Comparison of methodologies for determination of carboxylic and phenolic groups in humic acids

A nonlinear method for fitting acid–base potentiometric titration data was applied to commercial and vermicompost humic acids in order to determine the concentration and conditional pKaʹs of the ionizable sites. Data was also treated by a linear method based on modified Gran functions. In both cases the discrete site model was adopted, which means that the humic acid was treated as a mixture of monoprotic acids. Electrostatic effects were evaluated by performing titrations in media of 0.010, 0.10, and 1.0 mol l−1 NaCl. Results were compared with those obtained by conductimetric titration with NaOH and Ba(OH)2, infrared spectra of humic acid with different grades of protonation, and classical Ca-acetate and Ba(OH)2 exchange methods. Models with five and six binding sites were the best to represent experimental data of potentiometric titration using both the linear and nonlinear techniques.