The Surface Morphology of Mercury Plated Glassy-Carbon Electrodes and Stripping Voltammetry of Heavy Metals

This article deals with the investigation into the correlation between the surface morphology of the glassy carbon electrodes modified with mercury and the characteristics of voltammograms of copper, lead, cadmium and mercury. The methods of voltammetry and in-situ microscopy were used. It is shown that nucleation is the slow step of the mercury evolution on glassy carbon electrode. Overvoltage of the process is reduced after nuclei form. The factors, which determine the distribution of mercury on the electrode surface, are the degree of polishing of glassy carbon and exposure of the electrode to the air. Two types of insoluble compounds are formed on the electrode in solutions containing chloride-ions. One type of the compounds, a calomel, forms a reversible redox pair and is reduced to metallic mercury during cathodic polarization of the electrode. Well-defined voltammograms of the metals are observed in this case. The other type of the compounds appears during anodic polarization of the mercury-coated electrode in solutions containing chloride-ions when the mole ratio HCl/Hg2+ of the solution is less than 104. The formation of these compounds, which possess anomalous properties, is followed by appearance of a cathodic peak in the anodic voltammogram (‘reverse’ peak). A special electrochemical ‘conditioning’ of the electrode is needed to remove the precipitate of insoluble compounds. Otherwise, unusual ‘reverse’ peaks complicate the voltammograms of the elements to be determined.