Isotopic Tracers in Chemical Systems

Abstract

The study of chemical problems with radioactive and enriched stable isotopes as tracers is rapidly gaining in popularity, consonant with increasing availability of tracer nuclides and their compounds, and with increasing educational impetus to nuclear chemistry. Although the applications of tracers have been greatly diverse in nature, emphasis has been largely on studies which are peculiarly susceptible to tracer techniques, so that, for­ tunately, the often more precise conventional techniques of chemistry seem not likely to be supplanted in the near future. In keeping with this tendency, this review will consider, for the most part, those chemical reactions and interactions whose mechanisms, rates, or equilibria have been subject to elucidation only by the specific techniques of isotope tracing. Prominent among these are isotopic exchange reactions, isotope effects in chemical reactions, molecular decomposition and rearrangement, and the phenomena of sorption and diffusion. Although our theme will be the positive side of tracer chemistry, some of the material presented here clearly calls attention to pitfalls in the use of tracers in chemical systems. To indicate a few of the more important ones : (a) An undetected isotopic exchange can lead to erroneous conclusions ; many of these reactions have been found to proceed at surprising rates (e.g., the heterogeneous catalysis of some electron transfer reactions) , so that no prima facie assumptions seem safe; (b) differences in reaction rates of normal and tracer components can be seriously misleading, unless proper allowances are made ; in chemical reactions, these differences are, of course, insignificant with the heavier nuclides, but are appreciable with carbon and lower atomic number elements in specific types of reactions; in diffusion processes even the heavier elements may show significant isotope effects ; (c) too little at­ tention may be given to possible effects of the ubiquitous ionizing radiations and recoil atoms in radioactive tracer experiments ; even the softest beta particle or gamma ray can rupture many chemical bonds, producing highly reactive fragments ; (d) studies of chemical systems at extreme dilution are subject to careful examination until much more is known about the physical peculiarities of carrier-free tracers ; (e) the concept of purity assumes a quantitatively new meaning in tracer chemistry, since contamination of far less than 10-6 per cent, not detectable by conventional purity criteria, can suggest entirely erroneous conclusions ; this applies not only to contamina­ tion of one nuclide by another, but perhaps more significantly to the con­ tamination of a given nuclide in one chemical form by that in another. Some of these problems and their implications are discussed by Yankwich

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