Thermal Diffusion and Self-Diffusion in Ammonia

The temperature dependence of the isotopic thermal-diffusion factor α0 for ammonia has been investigated with the help of an equimolar mixture of 14NH3 and 15NH3. A 10-tube metal swing-separator as well as a two-bulb glass apparatus were used for these measurements. The value of α0 increases with temperature, its value being 0.00±0.02 at 242°K to 0.09±0.03 at 496°K, in disagreement with the values obtained by Watson and Woernley, who observed a definite inversion in the sign of α0 at temperature ∼290°K. The present set of observed values of α0 also disagrees with the theoretical calculation of Monchick and Mason. Possible causes of this disagreement are: (1) inadequacy of the potential model, (2) neglect of inelastic collisions, and (3) formation of dimers. The self-diffusion coefficient of ammonia has also been studied in the temperature range of 233° to 353°K with a two-bulb apparatus. There is marked disagreement between observed and calculated values at low temperatures. This is also explained in terms of dimerization.