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

The interaction between Poly(acrylic acid) and barium ions is analyzed usin g potentiometric (barium specific electrode) and conductometric titrations. At full ionization of the polyelectrolyte, in the presence of M+ counterio ns (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 i ons. When the added Ba2+ concentration does not exceed 0.2 x C-p, all bariu m ions bind with the polymer, i.e. none can be detected in the solution wit h the barium selective electrode. Assuming that monovalent counterions divi de the electrostatically condensed and. "atmospheric" ions and using Eisenb erg plots of the conductivity excess, the experimental data allows to calcu late the distribution of the different acrylic species on the fully deproto nated chain (free carboxylate groups, bound groups with M+ and with Ba2+ io ns). Assuming the formation of a bidentate Ba/COO)(2) species and taking in to account that part of the remaining groups bind with M+ ions, the calcula ted 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 betwe en 1.4 and 0.9 as r increases. The latter result is attributed to the chang e of the averaged electrostatic potential of the chain, in relation with th e binding of barium ions.