One‐ and two‐phase isochoric heat capacity measurements of aqueous solution of nacl near the critical point of pure water

Abstract

here is a very important theoretical aspect to near-critical and supercritical phenomena in ionic systems such as H20+NaCI T For this system, the Pc-x and Tc-x critical lines show rapid changes in slope near the critical point of pure water (Povodyrev et at., 1998). The derivatives dTc/dx and dPc/dx govern the thermodynamic (crossover) behaviour of fluid mixtures near the critical point of one component (Chang e t al., 1984;Kiselev et al., 1991; Levelt Sengers, 1993;Povodyrev et al., 1997). More precise experimental C , data are needed to confirm the basic ideas of modern crossover theory of critical phenomena in ionic systems. Therefore, our new experimental data C , for H,O+NaCI near the critical point of pure water may prove to be very important for the development of new theoretical equations of state which will more correctly predict the thermodynamic behaviour of ionic solutions near the solvent critical point (pure water) and improved existing models for aqueous electrolytes.A lot of attention has been focused on the system H20+NaCI because of its fundamental importance to the understanding of electrolyte behaviour and because of its importance in many geological and industrial processes. Thermodynamic properties of the H,O+NaCI system are required for the understanding of many hydrothermal processes. The development of an equation of state depends on experimental data, from which empirical parameters can be obtained. NaCl is a good example of a 1 :1 charge-type electrolyte. Theoretical modelling of the H,O+NaCI system will serve as an example for other ionic systems of 1 :1 charge-type electrolytes.The two-phase region of H20+NaCI solutions has been of particular interest because the phase equilibria properties are important in many hydrothermal processes Pitzer, 1985, 1989;Delany et al., 1987). Aqueous NaCl solutions at high temperatures and high pressures are encountered in deep earth, ocean and in electric power plants where an aqueous salt solution is used as the heat transfer fluid. Understanding various geochemical and industrial processes requires a thorough knowledge of thermodynamic properties of aqueous electrolyte solutions such as H20+NaCI.The main objective of the paper is to obtain new reliable experimental isochoric heat capacity data for NaCl in near subcritical and near supercritical water which can be used as a prospective medium in the new