| Popis: |
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 colloida/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. |
