Effect of fulvic acid concentration on modeling electrostatic and heterogeneity effects in proton binding reactions

Potentiometric titrations of two soil extracted fulvic acids (FA) were carried out at various FA concentrations (25mgl-1<[FA]<90mgl-1) and ionic strengths (0.005M<[KNO3]<1.0M). Experimental data have been analyzed by means of the master curve approach which includes an electrostatic spherical double layer model, and Langmuir-Freundlich isotherm was used for fitting the data. For one of the samples, the chemical heterogeneity of the fulvic acid has been described by means of a unique master curve for all the concentrations studied. For the other sample, two master curves were necessary to analyze the chemical heterogeneity: one for the lowest concentration and other for the higher concentrations; however, the calculated intrinsic proton binding parameters for both master curves are not quite different. The concentration of carboxylic sites obtained from the Ca-acetate titration method matched the maximum charge corresponding to the acid carboxylic sites of the Langmuir-Freundlich isotherm quite well.