THERMODYNAMICS OF AQUEOUS ELECTROLYTES AT HIGH TEMPERATURES BY THE ISOPIESTIC TECHNIQUE

Excess thermodynamic properties of a wide variety of aqueous electrolyte solutions at elevated temperatures have been determined with the ORNL high-temperature isopiestic facility. This unique experimental apparatus provides very precise results over the useful temperature range of 110 to 250°C. The isopiestic method is a comparative technique which, in the present case, uses the Pitzer-Peiper-Busey formulation for NaCl(aq) as the reference electrolyte. At elevated temperatures the ion-interaction model of Pitzer continues to be the most useful description of the experimental results. Our experimental program has included the alkali metal chlorides, sulfates, hydroxides, bromides, and bisulfates, the alkaline earth metal chlorides, and several specific compounds of scientific and practical interest. Numerous common-ion mixed electrolyte solutions have also been investigated in this program. In general, the activity coefficients of all the studied electrolytes decrease with increasing temperature. However, prominent differences between members of the same family remain, particularly when considered as the excess Gibbs free energy. The tendency for ion association increases with increasing temperature and becomes a factor at the higher temperatures. Trends and specific effects are illustrated with examples taken from the database generated in our high-temperature thermodynamic program.