Reductive coulometric stripping potentiometry, a technique not hitherto described, has been used to establish that the reduction of Ni(II) and Co(II) dimethylglyoximates, adsorbed on a mercury film electrode, is a 10-electron process. Exhaustive adsorption of Ni(II) or Co(II) complexes, in the 0-4 μg L(-)(1) concentration range, was achieved by vibrationally promoted electrolysis for 3 min of ∼25 μL volume samples, hanging under the working electrode in a nitrogen atmosphere. The adsorbed complexes were reduced by means of a constant current of 50 μA. The technique was successfully used for the calibration-free determination of Ni(II) in certified seawater and river water reference samples.
The paradoxical appearance of a cathodic reaction sometimes observed in anodic stripping voltammetry and stripping potentiometry when using mercury film electrodes in chloride media containing mercury(II) has been investigated by systematically varying relevant chemical and electrochemical parameters and comparing the results with thermodynamic equilibrium calculations. Microscopic observations of morphological changes on the electrode surface caused by potential variations were made possible by using a novel electrode design. Three conditions have to be fulfilled for the cathodic reaction to occur: (a) formation of calomel by reaction between elemental mercury on the electrode surface and mercury(II) in solution, (b) subsequent reduction of mercury(II) to elemental mercury on the calomelized electrode surface, and (c) a chloride concentration in the range 0.001-3.5 M. Different ways of avoiding the interference from the cathodic reaction in stripping voltammetry and stripping potentiometry are experimentally demonstrated, and a mechanism for the appearance of the cathodic reaction is proposed.
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