Equilibrium dialysis-ligand exchange: adaptation of the method for determination of conditional stability constants of radionuclide-fulvic acid complexes

The equilibrium dialysis-ligand exchange technique (EDLE) has been adapted to measure conditional stability constants for the complexation of metal ions with fulvic acid. Since fulvic acid permeates across the membrane during the experiment, the quantities involved have to be determined analytically and taken into account when calculating the conditional stability constants. It is shown here that the larger diffusion rates of fulvic acid compared to those of humic acid do not lead to enhanced errors in the results. Conditional stability constants (cKFA) of Laurentian Soil fulvic acid are determined at various pH values and ionic strengths for the binding of Co2+ and UO2+2. Over the whole pH range, at ionic strength of 0.2 M, log(cKFA) averages half a log unit lower than at ionic strength of 0.02 M. A comparison with the corresponding binding constants measured for Aldrich humic acid (cKHA) by the conventional EDLE technique shows that log(cKFA) values differ by not more than one order of magnitude from log(cKHA) values.