Redox Equilibria of Tin Ions in Alkali Silicate Melts

Abstract

Sn⁴⁺-Sn²⁺ redox in binary alkali silicate melts was studied by means of differential pulse voltammetry. The half-wave potential, E_1/2, of the reduction of Sn⁴⁺ to Sn²⁺ shifts toward the negative with changing the alkali metal ions from Li⁺ through Na⁺, K⁺ and Cs⁺, at a fixed concentration for each alkali metal oxide. Conversely, E_1/2 shifts to the positive with an increase in the content of each of the alkali oxide. This tendency for Sn⁴⁺-Sn²⁺ redox was different from those of Sb⁵⁺-Sb³⁺ and Cr⁶⁺-Cr³⁺ redox. The presence of Sb⁵⁺ and Cr⁶⁺ result in a strong acid, which leads to E_1/2 shift to the negative with an increase in basicity. These results for Sn ions can be interpreted in terms of complex formations of Sn⁴⁺ and Sn²⁺, in which the coordination numbers of the oxide ion for both ions do not differ significantly and their equilibrium constants for complex formation depend largely on the alkali species.