The mechanism of electroreduction of cystine at the mercury electrode

Abstract

Potential sweep oscillopolarography of solutions containing cystine and cysteine has been carried out. Mercury cysteinate is formed on the mercury electrode surface in contact with either cystine or cysteine solution at potentials more positive than the reduction potential of cystine. When a cathodic sweep is then applied, the adsorbed mercury cysteinate is reduced, producing a reversible reduction peak. The formation of mercury cysteinate from cysteine is always fast, but its formation from cystine is very slow at low pH values and its rate increases with increasing pH. The mercury in the mercury cysteinate may be in the monovalent or bivalent form but higher surface concentrations of cysteine favor the bivalent form. Heavy metal ions tend to displace mercury from mercury cysteinate shifting the reduction peaks to potentials more negative than that of mercury cysteinate or of the pure heavy metal ion.