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.
Received: 14 April 1998 Accepted: 23 April 1998 |