Determination of the zinc complexing capacity in seawater by cathodic stripping voltammetry of zinc—APDC complex ions |
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| Authors: | Constant MG Van den Berg |
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| Institution: | Department of Oceanography, University of Liverpool, Liverpool L69 3BX Gt. Britain |
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| Abstract: | A novel technique to determine complexing capacities for zinc is presented. The free zinc concentration is determined by cathodic stripping voltammetry preceded by adsorptive collection of complexes of zinc with ammonium pyrrolidine dithiocarbamate (APDC). The reduction peak of zinc is depressed as a result of ligand competition by natural organic material in the sample. Sufficient time is allowed to reach equilibrium between this material and added APDC, and equilibrium is maintained during the measurement. Both electrochemically reversible and irreversible complexes can therefore be investigated. Values for KZnAPDC′ are calibrated against NTA and EDTA in seawater of several salinities; log KZnAPDC′ was found to be 4.40 at 36‰, 4.36 at 24‰, 4.43 at 12‰, and 4.87 at 2.3‰. The ligand concentration and conditional stability constant, KZnL′, for complexing ligands in a sample from the Irish Sea were determined in the presence of 4 × 10?5 M APDC and with added zinc concentrations between 5 × 10?9 and 3 × 10?7 M. The data best fitted a complexation model containing two ligands with concentrations of 2.6 and 6.2 and 10?8 M, and with values for log KZnL′ of 8.4 and 7.5, respectively. These results are comparable to those obtained with other equilibrium techniques, but the values of the constants are greater than those from ASV measurements. |
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