The Effect of Solution pH on the Oxidation of Sulfite Ions and the Formation of Oxides on the Gold Electrode

Abstract

The results of studying the effect of solution pH on the electrode process that occurs on the gold electrode in solutions of sodium sulfite, sulfuric acid, and alkali, and also in the universal buffer of Britton–Robinson (pH 2–14) are shown. In sodium sulfite solutions, this electrode process represents a combination of the oxidation of sulfite species and the formation of oxides on the gold surface that proceeds simultaneously in the same potential region. It is shown that the solution pH and the oxidation of sulfite species have no effect on the amount of gold α-oxide formed. At the same time, the solution pH has a strong effect on the oxidation of sulfite species. Thus, the voltammograms measured in solutions with pH approximately between 2 and 11 are identical, i.e., the process rate is independent of the solution acidity and its partial composition. These results suggest that in this pH region, the oxidation of sulfite ions can be interpreted by the overall reaction $$2{\text{SO}}_{3}^{{2 - }} \to {{{\text{S}}}_{{\text{2}}}}{\text{O}}_{6}^{{2 - }} + 2{\text{e}}$$ to produce dithionate ions. In strongly alkaline solutions (pH 12.5–14), the oxidation potential shifts in the negative direction and the current decreases with increasing pH. These results suggest that in strongly alkaline solutions, the oxidation of sulfite ions can proceed on the partly blocked electrode surface by the reaction $${\text{SO}}_{3}^{{2 - }} + 2{\text{O}}{{{\text{H}}}^{ - }} \to {\text{SO}}_{4}^{{2 - }} + {{{\text{H}}}_{{\text{2}}}}{\text{O}} + 2{\text{e}}$$ to form sulfate ions. The changeover of the mechanism of oxidation of sulfite ions takes place in a narrow potential region in solutions with pH from 11 to 12.5 and is accompanied by anomalously sharp changes in the measured current. The latter anomalies are associated with the peculiar dynamics of the process of passivation/depassivation of the electrode surface by gold oxides.