Isotope Effect and Isotope Separation: A Chemist's View

Abstract

What causes the isotope effects (IE)? This presentation will be centered around the equilibrium isotope effects due to the differences in the nuclear masses. The occurrence of the equilibrium constant, K, of isotope exchange reactions which differ from the values predicted by the classical theory of statistical mechanics, K_cl , is explored. The nonclassical K corresponds to the unit-stage separation factor, α, that is different from unity and forms a basis of an isotope separation process involving the chemical exchange reaction. Here, the word “chemical exchange” includes not only the isotope exchange chemical reactions between two or more chemical species but also the isotope exchanges involving the equilibria between liquid and vapor phases and liquid-gas, liquid solution-gas, liquid-liquid, and solid-liquid phases. In Section I, origins of the isotope effect phenomena will be explored and, in the process, various quantities used in discussions of isotope effect that have often caused confusions will be unambiguously defined. This Section will also correlate equilibrium constant with separation factor. In Section II, various forms of temperature-dependence of IE and separation factor will be discussed.

KEYWORDS:

• isotope effects
• isotope separation
• partition function
• partition function ratio
• reduced partition function ratio
• equilibrium constant
• separation factor
• temperature dependence
• Bernoulli series
• orthogonal polynomial expansion