Explain:Zone refining (ii) Column chromatography.

7/8/2015 CBSE

Explain: (i) Zone
refining (ii) Column

(i) Zone refining:

This method is based on the principle that impurities are more soluble in the
molten state of metal (the melt) than in the solid state. In the process of zone
refining, a circular mobile heater is fixed at one end of a rod of impure metal.
As the heater moves, the molten zone of the rod also moves with it. As a result,
pure metal crystallizes out of the melt and the impurities pass onto the
adjacent molten zone. This process is repeated several times, which leads to the
segregation of impurities at one end of the rod. Then, the end with the
impurities is cut off. Silicon, boron, gallium, indium etc. can be purified by
this process.

(ii) Column chromatography:

Column chromatography is a technique used to separate different components of a
mixture. It is a very useful technique used for the purification of elements
available in minute quantities. It is also used to remove the impurities that
are not very different in chemical properties from the element to be purified.
Chromatography is based on the principle that different components of a mixture
are differently adsorbed on an adsorbent. In chromatography, there are two
phases: mobile phase and stationary phase. The stationary phase is immobile and
immiscible. Al2O3 column
is usually used as the stationary phase in column chromatography. The mobile
phase may be a gas, liquid, or supercritical fluid in which the sample extract
is dissolved. Then, the mobile phase is forced to move through the stationary
phase. The component that is more strongly adsorbed on the column takes a longer
time to travel through it than the component that is weakly adsorbed. The
adsorbed components are then removed (eluted) using a suitable solvent (eluant).

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