Isopiestic Measurements at High Temperatures and Concentrations: IV. The Ternary System CsCl–MgCl2–H2O at 428 K

Abstract Isopiestic molalities of the system CsCl–MgCl 2 –H 2 O at 428 K are reported. The isopiestic apparatus and details of the measuring procedure are described. MgCl 2 served as reference electrolyte. Its water activity was calculated with the extended Pitzer model of Wang and Pitzer [J Phys Chem Ref Data 27(5):971–991, 1998]. For the binary system, CsCl–H 2 O osmotic coefficients have been fitted at 298, 373, and 428 K using data from literature and of this work. The variation of the rational activity coefficient of water with temperature and concentration is explained by the effect of structure-breaking, ion association, and hydration. The isopiestic molalities in the ternary system show strong positive deviations from Zdanovskii’s rule. Empirical interpolation equations are applied to calculate the water activity at given total molality and mole fraction of CsCl. At molar ratios H 2 O/MgCl 2 < 8, water-anion exchange in the coordination sphere of Mg 2+ governs the variation of the water activity.