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 affin ity distribution function, derivatives of the overall proton adsorptio n isotherm are required. These derivatives are obtained by processing the basic data (pH as a function of added volume) by a smoothing splin e algorithm. Apparent affinity distributions, calculated on the basis of charge-pH curves, depend on ionic strength due to electrostatic eff ects. For the calculation of the intrinsic affinity distributions, the ''master curve' procedure is applied. To account for the electrostati c effects, an electrostatic double-layer model for rigid spheres is us ed. The resulting intrinsic affinity distribution can be described by a set of two semi-Gaussian peaks. Combination of the intrinsic affinit y distribution with the electrostatic double-layer model leads to a pe rfect prediction of the charge-pH curve for a third intermediate ionic strength.