Studies of the complexation properties of aquatic humic material by differential pulse polarography

Abstract

Amperometric titrations with copper and cadmium have been carried out for EDTA and two types of humic material of aquatic origin at pH 5.0-6.7. The free metal ion concentration was measured by differential pulse polarography. Accurate results were obtained for EDTA with both copper and cadmium as titrants and vice versa, the concentration of metal was determined accurately by titration with EDTA. The humic materials studied were a standard fulvic acid that had been isolated through a 12-step seperation procedure, and a natural fresh water sample. The formation of the metal-humic complexes was found to be complete within 3 min. The complexation capacity and stability constants were calculated using five different procedures; both inert and labile metal complexes were taken into consideration. The choice of calculation procedure influenced the results to some extent. The complexation capacities and stability constants were in most cases similar for the two types of humic matter. The complexation capacity, in terms of mmol g −1 TOC, was independent of the concentration of humic matter. The titrations of the two humic materials showed a more pronounced complexation of copper than of cadmium. The complexation capacity for cadmium was found to be less than one-tenth of that for copper. The interpretation of the amperometric titration curves is complicated by the uncertainty regarding the polarographic lability of the metal-humic complexes.