Because of the easy detection of their radiation, radioactive isotopes provide a use­ful approach to determine the local, time, and concentration distribution of sub­stances. Consequently, the radioisotopic tracer method can be used for reaction kinetic, mechanism, and equilibrium studies. In this chapter, some radiotracer meth­ods will be shown. In these studies, the radiotracer is most often in solution, so the most important properties of highly diluted solutions (solutions of carrier-free radioactive isotopes) and the basic rules of working with them will be discussed.

When an isotope has a relatively short half-life, the number of the radioactive nuclides is very few. For example, when the half-life of an isotope is 100 years, 1 kBq activity is emitted by about 7.5 X 10-12 mol radioactive nuclides. So, the preparation of a solution in so small concentration range demands very careful pro­cedures. The stock solution is usually kept in a 10-1 or 10-2 mol/dm3 concentrated acidic solution (e. g., in nitric or perchloric acid to avoid the formation of com­plexes with the radioactive isotopes), and this acidic stock solution is diluted step by step. In each dilution process, the solution has to be left to mix for at least 12 h before the next solution, since the carrier-free isotopes are mixed via the self-diffu­sion of the solvent and stirring does not increase considerably the rate of this pro­cess. To avoid the formation of radiocolloids, the pH of the solution may be increased gradually. The radioactivity of the solution must be checked regularly. Only very pure solvents (bidistilled or tridistilled water) are applied because even these pure solutions can contain more contaminants than the total quantity of the radioindicator.

The solutions containing carriers in macroconcentrations can be handled more easily; however, carrier-free radioisotopes have to be added to the solution of the carrier according to the rules mentioned previously.