Analysis of the effect of concentration on the acid–base properties of soil fulvic acid. Conformational changes

Potentiometric titrations of a sample of soil extracted fulvic acid (FA) were carried out using different concentrations (20 mg l−1<[FA]<80 mg l−1) and ionic strengths (0.005 M<[KNO3]<1.0 M). Experimental data were analysed by means of the master curve approach, which includes an electrostatic spherical double layer model, and a Langmuir–Freundlich isotherm was used for fitting the data. To describe the chemical heterogeneity of the sample, a master curve was obtained for each concentration of fulvic acid. The effect of fulvic acid concentration on the proton binding parameters was mainly reflected in the abundance of acid sites, and the protonation constant values were only slightly affected. The effect was attributed to conformational changes and a coiling factor was introduced. The value of this factor was calculated as the ratio between the abundances of acid sites for two concentrations of fulvic acid. The coiling factor varied with the fulvic acid concentration in a similar way to the fulvic acid radius, the values of which were obtained when applying the double layer model used to interpret the electrostatic effect. Identical behaviour was found for another previously studied fulvic acid.