A relation between the critical properties of aqueous salt solutions and the heat capacity of the solutions near the critical point using a single-fluid corresponding-states theory

Abstract

The effects of added salts on the heat capacity of water at constant pressure are extremely large near the critical point of water. A corresponding-states theory for the heat capacity of aqueous salt solutions is proposed. This theory is based on the assumption that the major effect of the salt is to shift the critical point of the water in the solution. Since the water has an infinite heat capacity at constant pressure at the critical point, this shift causes the very large effects observed. Tests of this theory with the available data on NaCl show that the theoretical predictions are surprisingly accurate. The heat capacity of the solution is predicted with an average error of ± 0.7 per cent and a maximum error of 3.4 per cent at temperatures from 320 to 600 K, molalities from 0.1 to 3.0 mol·kg −1, and at a pressure of 17.7 MPa. A similar equation for the enthalpy of the solutions is not accurate, and there are insufficient data in the literature to test the corresponding equations for thermal expansivity, compressibility, and the difference between heat capacities at constant pressure and at constant volume. Since the equation has been tested only for NaCl solutions at temperatures below the critical point, there is a pressing need for more experimental results. The theory predicts very large effects on the heat capacity of any solution of a strong electrolyte near the critical point of the solvent and that these effects can be predicted from a knowledge of the critical temperature and critical pressure of the solutions.