Homocysteine Voltammetry at a Mercury Electrode in the Presence of Nickel Ions

Abstract

At a mercury electrode, Hcy and Cys yield similar cathodic stripping peaks connected to the reduction of the pertinent mercury thiolate. However, due the different behavior as a ligand for nickel ion, the above compounds perform very differently in the presence of this ion. Whereas the nickel ion at a high enough concentration suppresses the Cys peak, in the case of Hcy it causes the cathodic peak to shift to more negative potentials. The peculiar behavior of Hcy is due to the stabilization of the mercury thiolate surface layer by intermolecular Ni 2þ bridges within the surface layer. Conversely, in the case of Cys, the nickel ion strongly competes with the mercury ion and leads to the formation of a surface adsorbed bis-cysteinatonickelate complex, which prevents the formation of mercury cysteinate. Such a difference allows determining Hcy by cathodic stripping voltammetry in the presence of nickel ion with no Cys interference.

Electroreduction of Bi(III) Ions in the Aspect of Expanding the ”Cap – Pair” Effect

“…It was indicated that active complexes are formed on the electrode's surface, which mediates electron transfer. It should also be noted that due to the mentioned electrochemical reactivity of both homocysteine and homocystine, which react with mercury in the same way as cysteine and cystine [39], we can discuss Bi -Hg2(SR)2 or Bi -Hg(SR)2 complexes. According to the literature reports [31], Bi(III) reacts with mercury cysteine thiolate -Hg(SR)2.…”

“…This form of anodic mercury oxidation in the presence of homocysteine or homocystine is adsorbed in the range of Bi(III) reduction potentials (~ 0 mV) and is loosely bonded to the electrode's surface [34,35]. However, in the case of polarographically non-active ethionine, complexes of type Bi -ethionine are formed on the electrode's surface [39,40].…”