Electrical conductivity of CaSO4 in aqueous solution under high pressure

Abstract

The electrical conductivity of aqueous solutions of 2-2 electrolytes, CaSO_4 and MgSO_4, has been measured within the ranges, 15～40℃, 1～1, 200kg/cm^2 and 10^-3～10^-4_M. The plots of Λ vs C^1/2 under high pressure show Kohlraush's linear relation in this concentration range. The equivalent conductance at infinite dilution Λ° has a maximum against pressure. The pressure at this maximum point has been found to be higher than that of the minimum viscosity of water at each temperature. The hydration numbers of Ca^2+, Mg^2+ and SO_4^2- -ions calculated by the Robinson-Stokes method are little changed by pressure and/or temperature. The dissociation constants of Ca^2+・SO_4^2- and Mg^2+・SO_4^2- ion-pairs are increased by pressure and decreased by temperature. The thermodynamic parameters, <ΔG>^^^-°, <ΔV>^^^-° <ΔH>^^^-° and <ΔS>^^^-°, are calculated from the coefficients of pressure and temperature of the dissociation constants. The closest approach distances of the ion-pairs are also estimated by using the theoretical equation of Fuoss. These ion-pairs would be solvent-separated ones, containing some water molecules between the cation and anion, and approach contact ones, liberating the water molecules with increasing temperature.