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THALLIUM
Natural Abundance, Stable Isobars Tl203, 29.5%, None Tl205, 70.5% Thallous chloride, thallous bromide, thallous iodide, and thallic iodide have been used as charge material in the separation of the isotopes of thallium. The monoiodide, TlI, is the preferred material from an operational standpoint. The usual charge consists of 500 g TlI in a style X-5 stainless steel charge bottle. Elemental thallium is used as a starting material for the preparation of TlI. Thallium metal is added in small portions to hot concentrated nitric acid until all the acid is consumed. When the solution is allowed to cool, a large deposit of thallous nitrate crystals is formed. After decanting the supernatant liquid, the crystals are dissolved in hot water and diluted to approximately twice the volume. The solubility of thallous nitrate in water at room temperature is about 9.6 g per 100 ml. The thallous nitrate solution is made basic with sodium hydroxide and a few milliliters of sulfurous acid are added to keep the thallium in a reduced state. Thallous iodide is precipitated quantitatively by the addition of a slight excess of potassium iodide solution. (Precipitation with hydriodic acid should not be used since it tends to form the sesquiiodide.) The solution is stirred vigorously for several minutes and allowed to stand. The yellow TlI precipitate settles quickly. A change in color of the TlI from yellow to orange to green to brown is observed when insufficient potassium iodide is used. However, the yellow color is restored after adding more potassium iodide solution and stirring. Thallous iodide is filtered on a coarse fritted-glass funnel and washed with cold water. The wash water must be cold since TlI is considerably more soluble in hot water. After air drying on the funnel, the solid is transferred to an evaporating dish and dried on a hot plate. The material is then vacuum dried at 325ºC until the yellow TlI turns a cinnabar red. This color change apparently is due to a crystal modification. Thallium is a cumulative poison and all thallium salts are toxic causing widespread damage to the nervous system, digestive tract, and, to a lesser extent, the kidneys and circulatory system. Thallium solutions are readily absorbed through the skin and the digestive tract, and it is imperative that these materials be kept off the skin and out of the respiratory and digestive systems. Respirators, chemical safety goggles, and rubber gloves must be worn when working with thallium or its compounds. |
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