| Summary: | The phase exchange process: (^10)BCl(_3)(1) + 11BCl(_3)(g) = (^11)BCl(_3)(l) + (^10)BCI(_3)(g) has been examined in a high efficiency packed fractionating column. The relative volatility of the isotopic boron chlorides has been found to be 0.9987(in favour of 10(_B)). A novel method has been developed for assaying the concentration in BC1(_3). This method is based upon the large capture cross section of for thermal neutrons as compared with any other nuclide likely to be present. The technique consisted in measuring the neutron transmission through an absorption cell, alternately empty- and filled with BCl(_3) gas. Standard nuclear physical apparatus and a BF(_3) filled proportional counter were used for measuring the neutron intensity. The value of the equilibrium constant obtained experimentally differs in sense and magnitude from that predicted by Urey from infra-red spectral data(l.014 at the boiling point). This discrepancy has been discussed in the light of a more general theory of the vapour pressure differences of isotopic liquid mixtures, and. it has been shown that reasonable agreement is obtained between theory and experiment if allowance is made for the existence, in the liquid phase, of infra-red dispersion forces'(analogous to the London dispersion forces), the magnitudes of which are isotope-dependent.
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