Radiometric titration is a two-phase titration method when the equivalence point (i. e., end point) is indicated by the disappearance of a radioactive isotope from one phase. It can be used if the unknown substance and the titrant form very insoluble precipitates or an easily extractable compound, and if one of the reagents has a suitable radioactive isotope. During the titration process, different volumes of the titrant are added to the unknown compound, the phases formed are separated by fil­tration and extraction, and the activity/intensity of any phases is measured. The main advantage of the radiometric titration is that the titration curve usually con­sists of linear portions; thus, few points are enough for drawing the titration curve. In addition, the linear character provides opportunities for automation.

Both the unknown compound and the titrant can be labeled. When precipitate is produced during the titration, the titration curves will be as follows:

• The titrant is labeled by a radioactive isotope; the activity of the solution shows the back­ground activity until the equivalence point. After the equivalence point, the activity of the solution containing the excess of the titrant increases (Figure 10.2, plot A).

• The unknown compound is labeled: the activity of the solution decreases until the equiva­lence point, and then it shows the activity determined by the solubility of the precipitate (Figure 10.2, plot B).

• Both the unknown compound and the titrant are labeled: the activity has a minimum at the equivalence point (Figure 10.3).

Two or more unknown compounds can be analyzed simultaneously by precipita­tion radiometric titration if the solubility of the precipitates is fairly different. For example, copper and zinc ions can be determined by titration with Fe(CN)64_. Zinc ions are labeled with a radioactive 65Zn isotope. The solubility of Cu2[Fe(CN)6] is much less than that of Zn2[Fe(CN)6]. By adding the solution of Fe(CN)64_ to the solution of copper and zinc ions, at first, Cu2[Fe(CN)6] precipitates, and then the activity of the solution remains constant. After the equivalence point of the precipi­tation of the copper ion, Zn2[Fe(CN)6] starts to precipitate and the activity of the solution decreases. After the precipitation of the total quantity of the zinc ions, the activity of the solution becomes constant again, determined by the solubility of Zn2[Fe(CN)6] (Figure 10.4).

Dual labeling gives additional analytical opportunities. For example, the quan­tity of sulfate and iodide ions in the same solution can be measured. Both sulfate and iodide ions are labeled by radioactive sulfur (35S) and iodine (131I) isotopes, respectively. The solution is titrated with barium chloride and barium sulfate preci­pitates. Then the activity of the solution decreases until the equivalence point of sulfate is reached. When barium chloride is added in excess, the activity becomes constant. Then, titration is continued using silver nitrate and precipitating silver iodide. Silver iodide is separated, and the activity of the solution decreases again until the equivalence point of iodide ions is reached, and then it becomes constant again (Figure 10.5).

Radiometric titration can also be made by extraction. For example, the unknown metal ion is titrated with a complex forming agent. The complex compound is extracted with an organic solvent. The concentration of zinc ions can be determined by titrating dithizone dissolved in chloroform. The zinc dithizone complex dis­solves in the chloroform. When zinc ions are labeled (e. g., by the 65Zn isotope), the activity of the aqueous phase decreases until the equivalence point is reached, and then it remains constant.