Acidity Measurements at Elevated Temperatures. VII. Dissociation of Water

The self-dissociation of water has been studied over the temperature range from 0 to 300 degrees C and in KCl media from 0.02 m to 2.7 m. Also, isothermal pressure coefficients of the dissociation quotients have been obtained in these same media up to 250 degrees C. A potentiometric method employing a hydrogen electrode concentration cell with flowing solutions was employed. The estimated accuracy of log Q(w) values tip to 250 degrees C is 0.02 log units and at 300 degrees C is 0.04 log units. Smoothing functions have been found which fit these data along with the precise potentiometric data of Harned and co-workers at low temperatures, the existing calorimetric data up to 55 degrees C and the recent conductimetric measurements of pure water up to 271 degrees C by Bignold et al., within about 1,5 times the estimated errors. Thermodynamic quantities for the dissociation reaction have been tabulated for rounded values of temperature and ionic strength at the saturation pressure of water. The isothermal pressure coefficients of log Q(w) varies approximately linearly with the square root of the ionic strength. This and the dependence of log K(w) on the density of the water is consistent with the assumption that the molal volumes of aqueous ions vary linearly with the compressibility coefficient of water. The heat for the dissociation reaction at infinite dilution is also shown to be strongly dependent on density.