Experimental measurements of the osmotic and activity coefficients, the enthalpy, and the heat capacity were used to derive a semiempirical equation for the thermodynamic properties of NaCl(aq) at constant pressure. This equation may be combined with results contained in the previous paper on the volumetric properties to yield a complete equation of state valid in the region 273 K≤T≤573 K, saturation pressure ≤P≤1 kbar, 0≤m≤6.0 mol kg−1. It is shown that this equation may be extrapolated to higher solute molalities at lower pressures. An estimation of uncertainties in various quantities is given. Tables of values for various thermodynamic properties are presented in the appendix.
The determination of the activity coefficient and related properties of sodium bicarbonate presents special problems because of the appreciable vapor pressure of C02 above such solutions. With the development of reliable equations for the thermodynamic properties of mixed electrolytes, it is possible to determine the parameters for NaHC03 from cell measurements or NaCl-NaHC03 mixtures. Literature data are analyzed to illustrate the method and provide interim values, but it is noted that further measurements over a wider range of concentrations would yield more definitive results. An estimate is also given for the activity coefficient of KHC03.
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