Study of the acid–base properties of three fulvic acids extracted from different horizons of a soil Original Research Article

Potentiometric titrations of three fulvic acids (FA) extracted from samples located at different horizons of a soil were carried out at various FA concentrations (40, 80 and 120 mg l−1) and ionic strengths (0.01, 0.1 and 1.0 M KNO3). Experimental data have been analysed by means of the master curve approach which includes an electrostatic double layer model. Spherical and cylindrical double layer models fitted the data equally well. Heterogeneity analysis of the master curves showed in all cases that the affinity distribution had two peaks centred at log KintH∼4 and log KintH∼8–9, respectively. A double Langmuir–Freundlich isotherm was used for fitting the data. The number of weak acid sites derived from the equivalence point of the experimental titration curves matched the maximum charge corresponding to the more acidic carboxylic sites of the double L–F isotherm quite well. Even though a decrease in log KintH was observed with increasing FA concentration, we can conclude that the FA samples from the three horizons exhibit an almost identical behaviour.