Equilibrium and kinetic isotope effects

Abstract

AbstractThe equilibrium distribution effects of deuterium and of tritium between liquid water, hydrogen gas and palladium hydride differ from one another in a characteristic manner. They are discussed in terms of zero point energies.Direct measurements of the deuterium distribution between liquid water and palladium hydride yielded an uncommonly high equilibrium separation factor of about 9.2 at room temperature. This value is in good agreement with the calculation. The corresponding value for the tritium distribution is calculated to be about 12.From equilibrium distribution measurements of tritium between hydrogen and β palladium hydride the β phase isotherms of the pure palladium tritium system could be obtained. The principal vibration frequency of the triton in the palladium lattice works out as 7.1 · 1012 sec−1 at 25 °C as to be expected.Combining the isotope effects of hydrogen diffusion in palladium ‐ which are discussed here ‐ with the equilibrium isotope effects mentioned above, one can calculate the ratio of the diffusion fluxes of two hydrogen isotopes in palladium electrodes.In the case of anodic dissolution the predicted anodic hydrogen/deuterium separation factor of about 1.7 is confirmed by direct measurements and nearly independent of temperature.The cathodic separation factor, however, which governs the ratio of the H/D fluxes in the case of cathodic deposition, is calculated much to low compared with the experiment, which yielded a value of nearly 10 at 25 °C. The discrepancy is based on an additional kinetic isotope effect of the surface reaction.