A statistical model of proton binding by humus

A model that considers both the carboxyl and weakly acidic groups of humus as continuous binding site distributions in which individual ligand concentrations are normally distributed with respect to the log K values for proton binding is proposed. The concentration, mean log K value, and variance for the log K distribution of each class of functional groups are estimated by nonlinear regression analysis of titration data. The values obtained for those parameters are chemically reasonable and the extent of proton binding between pH 4.0 and pH 10.8 is accurately described by the model.