What is photo chromatography?
Ans- Chromatography is a method of separating mixtures by using a moving solvent on filter paper. A drop of mixture solution is spotted near one end of the paper and then dried. The end of the paper, nearest the spot, is then dipped into the solvent without submerging the spot itself.
In ascending chromatography, the solvent is in a pool at the bottom and moves up by capillarity. In descending chromatography it is in a trough at the top and flows down by capillarity and gravity.
The solvent flows along the paper through the spots and on, carrying the substances from the spot. Each of these will, if the solvent mixture has been well chosen, move at a different rate from the others. After a time the paper is taken out and dried: the substances can be seen at once if coloured, or located by treating with a suitable locating agent.
The distance a substance travels depends upon the resultant between propelling and retarding forces.
Propellors
a) Solvent flow…..this carries all solutes along with it and is the same for all substances.
b) Solubility…..this is a propelling force which tends to displace a substance from the paper and to keep it moving with the solvent. Usually the more soluble a substance is in the solvent, the more rapidly it will move along the paper. Solvents are chosen for the greatest differential solubilities of the substances concerned.
Retarders
a) Adsorption…..this is an attraction between the cellulose of which the filter paper is made and the solutes. Adsorption is reversible and cellulose gradually releases most substances into the solvent as it flows over the spot. Adsorption is differential like solubility: some substances are more strongly adsorbed than others.
b) Partition…..chromatography depends upon two non-mixing liquid phases, the solvent and the water bound to the cellulose molecules of the filter paper. When a substance which is soluble in the two non-mixing solvents is exposed simultaneously to both, it will partition itself between them. The amount found in each solvent will depend upon the relative solubility of the solute in each. The degree of partition at equilibrium is known as the partition coefficient.
In fact the water forms the stationary phase and the solvent a moving phase. The water can be thought of as trapped in lots of little tubes over the tops of which the solvent is passing. When a drop is spotted on paper the solute dissolves in the water of the tubes. As the moving solvent runs over the tubes it picks up the solute by partition and redeposits some of it again by partition in succeeding tubes. As it moves, it is followed by fresh solvent and so the process repeats. As there are the equivalent of thousands of tubes, a vast number of partitions take place, so small differences in partition coefficient between different solutes of a mixture lead to good separation in the course of paper chromatography.