Thallium (Tl) is the fifth element in Group IIIA in the periodic table. It has an atomic number of 81, an atomic weight of 204.38, and valences of 1 and 3. The average abundance in the earth’s crust is 0.07 ppm, and in groundwaters it is <0.1 mg/L. The metal occurs chiefly in pyrites. Thallium is used in the production of glasses and rodenticides, in photoelectric applications, and in electrodes for dissolved oxygen meters.
The common aqueous species is Tl+. It is nonessential for plants and animals. Compounds of thallium are toxic on contact with moisture and by inhalation. The U.S. EPA primary drinking water standard MCL is 2 μg/L.
For analysis, use one of the atomic absorption spectrometric methods (Section 3111 B or 3113 B), or one of the inductively coupled plasma methods (Section 3120 or 3125), depending upon sensitivity requirements.