Speciation and equilibria in the H+–Al3+ - polyacrylic acid system

Abstract

Equilibria between Al3+ and the polyelectrolyte polyacrylic acid (PAA) were studied in 0.1 M NaCl medium at 25 °C (M = mol dm−3). The measurements were performed as precise potentiometric titrations in which OH− was in most cases generated coulometrically. The total concentration of Al(III) and PAA varied within the limits 6.23 × 10−5–2.01 × 10−4 M and 7.20 × 10−4–1.12 × 10−3 M, resp., with a 4–18 fold excess of ligand. The value of − log{[H+]/mol dm−3} varied between 3 and 5–7, the upper limit set by drifting EMF potentials caused by the onset of a precipitation reaction. The constant capacitance model was utilized to model the experimental data. With the objective of finding a model as simple as possible that gives an acceptable fit to data, the two species AlPAA2+ and Al(OH)2PAA are postulated. The polyelectrolyte effect resulting in pH-dependent formation constants is demonstrated. Furthermore a comparison between the stability constant of the 1:1 complexes of Al3+ with acrylate− and PAA−, respectively, shows the logK value of the latter to be five units higher and is ascribed to particular properties of the polyelectrolyte.