Kinetics of silver anodic dissolution in thiosulfate electrolytes

Abstract

The regularities of silver anodic dissolution are studied by using the voltammetry (at the potential scan rates from 5 to 1000 mV/s) on the electrode, which was renewed immediately in the solution by cutting-off a thin surface metal layer, and quartz microgravimetry, for various concentrations of sodium thiosulfate (0.05–0.2 M). It is shown that, in the potential range from 0 to 0.4 V (normal hydrogen electrode), the polarization curves reflect the silver dissolution, whereas the contribution of oxidation of S₂O ²⁻₃ ions is insignificant. At low potential scan rates, the process kinetics is of mixed nature. The kinetics and mechanism of anodic process are studied by using the measurements at high potential scan rates (100–200 mV/s) and the calculations of equilibrium composition of near-electrode layer. It is found that the exchange current in the electrolytes studied is 5 × 10⁻⁵ A/cm², the transfer coefficient α is approximately 0.5, and both parameters are virtually independent of the concentration of S₂O ²⁻₃ ions. The reaction order of silver dissolution with respect to the ligand $\left. {\frac{{\partial logi}} {{\partial logc}}} \right|_E$ is close to unity and is independent of potential. With regard for the literature data on the adsorption of thiosulfate ions on silver, this result is interpreted as the evidence for the involvement of one S₂O ²⁻₃ ion from bulk solution, along with adsorbed ligands, in the elementary act of metal dissolution.