DETERMINATION OF ENTHALPY OF IONIZATION OF WATER FROM 250-DEGREES-C TO 350-DEGREES-C

The enthalpy changes at zero ionic strength (AH) for the ionization of water (H2O = H+ + OH-) were determined by flow calorimetry from the heats of mixing of aqueous NaOH and HCl solutions in the temperature range 250 to 350-degrees-C. Pitzer ion-interaction models developed by other workers were used to calculate enthalpies of dilution of aqueous NaOH, HCl, and NaCl solutions for the extrapolation of DELTAH values from the conditions of the experiment to infinite dilution. Equations are derived for thermodynamic quantities (log K, DELTAH-degrees, DELTAS-degrees, DELTAC(p)-degrees, and DELTAV-degrees) for the ionization of water using the DELTAH-degrees values determined in this study from 250 to 350-degrees-C and literature log K and DELTAH-degrees values from 0 to 225-degrees-C. Smoothed values of log K, DELTAH-degrees, DELTAS-degrees, DELTAC(p)-degrees, and DELTAV-degrees are presented at rounded temperatures from 0 to 350-degrees-C and at the saturation pressure of water for each temperature. The equations in the present study provide a better representation of experimental thermodynamic data from 0 to 350-degrees-C than the Marshall-Franck evaluation.