Ionic solutions under high pressures II : pressure and temperature effects on the dissociation of the Co(NH3)6 3+ ・SO4 2- ion-pair

Abstract

The dissociation constant of the Co(NH_3)_6^3^+.SO_4^2^- ion-pair in water has been determined at high pressures up to 5, 000 aim at the temperatures of 15°, 25°and 40℃. using the previous electrochemical data. Then the closest approach distances of the ion-pair, α_D_-_R, α_F and α_S, were calculated from the theoretical equations of Denison-Ramsey, Fuoss and Bjerrum, respectively. All of them decrease almost linearly with increasing pressure. This decrease in α's with pressure could be understood, considering that these ion-pairs are solvent-separated ones in which both cations and anions are already extensively solvated in the loosely associated state. And this decrease in α turns out to be the reason why there appears a plateau in the curve of log K vs pressure above 2, 500 atm. Therefore, it would be concluded that pressure affects not only upon the solvation energies of the free ions through the dielectric constant of the solvent but also upon the Coulombic interaction energies between the cations and the anions in the ion-pairs through the closest-approach distance.