ANALYSIS OF COMPLEXATION EQUILIBRIA OF POLYACRYLIC-ACID BY A DONNAN-BASED CONCEPT

Complexation equilibria of uni- and divalent metal ions (Ag+, Ca2+, CU 2+, and Pb2+) with polyacrylic acid (PAA) have been studied at various degrees of dissociation (alpha) of PAA under different sodium salt co ncentration levels at 25 degrees C. Both pH and pM(M(Z+) = Ag+, Ca2+, Cu2+, and Pb2+) of equilibrium mixture solutions of M(Z+)/PAA/Na+ (exc ess) have been determined concurrently by a potentiometric titration m ethod. The electrostatic effect inherent in the polyion-metal ion bind ing equilibria has been evaluated by a Donnan-based concept and is cor rected for by the use of a nonideality term of acid dissociation equil ibria of the polyacid as a probe. For Ag+ -PAA and Ca2+ -PAA bindings, only monodentate ligand complexes, (MA)((Z-1)), have proven to be for med, whereas for Cu2+ and Pb2+ ion bindings, formation of both monoden tate and bidentate ligand complexes have been observed. For both CU2-PAA and Pb2+ -PAA systems, bidentate carboxylate complex formation is predominant at alpha > ca. 0.3, whereas at alpha < ca. 0.3, formation of monodentate carboxylate complexes becomes appreciable as cu decrea ses. Stability constants of these complexes together with the intra-mo lecular complexation equilibrium constants expressed by the ratio of t he concentrations of bidentate complexes to monodentate complexes have successfully been evaluated and are compared with each other in order to discuss the multidentate complexation properties of the polycarbox ylic acid. (C) 1997 Academic Press.