Interpretation of the acid-base equilibrium of humic acid by a continuous p K distribution and electrostatic model

Abstract

Acid-base equilibria of a humic acid were characterized by considering ionic group heterogeneity and electrostatic effects. The continuous p K distribution model, in which acidic functional groups having two centers of distribution were assumed in the humic molecule, was adapted to the experimental titration data by non-linear least square regression analysis. Excellent curve-fitting was obtained, and two centers of p K distribution (μ 1 and μ 2) and the total amounts of functional groups ( C A1 and C A2 ) were evaluated. It was suggested that there are strong and weak acidic groups in the humic molecule and that they continuously deprotonate according to their p K distribution. The μ values were evaluated in media of various ionic strength ( I = 0.004−1); μ 1 (strong groups): 3.4–5.1, μ 2 (weak groups): 6.0–7.9, and the ratio of C A1 to C A2 was ca. 5:2. The electrostatic model including the Debye-Hückel theory was adapted to evaluate the intrinsic dissociation constants, p K i, int . From the relationship between the μ values and I 1 2 , the intrinsic p K i values of two centers of distributions, μ int, could be evaluated; μ 1,int: 3.03, μ 2,int: 5.12. The intrinsic p K distribution and the species distribution curves were calculated under electrostatic-free conditions. Investigation of Fourier transform infrared spectra and the species distribution curves suggested that the two different types of carboxylic groups would contribute to the p K distribution of each functional group.