PH AND IONIC-STRENGTH EFFECTS ON NICKEL FULVIC-ACID DISSOCIATION KINETICS

The dissociation kinetics of Ni(II)-fulvic acid (FA) complexes as a function of FA concentration, pH, and ionic strength (I) were observed by ligand-exchange spectrophotometry. Lowering I and raising pH strongly decreased the dissociation rate; increasing the FA concentration caused a smaller rate decrease. Four data analysis methods estimated component concentrations Ci and rate constants ki for the Ni(II)-FA mixture. Methods that allowed only either Ci or ki to vary in the regression led to a more consistent relationship between these parameters and solution conditions. The most slowly reacting Ni(II)-FA complexes were least affected by changes in pH and I. The proportion of slowly reacting complexes increased with increasing FA. Consequently, experiments at environmental Ni:FA ratios are necessary to determine the importance of pH and I in controlling Ni(II)-FA dissociation in the environment. © 1990, American Chemical Society. All rights reserved.