Determination of metal ion binding parameters for humic substances

Abstract

In Part 1 of this series, a straightforward calculation method was applied to the extraction of metal ion binding parameters from reverse pulse polarograms for fulvic substances. In this paper we extend this approach to include ASV pseudo-polarograms. Pseudo-polarography allows measurements to be made at low (environmentally relevant) concentrations and overcomes problems associated with processes occurring during the stripping step. Data presented for simple systems (the labile Pb(II)–pyridine-2,6-dicarboxylic acid, and the inert Pb(II)– and Cu(II)–EDTA systems) establish the validity of the pseudo-polarographic approach. Application of this methodology to Pb(II) and Cu(II) complexation by Suwannee River fulvic acid (SRFA) reveals that the complexation of Pb(II) is less heterogeneous than that of Cu(II). As compared to reverse pulse polarography (RPP), the lability of the complexes was reduced under ASV conditions (greater ligand excess). Pb(II)–SRFA behaviour approached non-lability, and although binding parameters could be determined, the values are probably influenced by loss of lability. For Cu(II)–SRFA, considerable loss of lability occurred and binding parameters could not be calculated. For adequate sensitivity, minimisation of adsorption interferences, and elimination of stripping step effects, we recommend the use of pseudo-polarograms constructed from plots of DC-ASV peak area as a function of deposition potential.