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

Ion-selective electrode potentiometry was used to probe the Cu(II) com plexation 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 compl exes at pH 2.5-7.5 were determined. For FAs at pH 5.0, 6.3 and 7.0, co mplexation 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 we re 21-27%, 57-67% and 75-95%, respectively, indicating a more ''effici ent'' distribution of the fewer chelating moieties. Normalized to carb oxyl content, HAs were a much stronger Cu(II) complexant than FAs. Als o, the colloidal/particulate HA molecules were stronger complexants th an were the smaller (soluble) HA moieties. The apparent stability of C u(II)-humic complexes decreased with increasing metal-to-ligand ratio and with increasing ionic strength. The presence of other weakly compl exing metal ions, such as Mg(II), at relatively high concentrations in hibited 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 l igands, 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 consi dered as significant complexants in weakly acidic to near-neutral solu tions.