Determination and data evaluation of copper complexation by organic ligands in sea water using cathodic stripping voltammetry at varying detection windows

Constant M.G Van Den Berg,John R Donat

doi:10.1016/0003-2670(92)85181-5

Abstract

Published data of trace metals in sea water show very large differences in strength of organic complexation. These differences are incompatible with the expected order of complexation by reversible and nonspecific complexing ligands. Comparison revealed a linear relationship between the detection windows of the analytical techniques and the detected metal complexation. A possible explanation for this relationship is that the apparent differences between organic complexation of different metals are artifacts when the analytical techniques detect only one or two of many complexing ligands of greatly varying complexing ability in the natural water samples. Detailed complexation measurements of copper in samples from the Sargasso Sea and the North Sea were carried out at several detection windows using cathodic stripping voltammetry and confirmed the presence of several complexing ligands. The detected ligand concentration was found to decrease with increasing magnitude of the detection window, whereas the conditional stability constant was found to increase. The overall effect was that the degree of metal complexation [(Σα CuLi) − i + 1], where i is the number of α coefficients, increased with the detection window whereas the complexation due to the individual ligands (α CuLi) increased until a maximum had been reached when the ligand concentration was approximately equal to the metal concentration whereafter it decreased at very high detection windows. An additional observation was that the dissociation rate of organically complexed copper in sea water is very slow the process requiring more than 1 h to reach equilibrium with an added competing ligand. The complexation data were used to estimate the total number of ligands and the range of conditional stability constants. The overall complexation of copper in sea water was calculated giving values for pCu between 13.5 and 15.4.

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