pH effect on cysteine and cystine behaviour at hanging mercury drop electrode

The pH effects on the electrochemical reactions of thiol and disulphide groups on mercury electrodes have been studied. These groups facilitate the oxidation of mercury from the electrode and its conversion into mercury thiolates. Under the appropriate experimental conditions, these thiolates form a compact film around the electrode. The formation of this film can be detected by the appearance of a spike current by cyclic voltammetry. Adsorption at the mercury surface of these groups is conditioned by the charge of the molecule, which in turn, is a consequence of the pH. Therefore, in cysteine solution, the compact film appears only when pH lies between the pKa1 and pKa2, and between the pKa2 and pKa3 in cystine solutions. At these pH values cysteine and cystine carry a zero net charge.