Effects of solution pH on sulfite oxidation at a gold electrode

Abstract

The oxidation of sulfite at a renewable gold electrode in solutions with pH values ranging from 2 to 14 was studied using voltammetry. The voltammogram obtained for sulfite oxidation did not change when the solution pH was varied from 2 to 11, indicating that the process rate is independent of the acidity and partial composition of the solution. This result is in agreement with the overall reaction for the dimerization of sulfite ions, which leads to the formation of dithionate: $2{\mathrm{S}\mathrm{O}}_3^{2-}\to {\mathrm{S}}_2{\mathrm{O}}_6^{2-}+2{\mathrm{e}}^{-}$. Within the pH range 11–14, an increase in pH was accompanied by a shift in the sulfite oxidation potential and a decrease in the current. This result is concordant with the O-transfer reaction, which leads to the formation of sulfate: ${\mathrm{SO}}_3^{2-}+2{\mathrm{OH}}^{-}\to {\mathrm{SO}}_4^{2-}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}$. The mechanism of sulfite oxidation changes across the pH range 11–12.5; the reaction occurs in a narrow potential range (0.6–0.8 V) in this pH interval and is accompanied by anomalously sharp changes in the measured current.