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Atoms interchange frequently between molecules and ions and across phase boundaries. An example chosen for study in this laboratory exercise is the exchange of iodine (I) atoms between an organic iodine-containing molecule and iodide ions from dissolved sodium iodide (NaI). A continuous exchange of iodine atoms occurs whenever organic iodides and NaI are mixed. This process is not observable unless one of the species (the organic molecule or NaI) contains “tagged” iodine atoms. A common label is 131I or 125I. When one isotope of an element in a certain chemical compound is exchanged with the same or another isotope of the same element in another compound we call the process isotopic exchange.
Most of the exchanges occurring in a chemical mixture are between “normal” inactive and naturally occurring isotopes because the compounds consist mainly of inactive molecules and ions. In the case of iodine the exchange will be of 127I atoms between the various compounds. The exchange rate r is dependent on parameters such as concentration of the compounds and temperature, but is not affected by adding carrier free tracer amounts of 131I, say in the form of Na131I, because the iodine concentration is not changed to a measurable degree. Nor does r vary as 131I is exchanged with 127I. Therefore, the overall or total rate of iodine exchange between the compounds involving both tagged and untagged molecules can be determined from the observed behavior of the radioactive isotope.
In general, studies of isotopic exchange reactions in chemical and physical systems can provide useful information about reaction rates at equilibrium, reaction mechanisms and, for instance, the role of a reaction catalyst. Exchange processes are also applied to label compounds with radioactive nuclides, to study surface and interfacial phenomena and to determine the chemical stability (or rather liability) of selected atoms or functional groups in molecules.