In this article, a numerical procedure is proposed for prediction of activity and osmotic coefficients at elevated temperatures based on the vapor pressure lowering of electrolyte splutions available at room and intermediate temperatures.The vapor pressure lowering of electrolyte solutions can be correlated over a wide temperature range by equation, In AP=A+B/T+C(ln T-T/2TC), where A, B and C are adjustable parameters and T, is the critical temperature of pure water. The procedure is illustrated in the cases of KCI, CaCl, and especially LiCl solutions. An analytical expression for osmotic coefficients in the Meissner approach is derived by an integration of the Bjerrum equation.Thermodynamic properties of aqueous solutions of strong electrolytes at high temperatures are of considerable importance in many areas of chemical industry, in geochemistry, in desalination and steam power generation. However, with few exceptions, the available experimental data are confined to temperatures which are not far from room temperatures. In recent years, our knowledge about activity and osmotic coefficients comes mainly from isopiestic measurements performed at elevated temperatures by the Holmes and Mesmer group. Unfortunately, these measurements constitute only a small part of industrially important electrolyte-water systems and therefore procedures for prediction of activity and osmotic coefficients must be considered. Since in the case of electrolyte solutions, a linear temperature dependence of the coefficients is rarely observed, ordinary interpolation and extrapolation techniques have rather limited value. For example, if one molal solution is considered, osmotic coefficients as a function of temperature in the 273-573 K range have a distinct maximum for NaCI, KCl and CsC1, they decrease monotonically for LiCl and for CaCl, remain practically constant up to about temperature of 373 K but after they decrease with temperature increasing (Lindsay and Liu, 1971 ;Holmes et al., 1978;Holmes and Mesmer, 1981).In our previous works on the subject, estimation procedures were proposed for activity and osmotic coefficients at elevated temperatures based on data at one or two temperatures (Apelblat et al., 1988) and on the application of the Moriyama rule (Apelblat et al., 1987). In this work, it is considered a new extrapolation technique to predict activity and osmotic coefficients which is based on plentiful data available at room and . intermediate temperatures (for example, Robinson and Stokes, 1965;Hamer and Wu, 1972;Goldberg, 1981; Lob0 and Quaresma, 1989). The proposed numerical procedure is illustrated in the cases of KCI, CaCl, and especially LiCl solutions (lithium chloride is very soluble in water)..
Representation of Osmotic and Activity Coefficients as a Function of Temperature and Concentration