Some thermodynamic properties of liquid ammonia and trideuteroammonia

Article abstract of DOI:10.1016/0021-9614(87)90054-1

The orthobaric density of liquid NH3 has been measured from about 200 to 287 K, and for liquid ND3 from about 205 to 273 K.The molar volume of liquid NH3 exceeds that of ND3 by between 0.8 and 0.9 per cent.The difference of the vapour pressures of the two compounds has been measured from about 200 to 266 K, and the vapour pressure of NH3 from the triple-point temperature to 234 K.Liquid NH3 has the higher vapour pressure, the difference being relatively large for a pair of isotopic compounds.At 200 K, the ratio of the vapour pressure of NH3 to that of ND3 is about 1.2.The available vapour pressures for NH3 have been fitted to a Wagner equation.By combining vapour pressures derived from this equation with the differential measurements, values for the vapour pressure of ND3 have been obtained.These values have likewise been fitted to a Wagner equation.The material presented in this paper has been used to estimate the molar enthalpies of vaporization of NH3(l) and of ND3(l) from the triple-point temperatures to 290 K.The molar enthalpy of vaporization of ND3 exceeds that of NH3 throughout this range.The difference amounts to about 3.5 per cent at the triple-point temperatures, and decreases with rising temperature.