Thermodynamic characterization of electrolytic solutions using the Pitzer model: calculations of dilution enthalpy and heat capacity
Résumé
The aim of this study is to compute the dilution enthalpy and the heat capacity of aqueous solutions with the Pitzer's model. Indeed, the dilution enthalpy can be calculated using the apparent relative molal enthalpy, which is the temperature derivative of the excess Gibbs energy. The heat capacity of the solution is computed using the apparent molal heat capacity; the equation is obtained from the temperature derivative of the apparent relative molal enthalpy. Using the excess Gibbs energy defined by Pitzer [1], we obtain one equation for describing the apparent relative molal enthalpy and one for the apparent molal heat capacity. Parameters for these two equations are defined from the specific interaction parameters used to calculate the osmotic coefficient. As a consequence the apparent relative molal enthalpy can be expressed with the first derivative of interaction parameters with respect to temperature. The apparent molal heat capacity can be expressed with a combination of first and second temperature derivative of interaction parameters. Theses equations have been implemented in the Phreeqc-3 software [2] and tested on systems for which the temperature dependence of interaction parameters is known. Monovalent and divalent binary systems have been tested (NaCl, KCl, CaCl2 and MgCl2) as well as ternary systems (NaCl-CaCl2, NaCl-MgCl2 and CaCl2-MgCl2). For the ternary systems, we worked only on the heat capacity because of the lack of experimental data of dilution enthalpy. The parameters for monovalent electrolytes allow calculating data with a good accuracy whereas some discrepancies exist for divalent electrolytes. New interaction parameters were determined for these systems. References [1] K.S. Pitzer, Activity coefficients in electrolyte solutions, 2nd ed., 1991. [2] D.L. Parkhurst, C.A.J. Appelo, User's guide to PHREEQC (Version 2) : a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations, (1999).