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) 10⁷ s^-1; HSO₃ ^- + H₃O → SO₂ + H₂O, k_f ^H = (1.2 ± 0.15) 10⁹ M^-1 s^-1; HSO₃ ^- + CH₃COOH → SO₂ + H₂O + CH₃COO^-, k_f ^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.