Isotope and ortho-para separations

Isotope and Ortho-Para Separations of the Molecular Hydrogens by Adsorption at Low Temperatures... 

For a fixed temperature So.Do.p.Hs and the ortho-para separation factors Sp-D2.o Doy S0.H2.p-H2 are given by Equation 29 and are constants for a given surface. The dependence of the isotope separation factor on the gas phase ortho-para concentrations is shown in Figure 5. The maximum isotope separation factor is obtained when para-hydrogen is separated from para-deuterium. The minimum is obtained when ortho-hydrogen is separated from ortho-deuterium. 

In Table IV, separation factors for several of the isotopic hydrogens at considerably lower temperatures are given. The values of y and D in this Table were chosen to fit the experimental ortho-para separation factor of hydrogen at the lowest temperature, 27 °K. Even though fair agreement between theory and experiment is obtained for all of the... 

For those who are familiar with the statistical mechanical interpretation of entropy, which asserts that at 0 K substances are nonnally restricted to a single quantum state, and hence have zero entropy, it should be pointed out that the conventional thennodynamic zero of entropy is not quite that, since most elements and compounds are mixtures of isotopic species that in principle should separate at 0 K, but of course do not. The thennodynamic entropies reported in tables ignore the entropy of isotopic mixing, and m some cases ignore other complications as well, e.g. ortho- and para-hydrogen. 

Isotopes of hydrogen. Three isotopes of hydrogen are known H, 2H (deuterium or D), 3H (tritium or T). Isotope effects are greater for hydrogen than for any other elements (and this may by a justification for the different names), but practically the chemical properties of H, D and T are nearly identical except in matters such as rates and equilibrium constants of reactions (see Tables 5.1a and 5.1b). Molecular H2 and D2 have two forms, ortho and para forms in which the nuclear spins are aligned or opposed, respectively. This results in very slight differences in bulk physical properties the two forms can be separated by gas chromatography. 

Chromatography of various isotopomers of benzoic acid shows a moderate effect of number of deuterium atoms on the separation factor however, an important effect is exerted by the position of the label (Table 3). Isotopomers containing deuterium in the ortho positions display lower isotopic separation than isotopomers with deuterium in meta or para position. The isotope effects associated with the ortho positions are minimal, probably because interactions between ortho C- H or C- H bonds and the stationary phase are minimized by the steric shielding of the carboxyl group. 

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