Potentiometric Titration of Fulvic Acid: Equivalence Point Calculations and Acidic Functional Groups

Abstract

Gran's functions corrected for buffering and H₂O dissociation have been incorporated into the iterative calculation for the two equivalence points and the acid dissociation equilibria. The experimental and mathematical conditions required for accurate, dependable operation of the method have been examined. The resulting equivalence point calculations are based directly on a chemical interpretation of the system, without approximations. The first equivalence point is shigher than previously reported, and is displaced upward by KCl. Detailed chemical information has been obtained for some of the acidic functional groups, from plots of K_A = (m_Hδm_A/δm_AH) vs. mequiv./g. Carboxyl groups at the chelating sites are essentially all ionized for H⁺ concentrations less than 1 × 10⁻⁵ m.