The effect of sulfate and nitrate ions on the passivation and activation of silver in alkaline solutions

Abstract

Based on the concept of a competitive adsorption of hydroxide ions with SO 4 2− and NO 3 − activating anions, some peculiarities of the anodic behavior of silver in alkaline solutions containing sulfates or nitrates are explained. At the ascending branches of the anodic peaks, a co-adsorption of OH− and SO 4 2− or NO 3 − results in the formation of mixed adsorption complexes, which are soluble better than hydroxide ones. An increase in the part of the soluble oxidation products of silver is observed if a rotating disc electrode with a ring is used. Passivation of silver is explained by a change in the electronic structure of the adsorption complexes when certain potential values are reached, while a local activation, by the destruction of these complexes at the passive state potentials between the anodic peaks. At the depassivation, pH value of the solution in pits decreases, which results in the formation of Ag2SO4 or AgNO3 salts. The presence of the salts in a deposit on the electrode is confirmed by the appearance of a C 3 cathodic peak.