Abstract
Coulometric stripping potentiometry has been used to achieve calibration-free determinations of Cu(II), Zn(II), Cd(II) and Pb(II) in tap water samples. With the coulometric approach, interference from the formation of Cu-Zn intermetallic compounds (CuZnx) on the quantification of Zn(II) was circumvented and the average stoichiometry of CuZnx could be determined. For Zn to Cu concentration ratios below 1, CuZn is quantitatively formed. At higher ratios amalgamated Zn coexists with CuZn, CuZn2 and CuZn3. Both mercury soluble and mercury insoluble CuZnx exist. The former dissociates, with a half time of the order of one second, at potentials between the oxidation potentials for amalgamated Zn and that for CuZnx, while formation of the latter is irreversible. Due to interference from surface active groups on the glassy carbon surface, used as a substrate for the mercury film, determination of Zn(II) was not possible at Zn to Cu concentration ratios below approximately 0.1.
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