Explain: (i) Zone refining     (ii) Column chromatography. 

(i) Zone refining: The method is based on the principle that the impurities are more soluble in the melt than in the solid state of the metal. A circular mobile heater is fixed at one end of a rod of impure metal. The molten zone moves along with the heater which is moved forward. As the heater moves forward, the pure metal crystallizes out of the metal and the impurities pass on into the adjacent molten zone. The process is repeated several times and the heater is moved in the same direction. At one end impurities get con-centrated. This end is cut off. In this way, the impu-rities are swept from one end of the bar to the other. By repeating the process, ultra pure metal of silicon, germanium etc are obtained.



Fig. Zone Refining Process

(ii) Column Chromatography: In column chromatography an adsorbent (e.g., Al₂O₃) 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 suitale 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.