Calculation of molecular weights of humic substances from colligative data: Application to aquatic humus and its molecular size fractions

A rigorous mathematical expression for the dependence of colligative properties on acid dissociation of water soluble humic substances is presented. New data for number average molecular weights of a river derived humic material and its gel permeation Chromatographic fractions are compared with M̄n values obtained by a reevaluation of previously published experimental observations on soil and water fulvic acids. The results reveal a remarkable similarity of fulvic acids from widely different sources with respect to number-average molecular weight.