Potentiometric and conductometric analysis of the binding of Barium ions with alkali polyacrylate

Abstract

 The interaction between Poly(acrylic acid) and barium ions is analyzed using potentiometric (barium specific electrode) and conductometric titrations. At full ionization of the polyelectrolyte, in the presence of M⁺ counterions (M⁺=Li⁺, Na⁺ and K⁺), the binding ratio [Ba²⁺]_bound/C _p on the chain is determined, showing no significant difference between the three alkali ions. When the added Ba²⁺ concentration does not exceed 0.2×C _p, all barium ions bind with the polymer, i.e. none can be detected in the solution with the barium selective electrode. Assuming that monovalent counterions divide the electrostatically condensed and “atmospheric” ions and using Eisenberg plots of the conductivity excess, the experimental data allows to calculate the distribution of the different acrylic species on the fully deprotonated chain (free carboxylate groups, bound groups with M⁺ and with Ba²⁺ ions). Assuming the formation of a bidentate Ba(COO)₂ species and taking into account that part of the remaining groups bind with M⁺ ions, the calculated complexation constant (log K _c=6.5) is satisfactorily independent of the complexation ratio. The displacement ratio of M⁺ ions by attaching Ba²⁺ ions is also calculated, showing interestingly a continuous decrease between 1.4 and 0.9 as r increases. The latter result is attributed to the change of the averaged electrostatic potential of the chain, in relation with the binding of barium ions.