The anodic reactions at mercury electrodes due to cysteine

The detailed mechanism of anodic reactions of cysteine on mercury electrodes has been ascertained by means of electrocapillary measurements together with d.c. and a.c. polarography and voltammetry. The reactions consist of electro-oxidation of mercury in two separate steps to cysteine mercurous and mercuric thiolates respectively which are both strongly adsorbed at the electrode. At low coverages of the electrode by the mercurous product the mercuric thiolate is formed in the potential region of the second step by a non-faradaic surface process of disproportionation of the adsorbed mercurous thiolate; in consequence, only one faradaic step is then observed. The replacement of mercurous by mercuric thiolate at the electrode surface is accompanied by a pronounced change of electrode capacity. The path of the electrode reaction is determined by the form of mercury thiolates in the adsorbed state which depends strongly on the pH of the solution. In the first step in acidic solutions a compact film is formed of perpendicularly oriented mercurous cysteine thiolate molecules held together by strong lateral interaction, in alkaline solutions cysteine forms chelates with S and N atoms binding monomeric monovalent mercury. At positive potentials preceding the mass dissolution of mercury a direct formation of cysteine mercuric thiolate occurs in the solution, followed by its adsorption at the electrode surface. (C) 1997 Elsevier Science S.A.