Proton and metal ion binding to natural organic polyelectrolytes—I. Studies with synthetic model compounds

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 = p K a +p( ΔK a ) + log( α/1 − α) where K a is the intrinsic acid dissociation constant of the ionizable functional groups on the polymer, ΔK a 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 p K a 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( ΔK a )) 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 p K a of a cross-linked PMA gel, p K a = 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 ( β CoPMA+ = 11) was found to be in agreement with the linear polyelectrolyte analogue and the published data for cobalt-carboxylate monodentate complexes.