Para triple point

Properties of Light and Heavy Hydrogen. Vapor pressures from the triple point to the critical point for hydrogen, deuterium, tritium, and the various diatomic combinations are Hsted in Table 1 (15). Data are presented for the equiUbrium and normal states. The equiUbrium state for these substances is the low temperature ortho—para composition existing at 20.39 K, the normal boiling point of normal hydrogen. The normal state is the high (above 200 K) temperature ortho—para composition, which remains essentially constant. 

Not all of them are equally suitable as fixed points for thermometry. In (b) and (c), the points are characterized by a high equilibrium pressure and may be better considered as pressure fixed points (see, for example, ref. [9]). Types (d) and (e) are not generally referred to as triple points, but as solid-solid and liquid-liquid transitions, respectively. At low temperature, the last solid-liquid-vapour triple point is that of hydrogen (para 99.996%) at 13.8033 K. 

The relative amounts of the ortho and para forms are temperature dependent, as shown in Table 2.2. Most of the physical properties of hydrogen and deuterium, such as vapor pressure, density of the liquid, triple point temperature and pressure, etc., are mildly dependent upon the ortho-para composition. The transition from one form to the other involves a substantial energy change, as shown in Table 2.3. 

The triple points, boiling points, and critical constants for the isotopic forms of hydrogen and for several ortho-para concentrations are given in Table 2.4, and vapor pressure values are shown in Table 2,5. The vapor pressure equations of Table 2.5 agree with the experimental data for hydrogen to within the experimental error over most of the liquid range to the critical point. The values for deuterium and hydrogen deuteride should not be used at pressures much above atmospheric. 

Table 4.4 Physical properties of para-hydrogen (p-Ha) and normal-hydrogen (n-H2) at the triple- and normal boiling point...

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