A study of the electrolytic solution process using the scaled particle theory. 5. Heat capacity of aqueous ions at elevated temperatures

Abstract

UTPAL SEN. Can. J . Chem. 69,440 (1991). A theory composite of the scaled particle theory and the Born model of solvent continuum has been used to theoretically calculate the standard heat capacity of hydration as well as the partial molal heat capacity of aqueous ions and electrolytes at elevated temperatures. The uncertainties in the second temperature derivatives of solvent dielectric constant at various temperatures present a barrier to an accurate heat capacity prediction by the theory. Nevertheless, the agreement between the predicted standard heat capacity of electrolytes in solution and the corresponding experimental data, particularly at higher temperatures, is encouraging. Moreover, the composite theory seems to provide the most accurate thermodynamic predictions to date for aqueous electrolytes at higher temperatures without involving any arbitrary adjustable parameter. We therefore use this theory to find the proper ionic scale of the partial molal heat capacities at elevated temperatures. [Traduit par la rkdaction] IntroductionThe need for the data related to the thermodynamic behaviour of electrolytes in solution at elevated temperature is obvious, particularly in understanding corrosion problem in electric power industries and the behaviour of minerals in geothermal systems. A few recent experimental efforts to determine high temperature thermodynamic data for aqueous electrolytes suggest that the trend of the thermodynamic behaviour of electrolytes at elevated temperatures is quite unusual and interesting (1-4). However, such data are few and often only partially cover the temperature range between the normal melting point and the critical temperature of water (1-10). There are, of course, a few attempts to theoretically generate high temperature data through empirical equations and through using simplified models (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29). Horvath (30) compiled some of the advances made in the various attempts for theoretical calculation of entropy and heat capacity of aqueous at various temperatures. Recently, Tanger and Helgeson (31) revised their earlier empirical approach (29) of thermodynamic calculations by introducing additive correction terms to the effective electrostatic radii of aqueous ions to account for the pressure and temperature dependence of the solvent structure and presented the calculated values of various thermodynamic functions of aqueous ions at various temperatures and pressures (30). However, the arbitrariness of the correction terms could not be altogether done away with in spite of generous computer applications.Very recently, Tanger and Pitzer (32) revised an earlier semi