DETERMINATION OF PROTON AFFINITY DISTRIBUTIONS FOR HUMIC SUBSTANCES

Binding heterogeneity analysis is applied to charge-pH curves measured by Dempsey and O'Melia on fulvic acid samples. To calculate the affinity distribution function, derivatives of the overall proton adsorption isotherm are required. These derivatives are obtained by processing the basic data (pH as a function of added volume) by a smoothing spline algorithm. Apparent affinity distributions, calculated on the basis of charge-pH curves, depend on ionic strength due to electrostatic effects. For the calculation of the intrinsic affinity distributions, the ''master curve' procedure is applied. To account for the electrostatic effects, an electrostatic double-layer model for rigid spheres is used. The resulting intrinsic affinity distribution can be described by a set of two semi-Gaussian peaks. Combination of the intrinsic affinity distribution with the electrostatic double-layer model leads to a perfect prediction of the charge-pH curve for a third intermediate ionic strength.