Solubility Isotherms of Tritium in Palladium at Low Equilibrium Pressures

The phenomenon of hydrogen absorption by metals, discovered about 100 years ago, nowadays demands some interest for technological reasons (hydrogen storage, fuel cells, hydrogen embrittle­ ment of construction materials). The palladium/hydrogen system claims for an exceptional position among the metal hydrides of the protonic type: because of its relative simplicity the Pd/H2-system serves as a model revealing some discernible thermodynamic features, that govern the behaviour of those systems. From measurements of Nernst [1] and Frieske [2] the systems Pd/Ü2 and Pd/Ü2 are well known, whereas the Pd/T2-system is widely unknown; from separation factor measurements with traces of tritium [3] in the hydrogen-rich /5-phase of the Pd-hydride one can calculate the /?-PdTn isotherms [4]. Direct measurements on the PdTn-system (n = T/Pd) at low tritium pressures were performed by Favreau et al. [5] (temperature region 200 to 400 °C); the results of this research, however, are not in agreement with what can be extrapolated from the PdHnand PdDn-systems, assuming a normal run of the isotope effect. For this reason and since, because of the known tritium permeation and separation problems (nuclear power plants), the palladium-tritium system may become of particular interest, the solubility of 3H in Pd at low pressures (1.3 to 50 mbar) and tem­ peratures (25 to 70 °C) was analysed again [6]. For this purpose an apparatus was constructed permit­ ting the exact determination of absorptionand desorption-isotherms with only about 2 cm3 stp of tritium gas.