| Abstract: | A unified physico-chemical model, based on a modified Henderson-Hasselbalch equation, for the analysis of ion complexation reactions involving charged polymeric systems is presented and verified. In this model pH = pKa+p(ΔKa) + log(α/1 − α) where Ka is the intrinsic acid dissociation constant of the ionizable functional groups on the polymer, ΔKa is the deviation of the intrinsic constant due to electrostatic interaction between the hydrogen ion and the polyanion, and alpha (α) is the polyacid degree of ionization. Using this approach pKa values for repeating acidic units of polyacrylic (PAA) and polymethacrylic (PMA) acids were found to be 4.25 ± 0.03 and 4.8 ± 0.1, respectively. The polyion electrostatic deviation term derived from the potentiometric titration data (i.e. p(ΔKa)) is used to calculate metal ion concentration at the complexation site on the surface of the polyanion. Intrinsic cobalt-polycarboxylate binding constants (7.5 for PAA and 5.6 for PMA), obtained using this procedure, are consistent with the range of published binding constants for cobalt-monomer carboxylate complexes. In two phase systems incorporation of a Donnan membrane potential term allows determination of the intrinsic pKa of a cross-linked PMA gel, pKa = 4.83, in excellent agreement with the value obtained for the linear polyelectrolyte and the monomer. Similarly, the intrinsic stability constant for cobalt ion binding to a PMA-gel (βCoPMA+ = 11) was found to be in agreement with the linear polyelectrolyte analogue and the published data for cobalt-carboxylate monodentate complexes. |
