Thermal diffusion in hydrogen-deuterium mixtures

Abstract
The separation produced by thermal diffusion in hydrogen-deuterium mixtures has been measured at temperatures between — 183 and 425° C, and the thermal diffusion ratio R T found. Theoretical values of R T for the same mixture have been calculated for various values of the force index v from Chapman’s general expression appropriate to molecules behaving as point centres of repulsive force F = κr-v . The evaluation of R T in this way is possible only for isotopic mixtures, in which the interactions between like and between unlike molecules are the same. Comparison of the experimental value of with these theoretical values indicates that the fields of the hydrogen and deuterium molecules have an index v = 12.6. This agrees with the value v = 12.5 deduced from viscosity results. It is higher than the value v = 8.5, which is found if the theoretical values of R T for a Lorentz gas are assumed to apply—an assumption necessarily made in the case of the non-isotopic mixtures examined so far. This result suggests that the discrepancies between values of v hitherto deduced from thermal diffusion measurements and those expected from viscosity values are attributable to this assumption. The calculation shows that R T depends slightly on the concentration of the mixture, a point on which the experimental evidence is inconclusive.