KINETICS OF Cu(II) CATALYSED-OXIDATION OF CYSTEINE HYDROCHLORIDE BY METHYLENE BLUE IN ACIDIC MEDIUM

Abstract

The kinetics of Cu(II) catalysed-oxidation of cysteine hydrochloride under anaerobic conditions and in presence of hydrochloric acid have been described. The stoichiometry is found to be 2:1 and cystine is the oxidation product. The order is unity in cysteine as well as in methylene blue but at higher [cysteine] and low [MB], the order in MB changes to half. The rate increases on increasing [H⁺] and [copper sulphate] but at higher [catalyst] the rate attains a limiting value. Similarly, the rate tends to become independent of variation in [H⁺] at higher concentration: The increase in ionic strength increases the rate but it is not influenced by the addition of cystine or the leuco base. The reaction scheme postulating the interaction of cysteine with protonated MB as the rate determining step has been verified by studying the reaction under aerobic conditions.