Acid-basic and complexation properties of a sedimentary humic acid. A study on the Barra Bonita reservoir of Tiet(e)over-cap river, S(a)over-tildeo Paulo State, Brazil

Acid-base and complexation properties of humic acid (HA) isolated from a ri ver sediment were studied by potentiometric titration, adopting the discret e site distribution model and the modified Gran functions for data fitting. Six classes of titratable groups were characterized, with pKa values betwe en 2.4 and 10.2. Carboxylic groups accounted for 66% of the total of ioniza ble sites. The complexing properties were studied with regard to Cu2+, Pb2, Cd2+ and Zn2+ ions by potentiometric titration using Cu ion selective ele ctrode, or amalgam electrodes (Pb, Cd and Zn). The data treatment by the Sc atchard method revealed two binding sites for copper and lead and one bindi ng site for cadmium and zinc. The average stability constants were in the f ollowing order: log KHA-Cu > log KHA- Pb > log KHA- Cd congruent to log KHA - Zn, while the complexing capacity order, C-c, was: Pb > Cu > Cd congruent to Zn.