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 SO 2 proceeding via a general acid catalysis mechanism, and allowed values for the rate constants for the following reactions to be determined: SO 2 + H 2 O → HSO 3 - + H 3 O +, k b = (1.6 ± 0.2) 10 7 s -1; HSO 3 - + H 3 O + → SO 2 + H 2 O, k f H = (1.2 ± 0.15) 10 9 M -1 s -1; HSO 3 - + CH 3 COOH → SO 2 + H 2 O + CH 3 COO -, k f HA = (1.7 ± 0.5) 10 4 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.