S. Vavřička

The anodic reactions at mercury electrodes due to cysteine

Abstract 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 cystein 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.

Electrochemical reactivity of homocysteine at mercury electrodes as compared with cysteine.

Homocysteine differs from cysteine in electrochemical behaviour due to slight differences in hydrophobicity and in structure of their complexes with metals. This shows in adsorptivity, in anodic reactions with mercury, in catalytic reductions of oxygen mediated by mercurous thiolates and of thiol-cobalt(II) complexes, but most markedly in catalytic evolution of hydrogen from solutions containing cobalt ions.

Adsorption effects of electroactive species in d.c. polarography demonstrated in the case of the anodic waves of cysteine

Careful analysis of d.c. polarographic mean and instantaneous currents helps to explain why the anodic electrolytic reaction of cysteine on mercury electrodes proceeds differently in acetate and in borate buffer solutions, even though in both solutions cysteine as well as the product of its electrode reaction are adsorbed at the electrode. In acetate buffer of pH 4.7, cysteine is adsorbed through the interaction of its sulphur atom with mercury, which ultimately leads to a steeper than reversible slope of the first polarographic wave, to a transformation of that wave into an adsorption pseudo-prewave, and to a ready appearance of the second wave. The adsorption of cysteine from borate buffer of pH 9.4, in contrast, causes a marked increase of the electrode capacity but only little interference in the electrode process.

Adsorption in polarographic reversible electrode reactions

Summary A reinterpretation of the adsorption pre- and postwaves in polarographic reversible electrode reactions has been put forward. Both these anomalous waves are attributed to reversible surface redox systems where electron transfer takes place in the adsorbed state in contrast to the main wave, the half-wave potential of which corresponds to the normal redox potential. The half-wave potentials of these surface redox systems are determined by the interaction between the adsorbed species and the electrode. Supporting experimental evidence has been provided.

On the pH-dependence of polarographic half-wave potentials: II. Protonation of an electro-inactive group

Summary The pH-dependence of half-wave potentials of reversible and irreversible electrode reactions has been studied for a depolarizer bearing, besides the electroactive, an electroinactive acidic/basic group. Solution of two theoretical cases of an irreversible two-electron reduction indicates that the break which may occur on the E1/2 vs. pH plot does not denote the dissociation constant pKA of the acidic form of the electroinactive group. The qualitative conclusions from the solution have been discussed for some examples of the reduction of nitrobenzene derivatives.

Admittance measurements in dilute solutions: Re-examination of double-layer effects on the electrode reaction of Zn2+/Zn(Hg) In a large concentration range of the supporting electrolyte

A procedure is proposed enabling corrections to be made of the measured admittance components for the resistance uncompensated by the positive feedback, and so to determine the charge-transfer rate constants in dilute solutions of supporting electrolytes. The procedure was checked for the Zn2+/Zn(Hg) redox system in NaNO3 up to 10−2 M. A large increase of the measured rate constants with the dilution of the supporting electrolyte was found, obeying the Frumkin double-layer correction.

Anomalous Dependence of Peak Currents on Scanning Rate in Voltammetry of Co2+, Fe2+, and Ni2+Ions in Presence of Thiols

In the voltammetry of complexes of substituted thiols with Co, Fe and Ni, in which the metals are deposited on the electrode, a new faradaic current appears which increases with decreasing rate of potential scan; it is apparently due to a catalytic evolution of hydrogen.