Modeling the effects of ionic strength on ionization parameters for a soil fulvic acid at low concentrations

A soil-extracted fulvic acid of low concentration (10 and 30 mg l(-1)) was titrated potentiometrically in the presence of variable concentrations of KNO3 (0.01, 0.05, 0.1 and 1.0 M) as background electrolyte in such a way that the carbon content in the medium was the same as that of dissolved organic carbon present in natural freshwater. The data were analyzed following the master curve approach and local isotherm approximation methods. The affinity distribution calculated by using the spherical double layer model at higher fulvic acid concentration and the cylindrical double layer model at lower fulvic acid concentration were coincident. An electrostatic discrete ligand model was used to fit data to the presence of two types of acidic sites the intrinsic constants of which were computed by using the geometries estimated in constructing the master curve to calculate the electrostatic term. The intrinsic constants obtained, coincide with the positions of the maxima in the affinity distribution. (C) 1999 Elsevier Science B.V. All rights reserved.