The reduction of bisulfite on a bismuth rotating disk electrode (RDE) was studied in aqueous buffered electrolytes over the pH range 3-6. Clearly defined limiting currents were observed in all solutions examined; however, their magnitudes were not only smaller than those expected for a process limited by the diffusion of bisulfite from the bulk solution, but were also found to decrease as the media became less acidic. This behavior was attributed to a preceding homogeneous process that generates sulfur dioxide, the actual electroactive species. UV - visible absorption - reflection spectroscopy measurements at a RDE showed that in the potential region in which such limiting currents are observed, dithionite is produced with 100% faradaic efficiency, Results of rotation rate staircase scan amperometric RDE experiments were found to be consistent with the conversion of bisulfite into SO2 proceeding via a general acid catalysis mechanism, and allowed values for the rate constants for the following reactions to be determined: SO2 + H2O → HSO3 - + H3O+, kb = (1.6 ± 0.2) 107 s-1; HSO3 - + H3O → SO2 + H2O, kf H = (1.2 ± 0.15) 109 M-1 s-1; HSO3 - + CH3COOH → SO2 + H2O + CH3COO-, kf HA = (1.7 ± 0.5) 104 M-1 s-1. On this basis, and assuming diffusion-controlled rates for proton transfer from strong acids to oxygen bases, a more detailed mechanism involving formation of sulfurous acid as an intermediate is discussed and some thermodynamic and kinetic properties of the latter are estimated.
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