STABILITY-CONSTANTS OF COPPER(II) HUMATE COMPLEXES DETERMINED BY MODIFIED POTENTIOMETRIC TITRATION

Humic substances form highly stable complexes with micronutrient cations. Little is known, however, regarding the nature of the complexes or of their stability constants. 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. Furthermore, under the titration conditions employed (pH of 4 and 5; low ionic strength, 0.01 M; low Cu(II) concentration, 10(-5) M), precipitation did not occur and thus did not adversely affect the binding measurements. 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 (log K(bnl)) at pH 4 and an ionic strength of 0.01 M for the five humate preparations were 8.3, 7.9, 7.4, 7.2, and 6.9, respectively; constants at pH 5 were 8.5, 8.4, 7.9, 7.6, and 7.6, respectively.