Water Activity Measurement and Thermodynamic Modeling for the System Na2CO3–Cs2CO3–H2O at 308.15 K

The isopiestic equilibria in the system Na2CO3–Cs2CO3–H2O at 308.15 K were investigated with an improved isopiestic method with a modified gravimetric isopiestic apparatus. The water activities and osmotic coefficients were determined based on the calcium chloride standard reference solution. The equilibrium vapor pressure was calculated with the water activity. Combining the Pitzer binary parameters of Na2CO3, the Pitzer binary parameters β(0), β(1), and Cφ of Cs2CO3 and Pitzer mixing parameters θNa+, Cs+ and ψNa+, Cs+, CO32– were fitted with the experimental osmotic coefficients on the basis of the Pitzer model. The water activities were then calculated by the Pitzer parameters, and the good agreement between the calculated water activities and experimental results of this ternary system indicated that the Pitzer parameters obtained in this work were reliable. The thermodynamic data and Pitzer parameters obtained in this work can provide fundamental thermodynamic data for development of universal chemical models for brine systems containing cesium carbonate.