responsible for the various types of deposits would be adsorption and other inter-facial tension effects at the cathode, changes in conductivity due to the presence of other ions, and electrode reactions other than the simple discharge of silver ion. Taft and Horsley (5), carried on investigations with 169 addition (organic and inorganic) in an attempt to determine a relationship between the nature of the addition agent and the mass and form of the silver deposit. These deposits were classified as Control (no effect), Intermediate, Fine (macroscopically much smoother than the deposits in the control baths), Abnormal, and Striated. The deposits were weighed and analyzed for silver. They found that colloids of high molecular weight (above 250) produced abnormal or striated deposits. Smoother fine crystalline deposits resulted upon addition of certain inorganic salts. The lower fatty acids (formic, acetic and proprionic) did not produce a fine-grained deposit. Taft and Hodge (8) and Taft and Lutness (9) further studied addition agent action for silver deposition, at various conditions of temperature, concentration of addition agent, and current density. This study is an extension of the above investigations and is an attempt to determine the effect of various agents upon the form of electrolytically deposited silver. Extensive lists of wetting manufactured in America and commercially available, are to be found in the literature (10) (11).