DETERMINATION OF STABILITY CONSTANTS OF METAL-HUMIC ACID COMPLEXES BY POTENTIOMETRIC TITRATION AND ION-SELECTIVE ELECTRODES

Abstract

A new technique based on the combined use of potentiometric titration and ion-selective electrodes is proposed for determining apparent stability constants (successive stability constants and overall stability constants) f divalent metal cation (Cu2+, Pb2+, and Cd2+) complexes with humic acids. Fourteen humic acids from diverse sources, including humified rice straw, were examined. The stability constants increased with an increase in pH, and the order of values obtained was Cu2+ > Pb2+ >> Cd2+. The overall logarithmic stability constants with the soil humic acid examined were 8.65 for Cu2+, 8.35 for Pb2+, and 6.25 for Cd2+ at pH 5.0. The overall stability constants of Cd2+ complexes ranged from 4.3 × 105 to 4.6 × 107 at pH 6.0. The overall stability constants increased with increasing humification in the higher humification range, and the values correspond to the contents of very weakly acidic functional groups. The binding mechanisms of the metal cation complexes with humic acids seemed to accompany the coordinate bonds of undissociated very weakly acidic functional groups, in addition to the ionic bonds through COOH and/or OG groups.