Electrochemical behaviour of sodium dithionite and sulfite at a gold electrode in alkaline solution

The role of the surface condition of a gold electrode in the electrochemical behaviour of sodium dithionite and sulfite has been studied in alkaline solution. Dithionite is oxidised to sulfate in two steps, with sulfite as the intermediate. Both limiting currents of the corresponding waves obey the Levich equation. Hydroxide ions are consumed in both oxidation steps and when the free hydroxide ion–dithionite concentration ratio is lower than 8 the second wave, due to the oxidation of dithionite to sulfate, splits and a new wave appears, shifted by approximately 200 mV in the positive direction. In this wave hydroxide ions released by the dissociation of water are consumed. The oxidation of dithionite to sulfate becomes inhibited by the formation of Au(III)hydroxide at the electrode surface, while the oxidation of the free hydroxide ions is not inhibited. Hysteresis effects observed between the forward positive and a reverse negative scans can be explained by assuming that the inhibitory effect is eliminated by the formation and subsequent chemisorption of oxygen at the gold electrode surface.