The Triple-point Temperature of Pure Equilibrium Deuterium

A study specifically intended to measure the triple-point temperature of both n-D2 and e-D2 containing a volume fraction of HD not higher than 0.0001 has been completed. It was performed by measuring samples of gas in metal sealed cells. The cells, after being checked for a non-increasing HD contamination within the stated limits, were filled by directly connecting them to the outlet of a thermal-diffusion column for isotopic separation, known to produce D2 with a HD volume fraction not larger than 0.00003. In the case of e-D2, additional studies were performed on the effect of deuterating the catalyst or on high-temperature activation of the catalyst. Results proved that it is possible to obtain good results for e-D2 with partially deuterated catalyst, but this does not allow a reliable control of the contamination. An alternative procedure to full deuteration has been found, which uses commercial hydrous ferric oxide heated to about 400 °C, and commercially available deuterium, produced by isotopic separation and showing a purity better than 99.99%. Temperature values (18.7327 ± 0.0003) K for n-D2 and (18.6983 ± 0.0003) K for e-D2 on NPL-IPTS-68 were obtained at the liquidus point; the best thermodynamic values for these triple points are (18.7232 ± 0.001) K and (18.6888 ± 0.001) K respectively; e-D2 can be used as a first-quality temperature reference point.

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