Acid-base and complexation properties of humic acid (HA) isolated from a river sediment were studied by potentiometric titration, adopting the discrete site distribution model and the modified Gran functions for data fitting. Six classes of titratable groups were characterized, with pKa values between 2.4 and 10.2. Carboxylic groups accounted for 66% of the total of ionizable sites. The complexing properties were studied with regard to Cu2+, Pb2+, Cd2+ and Zn2+ ions by potentiometric titration using Cu ion selective electrode, or amalgam electrodes (Pb, Cd and Zn). The data treatment by the Scatchard method revealed two binding sites for copper and lead and one binding site for cadmium and zinc. The average stability constants were in the following order: log KHA-Cu > log KHA-Pb > log KHA-Cd <FONT FACE="Symbol">@</FONT> log KHA-Zn, while the complexing capacity order, Cc, was: Pb > Cu > Cd <FONT FACE="Symbol">@</FONT> Zn.
humic acid; acid-base titrimetry; river sediment; complexation capacity
