The Source of the Anomalous Cathodic Peak During ASV with  In Situ Mercury Film Formation in Chloride Solutions

Melissa A. Nolan and Samuel P. Kounaves*
Department of Chemistry, Tufts University, Medford, MA 02155 U.S.A.

*Author to whom correspondence should be addressed .

In situ mercury film depositions at a glassy carbon electrode at various concentrations of sodium chloride or hydrochloric acid were performed.  In the chloride concentration range of 0.001 to 0.5 M, an anomalous cathodic peak is observed during the anodic scan when performing square wave voltammetry.  This phenomenon has previously been attributed to the formation of mercury(I) chloride (calomel) on the electrode surface.  Cyclic voltammetry indicated that electrochemically-generated calomel, which forms due to the oxidation of the mercury film in solutions with greater than 0.001 M Cl-, is reversible and is reduced to elemental mercury during cathodic polarization of the electrode.  It is proposed that the cathodic peak is a result of the calomel which forms after Hg(0) is oxidized to Hg(I) by Hg(II) ions and subsequent reaction of Hg(I) with chloride ions.  This insoluble nonelectrochemically-generated calomel is formed under open circuit conditions and, not being reduced during the cathodic polarization, is reduced at more anodic potentials.  The most stable and reproducible responses of this system for ASV of Cd(II) and Pb(II) is obtained when the mercury is removed from the electrode surface by electrochemical oxidation at the end of each anodic scan.