Chemistry

Ultra-pure Metals :
In the present age of technological advancement, metals of high purity are required for special purposes. For example, pure germanium is needed for semiconductor devices. Uranium of high-grade purity is used as fuel in nuclear reactors.
Two special techniques have been devised to prepare metals of very very high purity.
1. Van Arkel method : This method was developed by van Arkel to obtain ultra-pure metals. It is based on the thermal decomposition of metal compounds. It is used for obtaining pure titanium which is used in space technology. The impure titanium metal is converted into titanium tetra-iodide.
The air in the barrel used in this process is removed to create a high vacuum. An iodine bulb is broken. Titanium metal is heated which reacts with iodine to form gaseous titanium tetra-iodide.
Ti + 2I₂ → TiI₄
The impurities do not react with iodine. The vapour of titanium tetra-iodide is passed over a heated tungsten filament (1674 K). Titanium tetra-iodide gets decomposed into titanium and iodine. Pure titanium is deposited upon the filament and can be removed. The regenerated iodine can be reused to react with more titanium. The process is repeated.
2. Zone refining method : This method is capable of producing metals of high purity. Germanium, which is used in semiconductor devices, is purified by this method. In this method, advantage is taken of the fact that impure molten metal, when allowed to cool, deposits crystals of pure metal.

An impure germanium rod is provided with a circular heater. The heater is slowly moved along the metal rod. A band of the rod melts. As the heater moves away, the metal crystallizes out of the melt. The impurities are swept along the molten zone. Finally, the impurities reach the other end of the rod, and are removed.