The anodic oxidation of zinc and cadmium in aqueous solution

Abstract

In the absence of oxidizing agents in the electrolyte solution, only the bipositive ions are formed at zinc and cadmium anodes on electrolysis. In the presence of certain oxidizing agents (e.g. nitrate ion), however, both metals dissolve anodically with an initial mean valence number between one and two; in the presence of chlorate ion zinc dissolves with an initial mean valence number between one and two, whereas cadmium exhibits an initial mean valence number of two. In those solutions in which the initial mean valence number of the metal is less than two, its value decreases further with increasing temperature of electrolysis. No unipositive ion of zinc or cadmium could be isolated, and flow experiments indicated that if the unipositive ion is formed it has a very short lifetime with respect to further oxidation to the bipositive state. All of the experimental data can be explained, however, on the hypothesis that the primary anode reaction is the formation of the unipositive ion, which can then be oxidized to the bipositive state either electrolytically or by an oxidizing agent in the solution. The second oxidation can be thought of as competitive between the oxidizing agent and the electrode. This hypothesis is supported by the relation between the initial mean valence number and the quantity of reduced electrolyte in the anolyte solution.