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Principles of Stable Isotope Distribution$
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Robert E. Criss

Print publication date: 1999

Print ISBN-13: 9780195117752

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780195117752.001.0001

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Isotopic Exchange and Equilibrium Fractionation

Isotopic Exchange and Equilibrium Fractionation

(p.40) 2 Isotopic Exchange and Equilibrium Fractionation
Principles of Stable Isotope Distribution

Robert E. Criss

Oxford University Press

Equilibrium isotopic fractionations are best understood in terms of reactions that involve the transfer of isotopes between two phases or molecular species that have a common element (M). These isotopic exchange reactions may be written in one of several standard forms, such as where AMb and BMd represent the chemical formulas of the phases or species, AM*b and BM*d represent the same phases or species in which the trace isotope has replaced some or all of the atoms of element M, and a, b, c, and d are stoichiometric coefficients. In the case where all of the molecules are homogeneous, that is, where AMb and BMd are composed solely of the common isotope of M, and where AM*b and BM*d are phases or species in which the trace isotope M* has replaced all atoms of element M, then the product a × b equals c × d and represents the total number of atoms exchanged in the reaction. The concept of the isotopic exchange reaction is best shown by an example. Consider the exchange of deuterium between water and hydrogen gas. This may be written as a reaction among isotopically homogeneous molecules; that is, or, alternatively, as exchange between homogeneous and heterogeneous molecules: Much of the utility of isotopic exchange reactions is that they may be described by equilibrium constants, defined in the standard way as the quotient of the activities of the products and reactants. Thus, the equilibrium condition for equation 2.2b becomes where K is the equilibrium constant. In equation 2.3, K has a particularly high value of 3.7 at 25°C.

Keywords:   Anharmonicity, Boltzmann constant, Calorimetry, Degeneracy, Entropy, Forams, Galena, Halogens, Ideal mixtures

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