The reduction of l-cystine hydrochloride at lead using static and rotating disc electrodes

The reduction of the disulphide, l-cystine hydrochloride to the l-cysteine hydrochloride thiol, in 0.1 mol dm−3 HCl at 298 K, has been studied at pre-treated, circular, 0.50 cm2 lead disc electrodes using steady state linear sweep voltammetry, non-steady state voltammetry and controlled potential coulometry. The diffusion coefficient for l-cystine hydrochloride was approximately 4.8 × 10−10 m2 s−1 from the three techniques. Reduction of the disulphide was irreversible and hydrogen evolution occurred as a competitive reaction at approximately −1.35 V vs. SCE. Analysis of the mixed control kinetics, using a Koutecky-Levich approach, allowed the relative roles of charge transfer and mass transport to be resolved. Anomalously high Tafel slopes, of typically −183 mV, were observed due to disulphide adsorption. The charge transfer kinetics are consistent with the first electron gain being rate determining while reaction orders are +1 with respect to both the disulphide and proton concentrations. The mechanism of l-cystine hydrochloride reduction has been critically discussed.