Stability constants of copper(II)-humate complexes determined by modified potentiometric titration

A ligand titration procedure using the Cu(II) ion-selective electrode was investigated as a means of determining stability constants of the Cu(II) complexes with humic and fulvic acids. The procedure was superior to conventional titration (metal ion as titrant) because constants for binding at the strongest sites are measured. A continuous-distribution model showed promise for modeling the binding data. The binding affinity of some humates for Cu(II) followed the order: soil humic acid (SHA) > peat humic acid (PHA) > lignite humic acid (LHA) > soil fulvic acid (SFA) ≃ fungal melanin. Intrinsic stability constants at pH 4 and at pH 5 were determined. -from Authors