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

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 Ni²⁺ 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.