EVALUATION OF NEGLECTING ELECTROSTATIC INTERACTIONS ON THE DETERMINATION AND CHARACTERIZATION OF THE IONIZABLE SITES IN HUMIC SUBSTANCES

The stoichiometry and pK(a) of the ionizable sites in humic acids has been determined by means of potentiometric titrations. The analytical implications of neglecting the free electrostatic energy that accompanies the proton dissociation in polyelectrolytes or humic acids is presented. Simulated titration curves of polyelectrolytes with known composition were generated according to an electrostatic model. The data of these titrations were treated by a linear calculation method developed for determination and characterization of weak monoprotic acids in mixtures. The influence of the polyelectrolyte molar mass was also studied. Besides, experimental titration curves of humic acids at ionic strength 0.010, 0.10 and 1.0 were performed and the titration data were treated by the same calculation method. The results showed that the indication of the existence of two or three kinds of carboxylic groups in high-molar-mass polyelectrolytes may have been affected by a mathematical fit as consequence of ignoring electrostatic interactions. However, the experimental titrations data treatment have characterized five kinds of titratable groups which have concentration and pK(a)s few affected by changes in the ionic strength. These results indicate that the chemical heterogeneity among groups with the same chemical nature is the major factor governing acid-base properties of humic acids having low charge accumulation.