Deviations from ideal behavior in isotopic mixtures of ammonia

The vapor pressure isotope effect between samples of isotopically substitut ed ammonia, NH3, ND3 and (NH3)-N-15, and their mixtures was measured as a f unction of temperature (225<T/K<259) and mixture composition. The p(x,T) re sults. were used to calculate the excess molar Gibbs energy function of the mixtures. The G(E)(x,T) data were fitted to a temperature dependent Redlic h-Kister equation and compared with theoretical calculations based on a mod ified version of the theory of isotope effects in mixtures. The theoretical estimates are in good agreement with the experimental data, confirming the mechanisms responsible for the deviations from ideality in such "almost id eal" systems. (C) 2001 American Institute of Physics.