Conductance of calcium sulphate and magnesium sulphate in aqueous solution at high pressure

Abstract

The conductance of calcium sulphate and that of magnesium sulphate were measured at different temperatures (15–35 °C), pressures (1–1500 bar) and concentrations (up to 10^–4 mol dm^–3). The association constant, K_A, equivalent conductance at infinite dilution, Λ°, partial molar volume change, ΔV, and the distance of closest approach, a, have been calculated. The maximum value of Λ°_p/Λ^°₁ for CaSO₄ and that for MgSO₄ occur at higher pressures than that for KCl; this observation is interpreted in terms of the dissociation of the ion pair at high pressure. With increasing pressure Walden's rule can no longer be applied, owing to a change in size of the hydrated ion. The phenomenon is closely related to the change in the dielectric constant of the solvent and the distance of closest approach, in agreement with the prediction of Bond's electrostatic model. The ion pair of calcium sulphate is more likely to be a contact ion pair than that of magnesium sulphate. Plots of the specific volume as a function of change in partial molar volume at three temperatures extrapolate to a point which may be taken to represent the situation where the structure of water around the ion is similar to that of bulk water.