Thorium (Th) is the first element in the actinium series of the periodic table. It has an atomic number of 90, an atomic weight of 232.04, and a valence of 4. The average abundance in the earth’s crust is 8.1 ppm; in soils it is 13 ppm; in streams it is 0.1 μg/L, and in groundwaters it is <0.1 mg/L. Thorium is a radioactive element, with 232Th having a half-life of 1.4 × 1010 years. It is widely distributed in the earth, with the principal mineral being monazite. Thorium is used in sun lamps, photoelectric cells, incandescent lighting, and gas mantles.
The aqueous chemistry of thorium is controlled by the Th4+ ion, which forms a set of complex species with hydroxides. Thorium’s radioactive decay isotopes are dangerous when inhaled or ingested as thorium dust particles.
Either of the flame atomic absorption spectrometric methods (Section 3111 D or E) may be used for analysis. The inductively coupled plasma mass spectrometric method (Section 3125) also may be applied successfully in most cases (with lower detection limits), even though thorium is not specifically listed as an analyte in the method.