Excess thermodynamic properties of aqueous electrolytes to high temperatures and pressures

Excess thermodynamic properties have been obtained for a number of pure and mixed aqueous electrolyte solutions at high temperatures and pressures through measurements of isopiestic ratios and enthalpies of dilution. The equipment and techniques used are described. Representations of the results used to correlate measured values and calculate other thermodynamic quantities are described. Isopiestic ratios relative to NaCl(aq) standard solutions have been measured for a number of classes of electrolytes to 250 C, including the alkali and alkaline earth chlorides, alkali sulfates and bisulfates, and a number of mixed electrolyte solutions. Flow calorimetric measurements of dilution enthalpies have been made as functions of temperature and pressure to 400 C and 400 bars on NaCl(aq), CaCl2(aq) and NaOH(aq). Results extend across the temperature range of transition between strong electrolyte behavior, as indicated by results at low temperatures, and ion association at high temperatures, as has been indicated in studies of electrical conductance. Modeling of the experimental results with other available data has given consistent sets of values of excess thermodynamic properties for a number of aqueous electrolyte systems.