Explain: Column chromatography.

Explanation

(i) Zone refining. This method is employed to get metals of very high purity. Germanium, silicon, boron, gallium, indium are purified by this method. The method is based on the principle that the impurities are more soluble in the melt than in the pure metal. A circular mobile heater is fixed at one end of a rod of the impure metal. The heater is slowly moved forward. The melted zone moves along with the heater. As the heater moves forward the pure metal crystallizes out of the melt while the impurities pass into the adjacent molten zone. The process is repeated a number of times until the desired state of purity is reached. The end of the rod where the impurities finally get concentrated is cut off. The method is specially useful for producing semiconductors of very high purity, Germanium, for example, purified by this method contains as little as 10 -7 ppm of the impurities (Fig).

(ii) Column chromatography : In this method of purification various components of a mixture are differently adsorbed on an adsorbent. In column chromatography an adsorbent (e.g., `Al_(2)O_(3))` is packed in a glass column. The mixture to be separated or purified, taken in a suitable solvent, is applied on the top of the column. The components of the mixture get adsorbed on the column. They are then eluted out with a suitable eluent (solvent). The weakly adsorbed component is eluted first followed by the more strongly adsorbed and so on. The method is especially suitable for such elements which are available only in minute quantities and the impurities are not very much different in their chemical behaviour from the element to be purified.