Potentiometric titration of humic acids

The acid-base properties of humic acids from the brown coal of the Aleksandriiskoe deposit (Ukraine) were investigated. The possibility of determining COOH and OH groups in humic acid macromolecules by different experimental methods was analyzed. It was found that the results of potentiometric titration depend on the method of titration (homogeneous or heterogeneous), the concentration of humic substances in the system, and the time taken to establish an equilibrium value of pH in the titrate. It was shown that the amount of accessible functional groups (COOH and OH) in humic acid macromolecules, which can be determined by a potentiometric method, is greater in homogeneous titration and at low concentrations of humic substances in solution. It was established that the total amount of acid groups determined by a barium method is higher than that measured with the use of potentiometric titration. The concentration of carboxyl groups determined by an acetate method is much lower than the results of potentiometric titration and the barium method. It was shown that the potentiometric titration of homogeneous systems (sodium humate + HCl) is preferable in the analysis of the acid-base properties of humic acids.