Humic acid–proton equilibria: A comparison of two models and assessment of titration error

Summary Charge development curves, obtained by potentiometric titration of ten soil–derived humic acids, are described using two modelling approaches: a simple electrostatic model with limited functional group heterogeneity (model A), and an implementation of an alternative from the literature, model V, incorporating up to 12 functional group types. Both models provide excellent descriptions of charge dependence on pH and ionic strength. However, for model A, 11 fitted parameters were required to describe the data across the full range of ionic strength (0.01–3.0), compared with seven for model V. Although model A required more parameters, it was conceptually simpler than model V, requiring only two functional group types, which may have advantages in describing the formation of metal complexes. Error in the determination of humic charge from titration data was shown to increase markedly for data obtained at pH<3. The apparently anomalous trends in humic charge at low pH reported in the literature are explained in terms of this error. Investigation of the kinetics of protonation and deprotonation reactions of humic acids suggested that titration hysteresis was the result of dis–equilibrium, especially at high pH. The use of a 7-day batch titration technique, instead of continuous titration apparently eliminated hysteresis from the data.