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 cm(2) lead disc electrodes using steady state linear sweep voltammetry, non-steady state voltarnmetry and controlled potential coulometry. The diffusion coefficient for L-cystine hydrochloride was approximately 4.8 x 10(-10) m(2) 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. (c) 2005 Elsevier B.V. All rights reserved.