Ionic solutions under high pressures VII : mobility of hydrogen ion in water at 15,25 and 40C up to 5000atm

Abstract

The electrical conductivities of hydrochloric acid in water have been measured in the dilute-concentration range of 10^-4 to 10^-3 mol dm^-3 at 15, 25 and 40℃ up to 5000 atm and the limiting equivalent conductivity of hydrochloric acid at high pressure, Λ°^(p) (HCl) has been obtained with the aid of Onsager's equation of conductivity. The ratios, Λ^(p) (HCl)/Λ^(1) (HCl) at 25℃ have a slight concentration dependence in the concentration range of 10^-4 to 10^-2 mol dm^-3. They are compared with the literature values available; Zisman's early data do not seem erroneous. The isotherms of Λ°^(p) (HCl) vs. pressure have a maximum around 3500 atm; the maximum pressure is 3550 atm at 15℃, 3500 atm at 25℃ and 3400 atm at 40℃. The anomalous conductivity of hydrogen ion {λ°^E(p)=Λ°^(p)， (HCl)-Λ°^(p) (KCl)} is obtained by using Λ°^(p) (KCl) in the previous paper. The value of λ°^E(p) continues to rise with increasing pressure up to 5000 atm at each temperature and does so that of λ°^(p) (H^+) estimated from the assumption that the Walden product of bulky monovalent tetrabutylammonium ion is approximately independent of pressure. The present conductivity data and the NMR reorientational correlation time measured by Jonas et al. confirm that the slow step in the proton jump mechanism is the reorientation of water molecules and that λ°^E reflects the anomalous proton mobility better than λ°(H^+). The Debye relaxation equation is inapplicable to real molecular processes in compressed water without any correction parameter. The activation enthalpy and energy at 25℃ for λ°^(E) and λ°(H^+) fall rapidly in the lower pressure range of <～1000 atm. The influence of pressure on the water structure appears very strong at the lower pressures in view of these activation energies. Both activation energies for λ°^(E) and for λ°(H^+) at higher pressures than ～3000 atm diminish very slowly as pressure increases. On the other hand, the activation energy for viscous flow of water calculated from Cappi's results decrease up to ca. 3000 atm and goes up at higher pressures. The fact that λ°^(E) and λ°(H^+) continue to increase up to 5000 atm in the temperature range of 15 to 40℃ seems to indicate that the hydrogen bonds in water are not completely broken down by pressure but persist in some way under the experimental condition.