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The H/D and  14 N/ 15 N vapor pressure isotope effects in liquid and solid ammonia have been measured at temperatures between 163 K and 243 K. The isotopic vapor pressure data have been fitted to T ln (P′P) = A/T- B for the liquid/liquid, liquid/solid and solid/solid ranges of temperature. The triple points are; 195.41 K (45.49 torr) for  14 NH 3 , 198.96 K (48.35 torr) for  14 ND 3 , and 195.58 K (48.83 torr) for  15 NH 3 . The isotopic difference in the vapor pressures of NH 3  and ND 3  at temperatures between 195.41 K and 198.96 K is nearly independent of temperature within the present experimental uncertainty. The phase ratios of the reduced partition function ratios, f liq /  f gas  and f sol /f gas , deduced from these results are well represented by Tin (f c /f g ) = A/T-B. Molecular forces in the liquid and solid ammonias are discussed using a simple cell and a 4-molecular unit cell model, respectively. The librational motions in the liquid are almost as highly hindered as they are in the solid, but the directionality of the external forces on nitrogen atoms in liquid ammonia is not as well defined as in the solid.

Vapor Pressure Isotope Effects in Liquidand Solid Ammonia