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

Abstract

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 p K a's 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.