Isotope Separation

Ion-exchange chromatography is able to combine the effects of many single-stage separations due to the small height of the equivalent theoretical plates, 0.1–10 mm. Hence, it can be used for the (partial) separation of isotopes for which the separation factors are of the order of 1.001 only. This method has been used for many elements, and in some cases, e.g., boron and uranium, has led to industrial isotope enrichment. It is extremely difficult to separate isotopes by chemical means, although some of their properties are mass dependent. The differences in property are reflected in the equilibrium constants pertaining to the different isotopic species, corresponding to 0.1–10 J mol ^{- 1} in Gibbs free energy. In order to achieve useful isotope separations it is necessary to multiply the single equilibrium effect many-fold and ion-exchange chromatography and related techniques are excellent methods for this purpose. In contrast to the equilibrium effect, the kinetic isotope effect and the difference in the diffusion coefficients between species in solution containing different isotopes have not found much use in isotope separation by ion exchange. In solution the ions are solvated and the mass difference between the different isotopes of the ions becomes ‘diluted’ by the masses of the solvent molecules, so that this effect is much smaller than for the ‘bare’ ions. Taylor and Urey were the first, in 1937, to separate isotopes (⁶Li and ⁷Li as Li ⁺ , and ¹⁴N and ¹⁵N as NH₄⁺ ) by ion-exchange chromatography, using a zeolite cation....