ENTHALPIES OF DILUTION OF AQUEOUS-SOLUTIONS OF NAOH, KOH, AND CSOH AT300, 325, AND 350-DEGREES-C

Dilution enthalpies, measured using isothermal flow calorimetry, are r eported for aqueous solutions of KOH and CsOH at 300 degrees C and 11. 0 MPa, 325 degrees C and 14.8 MPa, and for aqueous solutions of NaOH, KOH, and CsOH at 350 degrees C and 17.6 MPa. Previously collected dilu tion enthalpies for aqueous solutions of NaOH at 300 degrees C and 9.3 MPa and at 325 degrees C and 12.4 MPa were included when fitting the Pitzer parameters. The concentration range of the hydroxide solutions was 0.5-0.02 metal. Parameters for the Pitzer excess Gibbs ion-interac tion equation were determined from the fits of the experimental heat d ata. Equilibrium constants, enthalpy changes, entropy changes, and hea t capacity changes for alkali metal ion association with hydroxide ion were estimated from the heat data. For all systems, the enthalpy chan ges and entropy changes were positive and had accelerating increases w ith temperature. The resulting equilibrium constants show significant, but smaller, increases with temperature.