The spccific conductivity of high-purity water has been measured at temperatures between 51 and 271°C along the saturated vapour pressure curve. Correction has been made for traces of contamination. The frequency dispersion of the impedance of the water-filled cell has been analyzed in terms of an equivalent electrical circuit. The data have been used to calculate the ionic product constant of water over this temperature range.Accurate values of the dissociation constant K, for water are required when setting up thermodynamic diagrams, and pH scales, and for kinetic studies involving hydrogen or hydroxyl ions. Clever ' and Holzapfel have reviewed the experimental determinations of K,. The measurements of Harned and Robinson provide the generally accepted values of K, from 0 to 60°C. At higher temperatures, up to 306"C, K, has been determined, along the saturated vapour pressure curve, by Noyes, et aZ.,4 A~kerrnann,~ Dobson and Thirsk and Mesmer, Baes and Sweeton.' In the single-phase water region, David and Harnann 8 9 and Hamann and Linton l o * l 1 determined IC, at pressures up to 13.3 GN m-2 (133 kbar), and temperatures up to 804"C, while Holzapfel and Franck measured the dissociation of supercritical water at 500,750 and 1000°C. Quist has reported extensive measurements between 300 and 800°C (at pressures up to 4000 bar).There is considerable technological interest in the intermediate temperature range 60 to 370"C, which requires that relevant values of K, are established. This paper presents values of K,, along the saturated vapour pressure curve, in the range 51-271"C, which were obtained from the experimental measurement of the specific conductivity of pure water. were derived from e m f . measurements using cells of the form : H2, Pt/HCI/AgCI, Ag ; H2, Pt/NaOH, NaCI/AgCl, Ag. The extension of this method to high temperatures is an attractive possibility in view of the work in acid solutions by Lietzke and Stoughton l4 ; it has been used with some success (Dobson and Perkovets and Kryukov '9. It is, however, prone to difficulties associated with the establishment of the Ag/Ag,O equilibrium in mixed hydroxide/chloride systems (Case and Bignold 16) and the tendency for hydrogen to be oxidized on the silver surface at higher temperatures.The higher temperature values of Noyes et aL4 were derived from the conductimetric determination of the hydrolysis and dissociation constants of ammonium acetate, ammonium hydroxide and acetic acid. These values are subject to correction in the light of modern theories of ionic solutions (Fisher 17) but the method is clearly capable of producing reliable data.The determination of K,, at low temperatures, by the direct measurement of the specific conductivity ofpure water (Kohlrausch and Heydweiller l'), is subject to errors
