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

RODOLFO D. PORASSO; JULIO C. BENEGAS; MARC A. G. T. VAN DEN HOOP

JOURNAL OF PHYSICAL CHEMISTRY B - (Print)

AMER CHEMICAL SOC

Año: 1999 vol. 103 p. 2361 - 2365

1520-6106

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 (pKa) 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 pKa was observed at higher degrees of dissociation. Analytical expressions for the change in apparent pKa 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 theintrinsic reduced free energy of binding (gb) 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.