The reduction of l-cystine hydrochloride at stationary and rotating disc mercury electrodes

The kinetics of l-cystine hydrochloride reduction have been studied at a mercury-plated copper rotating disc electrode (RDE) and at a stationary mercury disc electrode (SMDE) in 0.1 mol dm−3 HCl at 298 K. The reduction of the disulphide is irreversible and hydrogen evolution is the major side reaction. In contrast to steady state electrode kinetic studies at a mercury drop electrode (which shows a well-defined limiting current), the mercury-plated Cu RDE shows overlap between disulphide reduction and hydrogen evolution. These effects are attributable to strong reactant adsorption with a calculated surface coverage close to 100%. A Tafel slope of −185 mV per decade is found with a cathodic transfer coefficient of 0.32 and a formal rate constant of 6.7 × 10−9 m s−1. The relative merits of steady state voltammetry at a mercury-plated copper RDE and linear sweep voltammetry at the SMDE are discussed, as is the mechanism of l-cysteine hydrochloride formation.