Isotopic Exchange

a chemical process involving the redistribution of the isotopes of an element between reacting substances. Isotopic exchange involves the substitution of one isotope of an element by another isotope of the element in the molecules of a given substance without a change of their elemental composition. For example, if hydrogen chloride HC1, enriched by the heavy isotope of chlorine ³⁷C1, is mixed with chlorine Cl₂ with the normal isotope composition (75.53 percent ³⁵C1 and 24.47 percent ³⁷C1), then as a result of the isotopic exchange reactions

H³⁷Cl+³⁵Cl₂ = H³⁵Cl+³⁵Cl³⁷Cl

H³⁷Cl+³⁵Cl³⁷Cl = H³⁵Cl+³⁷Cl₂

the chlorine will become enriched by the heavy isotope and the hydrogen chloride will become depleted by the heavy isotope.

Isotopic exchange reactions may take place under various conditions: under homogeneous conditions (for example, between solute and solvent, in gas mixtures) and under heterogeneous conditions (for example, between solids or liquids and insoluble gases). The mechanisms of isotopic exchange reactions are not different from the mechanisms of other chemical reactions.

Since the chemical properties of isotopes of the same element are almost identical and the relative differences in their atomic masses are not large (with the exception of hydrogen isotopes), each isotope on attainment of chemical equilibrium will be distributed almost uniformly between the reactants. The nonuni-formity for the isotopes of heavy elements does not exceed several tenths of a percent, whereas that of the isotopes of light elements (from Li to CI) does not exceed 10 percent. Only for hydrogen isotopes is the nonuniformity of the distribution.between some substances as high as several hundred percent. The distribution of isotopes between substances in the state of equilibrium is characterized by the distribution coefficient a, which determines the ratio of the equilibrium concentrations of isotopes in the reactants; a = 1 for a uniform distribution of isotopes.

Deviation from the equilibrium distribution depends not only on the mass of the isotopes but also on the chemical composition of the substances undergoing isotopic exchange. In addition, a depends on the temperature and approaches unity with increasing temperature in all cases. The rate of isotopic exchange is wholly determined by the mechanism of the corresponding reactions. In some cases the exchange is practically instantaneous (for example, in ionic reactions in solution), while in other cases it proceeds very slowly or not at all. Various catalysts may be used to accelerate isotopic exchange as other chemical reactions.

Isotopic exchange is used for the concentration of the desired isotope. For this purpose, the enrichment process involving the isotope and one of the substances is repeated numerous times under the conditions of a nonuniform isotopic distribution between the substances. Such methods have found industrial application in the case of the production of hydrogen and lithium isotopes, which are being used in atomic and thermonuclear energy production. These processes include, for example, the production of heavy water by way of isotopic exchange between water and hydrogen sulfide or between water and hydrogen:

¹H²HS + ¹H₂O = ¹H²HO+ ¹H₂S

or

¹H²H + ¹H₂O = ‘H₂ + ‘H²HO

Isotopic exchange is being used in chemical studies to elucidate the nature of the elementary steps of various reactions. The rate of an isotopic exchange sometimes serves as a better indicator than other reactions of the mobility of atoms in molecules and of the reactivity of chemical compounds. Isotopic exchange is also being used in the synthesis of labeled compounds.