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 h eats of mixing of aqueous NaOH and HCl solutions in the temperature ra nge 250 to 350-degrees-C. Pitzer ion-interaction models developed by o ther workers were used to calculate enthalpies of dilution of aqueous NaOH, HCl, and NaCl solutions for the extrapolation of DELTAH values f rom the conditions of the experiment to infinite dilution. Equations a re 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 2 50 to 350-degrees-C and literature log K and DELTAH-degrees values fro m 0 to 225-degrees-C. Smoothed values of log K, DELTAH-degrees, DELTAS -degrees, DELTAC(p)-degrees, and DELTAV-degrees are presented at round ed temperatures from 0 to 350-degrees-C and at the saturation pressure of water for each temperature. The equations in the present study pro vide a better representation of experimental thermodynamic data from 0 to 350-degrees-C than the Marshall-Franck evaluation.