Vapor Pressure Isotope Effects in Liquidand Solid Ammonia

Abstract

Dedicated to Professor Jacob Bigeleisen on the occasion of his 70tli birthdayThe H/D and 14N/15N 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 14NH3, 198.96 K (48.35 torr) for 14ND3, and 195.58 K (48.83 torr) for 15NH3. The isotopic difference in the vapor pressures of NH3 and ND3 at temper atures 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, / it,//gas and /soi//gas, deduced from these results are well represented by Tin (fc/fg) = 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.

Thermodynamic Analysis of Isotope Effects on Triple Points and/or Melting Temperatures

“…The calculated and observed TPIE's agree within the experimental error. H/D and 15N /14N TPIEs and VPIEs for am monia were reported by Ishida and coworkers [39]. The H/D effects are in good agreement with measure ments published in 1962 by Kiss et al [40], Ishida et al determined (In (/T/.^))nh3/nd3 in the region between the triple points.…”

“…The H/D effects are in good agreement with measure ments published in 1962 by Kiss et al [40], Ishida et al determined (In (/T/.^))nh3/nd3 in the region between the triple points. TPIE calculated from the intersections of that fit with the ones for ln(P'/P)s and In (P'/P), is -3.19 + 0.15 K, in good agreement with the value calculated from Table 2, -3 .2 3 +0.15 K. Both re sults, however, are significantly below the directly ob served TPIE, -3.55 K [39,41], and we find no appar ent reason for the discrepancy. Calculated and observed 15N /14N effects also disagree by more than 10"1 K. Appleton and Van Hook [42] reported Ö In (P'/P) for HCN/DCN and HC14N/HC15N isoto pomer pairs but did not measure TPIE.…”

Photochemistry on Pluto: part II HCN and nitrogen isotope fractionation

“…The saturation vapour pressure is also likely to be different for HC 15 N than it is for HC 14 N, but we were not able to find any laboratory measurements of the HC 15 N vapour pressure as a function of temperature. Studies of 15 NH 3 compared to 14 NH 3 find that the saturation vapour pressure of 15 NH 3 is 72-85 per cent lower than the saturation vapour pressure of 14 NH 3 (King et al 1989) de-pending on the temperature. In our simulations, we will assume that a similar difference between the saturation vapour pressures of HC 14 N and HC 15 N exists, and evaluate whether this fractionation could effectively counter the enhanced loss rate of HC 14 N com-pared to HC 15 N due to thermal velocity and greater abundances in the atmosphere.…”

Isotope effects on VLE properties of fluids and corresponding states: Critical point shifts on isotopic substitution

“…and ln(P C /P C ) are cited by Landolt-Bornstein [41]. The vapor pressure IE is known over a broad range, and lower temperature liquid molar density IE data are available [69][70][71]. The value T C = 0.2 K is in poor agreement with the correlation in Fig.…”

“…Also ln(T C /T C ) is known, but not ln(P C /P C ) or ln(ρ C /ρ C ). For NH 3 /ND 3 the vapor pressure IE is known over most of the liquid range, but the molar density IE only between 198 and 290 K [69][70][71]. Two very different values of ln(T C /T C ) have been reported [41,60,72] and Landolt-Bornstein [41] gives values of ln(P C /P C ) and ln(ρ C /ρ C ).…”