Thermodynamics of aqueous sodium sulfate

The activity coefficient of saturated aqueous Na₂SO₄ is calculated from the properties of the solid and the infinitely dilute solution as well as the solubility. These values are compared with those given by the equation of Rogers and Pitzer which is based on the measured dependence of heat capacity upon molality together with other solution properties at low temperature. Excellent agreement is found from 30 to 280°C. Consequently the equation of Rogers and Pitzer is given an extended range of validity to saturated molality and to 280°C. The trend of solubility with temperature is discussed in relation to the C_p of solution.