At high temperatures, the properties of aqueous electrolyte systems differ markedly from those at 25°C. For mixed-electrolyte dilute solutions at high temperatures, the degree of ion-association is sufficiently large that the association equilibrium must be incorporated in any model describing the solutions. These association reactions usually do not occur to a measurable extent at room temperature. Oscarson and co-workers have designed a correlation model based on the excess Gibbs energy which can correlate log K and ∆H values as well as the heats of dilution for aqueous electrolyte systems as a function of temperature (T) and ionic strength (I) from 275 to 350°C. Use of calorimetric data to develop the model has been shown to be more accurate than using ∆dilH values from the variation of log K with temperature because one less differentiation with respect to temperature is required. In this study, the computer program developed by Oscarson and co-workers has been modified by incorporating the IAPWS-95 water equation of state and Archer and Wang's correlation of the dielectric constant of water into the excess Gibbs engergy model. The difference between the present work and prior work is that it uses a more accurate equation of state for water, a more accurate dielectric constant for water and the best equilibrium constants currently available. The properties of water play a very important role in the calculation of ∆dilH values, and the modified program developed here using improved water equations is shown to be superior to the previous one. The results of this modified model were tested by comparing the predicted heats of dilution with experimental measurements from Oscarson's work. These experimental data cover the range from 523.15 K to 623.15 K and 103 bar to 128 bar. The nominal concentrations of the solutions used for the ∆dilH experiments were 0.25, 0.5 and 1.0 m. Equilibrium constants K for Na2SO4 (aq), H2SO4 (aq), NaAc (aq), and HCl (aq) association were taken from conductivity values measured by Wood and co-workers using a flow conductance apparatus. These log K values were used to compare the predicted log K values from Oscarson's model and those from this modified model.



College and Department

Ira A. Fulton College of Engineering and Technology; Chemical Engineering



Date Submitted


Document Type





model, heats of dilution, equilibrium constants, high temperature