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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Equilibrium fractionation ： ウィキペディア英語版 Equilibrium fractionation Equilibrium isotope fractionation is the partial separation of isotopes between two or more substances in chemical equilibrium. Equilibrium fractionation is strongest at low temperatures, and (along with kinetic isotope effects) forms the basis of the most widely used isotopic paleothermometers (or climate proxies): D/H and 18O/16O records from ice cores, and 18O/16O records from calcium carbonate. It is thus important for the construction of geologic temperature records. Isotopic fractionations attributed to equilibrium processes have been observed in many elements, from hydrogen (D/H) to uranium (238U/235U). In general, the light elements (especially hydrogen, boron, carbon, nitrogen, oxygen and sulfur) are most susceptible to fractionation, and their isotopes tend to be separated to a greater degree than heavier elements. == Definition == Most equilibrium fractionations are thought to result from the reduction in vibrational energy (especially zero-point energy) when a more massive isotope is substituted for a less massive one. This leads to higher concentrations of the massive isotopes in substances where the vibrational energy is most sensitive to isotope substitution, i.e., those with the highest bond force constants. In a reaction involving the exchange of two isotopes, lX and hX, of element "X" in molecules AX and BX, :$A^lX\; +\; B^hX\; \backslash rightleftharpoons\; A^hX\; +\; B^lX$ each reactant molecule is identical to a product except for the distribution of isotopes (i.e., they are isotopologues). The amount of isotopic fractionation in an exchange reaction can be expressed as a fractionation factor: :$\backslash alpha\; =\; \backslash frac\}$ $\backslash alpha\; =\; 1$ indicates that the isotopes are distributed evenly between AX and BX, with no isotopic fractionation. $\backslash alpha\; >\; 1$ indicates that hX is concentrated in substance AX, and indicates hX is concentrated in substance BX. $\backslash alpha$ is closely related to the equilibrium constant (Keq): :$\backslash alpha\; =\; (K\_\; \backslash cdot\; \backslash Pi\; \backslash sigma\_/\; \backslash Pi\; \backslash sigma\_)^$ where $\backslash Pi\backslash sigma\_$ is the product of the rotational symmetry numbers of the products (right side of the exchange reaction), $\backslash Pi\backslash sigma\_$ is the product of the rotational symmetry numbers of the reactants (left side of the exchange reaction), and $n$ is the number of atoms exchanged. An example of equilibrium isotope fractionation is the concentration of heavy isotopes of oxygen in liquid water, relative to water vapor, :$H\_2^O\_\; +\; H\_2^O\_\; \backslash rightleftharpoons\; H\_2^O\_\; +\; H\_2^O\_$ At 20 °C, the equilibrium fractionation factor for this reaction is :$\backslash alpha\; =\; \backslash fracO)\_\}O)\_\}\; =\; 1.0098$ Equilibrium fractionation is a type of mass-dependent isotope fractionation, while mass-independent fractionation is usually assumed to be a non-equilibrium process. For non-equilibrium reactions, isotopic effects are better described by the GEBIK and GEBIF equations for transient kinetic isotope fractionation, which generalize non-steady isotopic effects in any chemical and biochemical reactions.〔Maggi F. and W. J. Riley, (2010), Mathematical treatment of isotopologue and isotopomer speciation and fractionation in biochemical kinetics, Geochim. Cosmochim. Acta, 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Equilibrium fractionation」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.