Thermal Diffusion with Ammonia

With a "two-bulb" thermal diffusion experiment in which is used NH₃ having about a 15 percent N¹⁵ content to increase the accuracy of the mass spectrometer analyses, it has been found that with decreasing temperature the thermal diffusion constant α of ammonia changes from + to - values at about room temperature. The value of α varies linearly with the logarithm of the absolute temperature, the rate of decrease being, however, nearly eight times that for neon and argon. A qualitative discussion is presented, attributing this effect largely to the strong first-order dipole-dipole intermolecular forces which are proportional to 1R⁴.