Chemical and Electrostatic Association of Various Metal Ions by Poly(acrylic acid) and Poly(methacrylic acid) As Studied by Potentiometry (original) (raw)

Potentiometric titrations have been performed for poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA) in solution in the presence of different metals (Ca, Mg, Zn, and Cu) by titrating with KOH without additional salt. For the Me/PAA system, experimentally obtained apparent dissociation constants (pK a) appear to decrease in the initial part of the titration curve, which 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 observed at higher degrees of dissociation. Analytical expressions for the change in apparent pK a upon ionization have been derived on the basis of counterion condensation theory taking into account both electrostatic and chemical binding of counterions of different valences. Taking also into account the effects of the flexibility of the polymer, the agreement between calculated and experimentally obtained data is certainly satisfying for the larger part of the titration curve. The agreement is completely lost by considering only electrostatic interactions between metal ions and the polyanions. An 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 Mg ≈ Ca < Zn < Cu for both polyelectrolytes. The present approach allows us to quantify the contributions of the individual physicochemical processes related to the overall distribution of counterions around charged macromolecules. Introduction Knowledge of association phenomena of metal ions with charged macromolecules is of importance for the understanding of their physicochemical behavior in environmental and biological systems. From an experimental point of view, information is usually obtained with respect to the oVerall distribution of counterions around the charged macromolecules without having the ability to distinguish between different fractions which might be attributed to various underlying interaction processes, like electrostatic and chemical binding. Recently, we have presented an extension of the counterion condensation theory, which takes into account a number of association phenomena of counterions around linear charged polyelectrolytes. 1 The contribution of the territorial and site-specific association processes to the overall distribution has been discussed and illustrated from the theory for a wide range of different chemical conditions, including polyelectrolyte charge density, concentrations of polymer and monovalent and divalent counterions, etc. The proton dissociation of weak polyacids in solutions with its own counterions and with added 1:1 simple salt is one of the physicochemical processes largely studied theoretically and used experimentally. 2-7 However, potentiometric analysis for the case of polyelectrolyte solutions containing a mixture of counterions of different valence has received little attention. 8-13 Ishikawa 9 and Zhang and Nilsson 11 have used the solution of the Poisson-Boltzmann equation to interpret their results, assuming only electrostatic interactions between the polyelec-trolyte and the low molecular ions in the solution. On the other hand, and on the basis of potentiometric experiments, chemical binding of Cu to polymethacrylate polyion was proposed as part