The reduction of l-cystine in hydrochloric acid at mercury drop electrodes

The reduction of l-cystine in 0.1 mol dm−3 HCl at 298 K has been studied at mercury electrodes. Dropping mercury electrode (DME), static mercury drop electrode (SMDE) and hanging mercury drop electrode (HMDE) modes were used with normal, sampled d.c. and differential pulse polarographic detection. The charge transfer kinetics for the irreversible reduction of l-cystine were complicated by reactant and product adsorption, by the formation of cysteinate complexes between mercury and the product thiol as well as by formation of Hg2Cl2. High Tafel slopes of −182 mV per decade were observed with a cathodic transfer coefficient of 0.32. The diffusion coefficient of l-cystine was found to be 5.3 × 10−10 m2 s−1 in 0.1 mol dm−3 HCl and 4.2 × 10−10 m2 s−1 in 2.0 mol dm−3 HCl at 298 K. The mechanism of l-cystine reduction at mercury has been discussed and the effects of pH and reactant concentration have been quantified.