Square-wave adsorptive stripping voltammetric behaviour of fresh and aged cystine solutions at physiological pH1

Different cathodic square-wave stripping voltammetric behaviour for fresh and aged cystine solutions at slightly alkaline buffered medium (pH 7.4) is observed at distinct pH values. The fresh solutions of cystine at pH 7.4 exhibited three reduction peaks at -0.29 (mercuric cysteine thiolate), -0.64 (mercurous cysteine thiolate) and -0.75 V (free cystine). The peak at -0.75 V is assigned as the reduction of the -S-S- bond of free cystine which is not detected for aged cystine solution (a few hours) but a new peak at -0.68 V is seen. The peak at -0.68 V is attributed to the reduction of the disulfidic anion S2-2, the decomposition product of cystine at physiological pH (7.4).