The Difference in Vapor Pressures of Ortho and Para Deuterium

The difference between the vapor pressures, ΔP(e — n), of the 20.4°K equilibrium mixture and the normal mixture of the ortho and para varieties of D2 were determined from 15° to 20.4°K. ΔP(e — n) varied from 0.3 mm of Hg at 15°K to 3.8 mm at 20.4°. ΔP(e — n) for deuterium is small as compared with ΔP(e — n) hydrogen, but ΔP(orthopara)/P(n) for deuterium is approximately equal to ΔP(para-ortho)/P(n) for hydrogen at the same temperature. Further measurements were made on the uncatalyzed change with time of the vapor pressure of liquid normal deuterium. The change for deuterium is less than one mm of Hg in 200 hours, whereas the vapor pressure of liquid normal hydrogen increases one mm in 4 hours. This large difference in rates is attributable to the difference in magnetic moments of the proton and deuteron. If Wigner's theory of the ortho-para conversion by paramagnetic molecules in the gaseous phase is extended to the liquid phase to calculate the relative rates of change of the vapor pressures of liquid normal deuterium and liquid normal hydrogen, a ratio of 1/1000 is obtained for the ratio of the rate of change for deuterium to the rate of change for hydrogen.