The electrochemistry of sulfite in aqueous solutions: UV-Visible reflectance spectroscopy studies at rotating disk electrodes

Certain aspects of the reduction and oxidation of sulfite in aqueous electrolytes have been examined using near normal incidence UV-Visible reflectance spectroscopy at rotating disk electrodes. Experiments performed in solutions of pH 3-6, in which the spectrum of the diffusion boundary layer using both Au and Bi electrodes was monitored as a function of the applied potential provided evidence that sulfite is reduced quantitatively over a significant potential range to generate dithionite with 100% faradaic efficiency. Analysis of the same type of measurements on Bi electrodes was consistent with the reduction of sulfite proceeding via a chemical-electrochemical (CE) mechanism involving formation of sulfur dioxide as the electrochemically active species. Furthermore, this methodology made it possible to estimate the dissociation constant for the thus far elusive sulfurous acid yielding values of pK _a ca. 0.6 and dehydration rates on the order of a ns. At very negative potentials, -1.8 V vs SCE, spectroscopic evidence was obtained for the direct reduction of sulfite on Bi. In another series of experiments involving oxidation of sulfite on Au, correlations between the state of oxidation of the surface and the rates of the faradaic process could be elucidated by monitoring the reflectance of the electrode as a function of the applied potential with the disk under rotation. The results obtained were found to be consistent with a competition between formation of a placed exchanged oxide from adsorbed oxygen on the Au surface and removal of the latter by sulfite.