In Situ Spectroscopic Determination of Faradaic Efficiencies in Systems with Forced Convection under Steady State: Electroreduction of Bisulfite to Dithionite on Gold in an Aqueous Electrolyte

The reduction of bisulfite on Au electrodes in buffered aqueous solutions (pH = 5.25) was examined by in situ near-normal-incidence UV-visible reflection absorption spectroscopy on a rotating disk electrode (RDE) and in situ transmission UV-visible spectroscopy downstream from a channel-type electrode. The currents associated with dithionite (S₂O₄^2-) formation obtained from the spectroscopic measurements were found to be lower than those expected for a one-electron, diffusion-controlled process. Faradaic efficiencies for S₂O₄^2- generation derived from RDE data yielded, in the range -0.60 to -0.75 V vs Ag/AgCl, values close to 100%. The advantages of rotating disk over channel-type electrodes for determining faradaic efficiencies by UV-visible spectroscopic techniques are discussed.