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

Acid-base equilibria of a humic acid were characterized by considering ionic group heterogeneity and electrostatic effects. The continuous pK 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 pK distribution (μ1 and μ2) and the total amounts of functional groups (CA1 and CA2) 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 pK distribution. The μ values were evaluated in media of various ionic strength (I = 0.004−1); μ1 (strong groups): 3.4–5.1, μ2 (weak groups): 6.0–7.9, and the ratio of CA1 to CA2 was ca. 5:2. The electrostatic model including the Debye-Hückel theory was adapted to evaluate the intrinsic dissociation constants, pKi,int. From the relationship between the μ values and I12, the intrinsic pKi values of two centers of distributions, μint, could be evaluated; μ1,int: 3.03, μ2,int: 5.12. The intrinsic pK 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 pK distribution of each functional group.