ION-SELECTIVE ELECTRODE POTENTIOMETRIC STUDIES ON THE COMPLEXATION OF COPPER(II) BY SOIL-DERIVED HUMIC AND FULVIC-ACIDS

Ion-selective electrode potentiometry was used to probe the Cu(II) complexation capacity of soil-derived humic acids (HAs) and fulvic acids (FAs) at pH 5.0, 6.3, and 7.0. The relative stabilities of these complexes at pH 2.5-7.5 were determined. For FAs at pH 5.0, 6.3 and 7.0, complexation capacity measurements indicated that 15-18%, 28-33% and 40-50% of carboxyl groups, respectively, may be involved in strong Cu(II) binding (assuming bidentate coordination). For HAs the proportions were 21-27%, 57-67% and 75-95%, respectively, indicating a more ''efficient'' distribution of the fewer chelating moieties. Normalized to carboxyl content, HAs were a much stronger Cu(II) complexant than FAs. Also, the colloidal/particulate HA molecules were stronger complexants than were the smaller (soluble) HA moieties. The apparent stability of Cu(II)-humic complexes decreased with increasing metal-to-ligand ratio and with increasing ionic strength. The presence of other weakly complexing metal ions, such as Mg(II), at relatively high concentrations inhibited Cu(II) complexation; Al(III) was bound in preference to copper(II) in the pH range 3.5-5.5. By comparison with binding by discrete ligands, aliphatic carboxyl moieties, e.g., malonate and citrate, were established as appropriate models for humic substance chelating groups; salicylate and phthalate complexed Cu(II) far too weakly to be considered as significant complexants in weakly acidic to near-neutral solutions.