Stable isotopes exchange reactions

The molar volumes of isotopically substituted compounds are virtually indistinguishable for most common compounds so that typical light stable isotopic exchange reactions involving solid materials are insensitive to changes in pressure normally encoimtered imder lithospheric conditions making them suitable for geothermometry (the study of temperature within the Earth). 

In summary, the application of stable isotopes to groundwater studies is based on the fact that the isotopic composition of water behaves conservatively in low-temperature environments where water-rock contact times are short relative to the kinetics of mineral-water isotope exchange reactions. 

There were several key observations, which were made almost around the same time, that led to studies in the fields of stable isotope geochemistry and cosmochemistry. First, it was the physicochemical formalism for isotope effects, particularly the determination of the position of equilibrium in isotopic exchange reactions. Urey (1947) and Bigeleisen and Mayer (1947) demonstrated that the position of isotope exchange in a chemical reaction may be calculated with high precision. The difference in chemical behavior for... 

An illustration of an isotope exchange reaction is the distribution of stable carbon isotopes between equilibrated C02(g) and aqueous HCO3 species of the carbonate buffer system. The hydration reaction for CO2, Eq. (4.11)... 

Isotopic exchange reactions of oxygen have been studied ever since water enriched in the stable isotopes of oxygen (O18 and O17) became available in the 1930 s (Lewis and Cornish, 1933 Urey et al., 1936). 

Bickle JM, Baker J (1990b) Migration of reaction and isotopic fronts in infiltration zones Assessments of fluid flux in metamorphic terrains. Earth Planet Sci Lett 98 1-13 Blattner P, Lassey R (1989) Stable isotope exchange fronts, Damkohler numbers, and flnid-rock ratios. ChemGeol 78 381-392... 

Our own calculations [25], which predict that the H20 molecule is a thermodynamically stable species up to at least 4000 K, are in agreement with these subsequent results. We, therefore, believe that it is possible to employ the methods described in this paper to make fully converged quantum-mechanical predictions of equilibrium constants for chemical and isotope exchange reactions within a sunspot. 

Potentially, stable isotopic differences between phases reflect temperatures at the time the ratio is locked in under equilibrium condihons. This typically occurs when some chemical reaction occurs because stable isotopic exchange kinehcs are very sluggish (joule-scale driving forces instead of kilojoule driving forces for chemical reactions). 

At Bayer CropScience, the use of a stable isotope IS has become common practice to eliminate the effects of matrix suppression on instrument signals. The stable isotopes are synthesized by deuterium exchange reactions on authentic native standards or the... 

We briefly review processes in which isotopic fractionations may be recorded in isotopically distinct reservoirs that are preserved in nature. These concepts have been extensively covered in the H, C, O, and S isotope literature, and we illustrate several examples for the non-traditional stable isotope systems discussed in this volume. One of the simplest processes that produces isotopically distinct reservoirs would be slow reaction of substance A to B, where A and B remain open to complete isotopic exchange during the process. This is commonly referred to as closed system equilibrium, and the changes in isotopic compositions that occur may be defined by the exact relation ... 

Fractionations are typically very small, on the order of parts per thousand or parts per ten thousand, so it is common to see expressions like 1000 ln(a) or 1000 (a-l) that magnify the difference between a and 1. a =1.001(1000 [a-l] = 1) is equivalent to a 1 per mil (%o) fractionation. Readers of the primary theoretical literature on stable isotope fractionations will frequently encounter results tabulated in terms of P-factors or equilibrium constants. For present purposes, we can think of Pjjh as simply a theoretical fractionation calculated between some substance JiR containing the elementX, and dissociated, non-interacting atoms ofX. In the present review the synonymous term Uxr-x is used. This type of fractionation factor is a convenient way to tabulate theoretical fractionations relative to a common exchange partner (dissociated, isolated atoms), and can easily be converted into fractionation factors for any exchange reaction ... 

Kinetic fractionations can occur when there is incomplete isotopic exchange between the different phases present in a system. A thorough introduction to kinetic stable isotope fractionation theory is unfortunately beyond the scope of the present review. Flowever, it is useful to include a brief discussion of some basic aspects, particularly in comparison to equilibrium fractionation theory. A simple example of kinetic fractionation is the evaporation of a liquid water droplet into a vacuum, in this example FljO molecules entering the gas phase are physically removed from the vicinity of the droplet, so there is no chance for isotopic equilibration between vapor-phase molecules and the residual liquid. Isotopic fractionation in this case is determined by a one-way reaction path, and will not, in general, be the same as the fractionation in a system where vapor-phase molecules are able to equilibrate and exchange with the liquid. In other reactions, isotopic exchange is limited by an energy barrier—an... 

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