Potentiometric Trtrations of Humic Substances: Do Ionic Strength Effects Depend on the Molecular Weight?

摘要

That the solution chemistry of humic substances is perturbed by ionic strength effects and functional group heterogeneity is now widely accepted. Application of the two-phase approach employing concepts from the Donnan equilibrium and the electrochemical unit cell had earlier postulated the ionic strength effects to be due to the presence of higher molecular weight moieties in the fulvic acid "assemblage". Potentiometric titrations of a number of aquatic fulvic and humic acids (isolated by an initial preconcen-tration on diethylaminoethyl (DEAE) cellulose prior to their adsorption on XAD-8 resins) are presented as a function of ionic strength and their molecular weight distributions. Even though an apparent correlation between molecular weight distribution and ionic strength effects was observed, for humic substances of similar molecular weight, ionic strength effects seem to increase with an increase in the polydispersity of the molecule.