Acid-base properties of humic acids of different origin as seen from the potentiometric titration data

Potentiometric titration was used for obtaining detailed information on the acid-base properties of humic acids. Humic acids isolated from peat, brown coal, soddy-podzolic soil, and chernozem were studied. Back titration (0.1 M HCl as the titrant) and direct titration (0.1 M NaOH as the titrant) of humic acid solutions (2 mg/ml) at the ionic strength of 0.1 were performed. The humic acids studied were found to be similar in acid-base properties. The dissociation constants were in the ranges. pK(a)(1)= 4.6-4.9, pK(a)(2)= 7.7-8.2, and pK(a)(3) = 9.8-10.0 from the back titration data; pK(a)(1) = 4.8-5.2 and pK(a)(2) = 9.4-9.6 from the direct titration data. A hysteresis between the back and direct titration curves was observed for all samples; this was apparently due to the conformational changes in the polyelectroly te molecules, The reprotonation of weakly and groups is more active than th eir subsequent dissociation (5.64-6.09 and 3.6-3.95 meq/g of HA, respective ly), which can be explained by the formation of inter- and intramolecular h ydrogen bonds affecting the reactivity of functional groups. The results ob tained can form the basis for studying interactions between humic acids and metal cations.