RESPONSE OF COPPER (II) HUMIC-ACID DISSOCIATION KINETICS TO FACTORS INFLUENCING COMPLEX STABILITY AND MACROMOLECULAR CONFORMATION

The dissociation reactions of copper(II)-humic acid complexes were inv estigated using a competing ligand spectrophotometric technique. The e ffects of pH, initial Cu: HA ratio, electrolyte concentration, and pre dissociation equilibration time on the rates of complex dissociation w ere studied and are discussed in terms of equilibrium speciation and m acromolecular conformation. In general, increasing pH or predissociati on equilibration time decreased Cu-HA complex dissociation rates; incr easing Cu: HA ratio or electrolyte concentration increased Cu-HA disso ciation rates. These trends are shown to be consistent with slower dis sociation from more thermodynamically stable complexes. The response o f the rate of these reactions to the different experimental conditions did not imply an effect on reaction rates due to the degree of macrom olecular contraction or expansion. The effect of predissociation equil ibration time was attributed to slow, complexation-induced conformatio nal change which promoted formation of more stable complexes and retar ded subsequent dissociation. Data analysis was based on proportions of Cu in operationally defined kinetic fractions and on a first-order ex ponential decay function modified by a log-normal distribution.