Chemical and electrostatic association of various metal ions by poly(acrylic acid) and poly(methacrylic acid) as studied by potentiometry

Potentiometric titrations have been performed for poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA) in solution in the presence of different m etals (Ca, Mg, Zn, and Cu) by titrating with KOH without additional salt. F or the Me/PAA system, experimentally obtained apparent dissociation constan ts (pK(a)) appear to decrease in the initial part of the titration curve, w hich was found to be most pronounced in case of Cu. This was also found for the Cu/PMA system. For all systems studied, an increase in pK(a) was obser ved at higher degrees of dissociation. Analytical expressions for the chang e in apparent pK(a) upon ionization have been derived on the basis of count erion condensation theory taking into account both electrostatic and chemic al binding of counterions of different valences. Taking also into account t he effects of the flexibility of the polymer, the agreement between calcula ted and experimentally obtained data is certainly satisfying for the larger part of the titration curve. The agreement is completely lost by consideri ng only electrostatic interactions between metal ions and the polyanions. A n increase in chemical binding as indicated by a more negative value of the intrinsic reduced free energy of binding (g(b)) is observed in the order M g approximate to Ca < Zn < Cu for both polyelectrolytes. The present approa ch allows us to quantify the contributions of the individual physicochemica l processes related to the overall distribution of counterions around charg ed macromolecules.