M-2729 J. Chem. Thermodynamics 1992, 24, 993-1000 Vapour pressure of trideuterioammonia J. C. G. CALADO, J. N. C. LOPES, and L. P. N. REBELO Centro de Qulrnica Estrutural, Complexo L Instituto Superior Tkcnico, 1096 Lisboa Codex, Portugal (Received 11 October 1991; in final form 14 February 1992) The H-to-D vapour-pressure isotope effect in liquid ammonia has been measured at 62 temperatures between 228 K and 260 K. The vapour pressures, corrected to 100 per cent nuclidic purity, have been fitted to the equation: T In r = A + BIT + CT, where r is the vapour- pressure ratio p(NH3)/p(ND3). The fit yielded the parameters: A - - 8.22508 K, B = 12338.2 K 2, and C = -0.05544. Comparisons with the results of other authors were made in order to clarify some discrepanciesfound in the literature. Our values are in accord with the previous results of King et al, and an extrapolation of the fitted equation down to the triple- point temperature gave good agreement with the published results. The fitted equation was used in conjunction with the Clapeyron equation to calculate the difference in the molar enthalpies of vaporization between NH 3 and ND 3. At T = 230K that difference is - 846 J. tool 1 decreasing to - 747 J. mol 1 at ~260 K. 1. Introduction Recently we started an investigation on the thermodynamics of a variety of nuclidically pure species and their mixtures. We have already reported results for the difference between the vapour pressures of methane and tetradeuteriomethane. ~1) Ammonia and trideuterioammonia exhibit an unusually large isotope effect and thus constitute a suitable system to study from an experimental point of view. However, significant discrepancies were found between the experimental results reported by different authors, ~2 6) for the saturation vapour pressure of the liquid. Theoretical studies have been made very recently ~z) in the light of Bigeleisen's theory of vapour-pressure isotope effects. It thus seemed worthwhile to carry out accurate measurements of the vapour pressure p(NH3) of ammonia and of the difference iAp=p(NH3)--p(ND3)} between the vapour pressures of ammonia and trideuterioammonia, from T = 228 K to 260K, as an attempt to sort out the discrepancies between the published values. The results will also be used in a study of {xNH 3 + (1-x)ND3} now in progress. The differences A(A~Hm) of the molar enthalpies of vaporization between the two nuclidic species were also calculated. 0021 9614/92/090993+08 $02.00/0 © 1992 Academic Press Limited