Because of their fairly high sensitivity, radiotracer methods are widely applied in all fields where the interface plays an important role in the reactions (interfacial chemistry, colloid chemistry, heterogeneous catalysis, etc.). The surface quantity of the substances is about 10-9—10-8 mol/cm2; thus, the study of the interfacial reac­tions requires analytical methods that are able to detect and precisely measure the change of these small quantities. The radioactive isotopes fulfill this requirement. In addition, radiotracer studies are applicable in a very broad concentration range of the solution or gas interacting with the interface: from carrier-free to saturation concentrations or to critical pressure. The high concentrations are reached using inactive carriers (isotopic effects can usually be disregarded).

Another advantage is the possibility of multiple indications if the radioactive isotopes can be separated using their radiochemical properties (type and/or energy of radiation and half-life, as discussed in Section 8.3). By multiple indications, interfacial processes can be studied from the direction of both bulk phases. For example, in ion exchange processes, both ions can be labeled. In this way, the equivalency of the process can be checked or the effect of other interfacial processes (e. g., adsorption) or the influence of the exchange of additional ions (e. g., hydrogen and hydroxide ions of the water) can be studied. These experiments

give significant information on the interfacial processes of ion exchangers, includ­ing natural ion exchangers such as clay minerals, rocks, and soils.

The study of the cation exchange of calcium montmorillonite clay mineral and manganese(II) ions illustrates the difficulties of multiple radioactive indication. Calcium ions can be labeled as weak beta emitter 45Ca isotopes. For labeling man — ganese(II) ions, the 54Mn isotope can be used, which disintegrates via electron cap­ture and emits X-ray and gamma radiation. The gamma radiation of the 54Mn isotope can be measured by scintillation (see Section 14.2) as well as semiconduc­tor (see Section 14.3) detectors, and the beta radiation of the calcium isotope does not disturb the measurement of gamma radiation. However, the radiation of the 54Mn isotope disturbs the measurement of beta emitter isotopes. The weak beta particles of 45Ca can be measured by the liquid scintillation technique (discussed in Section 14.2). The liquid scintillation beta spectrum of 45Ca is shown in Figure 9.16. The spectrum of 54Mn obtained by a liquid scintillation spectrometer can be seen in Figure 9.17. The sharp peak originates from the electrons emitted after the electron capture from the K orbital. The energy of these electrons is 4.7 keV. When both isotopes, 45Ca and 54Mn, are present simultaneously, the shape of the spectrum is determined by their ratio (Figures 9.18 and 9.19). In Figures 9.16—9.19, the horizontal axis is the channel number proportional to the logarithm of the energy; the vertical axis shows the intensity.

As seen in Figures 9.18 and 9.19, the sharp peak of 54Mn can be eliminated when its activity is relatively large. Thus, the activity of 45Ca can be determined. When the radioactivity of 54Mn is small, however, the peaks corresponding to 45Ca and 54Mn cannot be separated, and the activity of 45Ca cannot be measured. This is a rare and interesting case in chemical analysis when the interfering effect of the “impurity” (54Mn) is higher if it is present in small quantities than if it is present in large quantities. The degree of interference has been experimentally determined as follows: the gamma and “beta” activities of solutions containing 54Mn isotopes

Figure 9.16 Liquid scintillation beta spectrum of the 45Ca isotope.

Figure 9.17 Liquid scintillation beta spectrum of the 54Mn isotope. The sharp peak originates from the electrons emitting after the electron capture from K orbital.

Figure 9.18 Liquid scintillation spectrum of 45Ca and 54Mn in small quantities.

Figure 9.19 Liquid scintillation spectrum of 45Ca and 54Mn in large quantities.

with different activities are determined; the beta activity is plotted as a function of gamma activity. When the solutions contain both 45Ca and 54Mn isotopes, the beta activity of the 54Mn isotope is calculated on the basis of gamma activity using a predetermined plot, and then this value is subtracted from the total beta activity. The difference is treated as the beta activity of 45Ca isotope.