Electrochemical reduction of bisulfite in mildly acidic buffers: Kinetics of sulfur dioxide bisulfite interconversion

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+, k(b)= (1.6 +/- 0.2) 10(7) s(-1). HSO3- + H3O+ --> SO2 + H2O k(f)(H) = (1.2 +/- 0.15) 10(9) M-1 s(-1) HSO3- + CH3COOH --> SO2 + H2O + CH3COO-, k(1) (HA) = (1.7 +/- 0.5) 10(4) M-1 s(-1) On this basis, and assuming diffusion-controlled rates fur 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.