The paper is then placed upright in a suitable solvent , such as water. As the solvent soaks up the paper, it carries the mixtures with it. Different components of the mixture will move at different rates. This separates the mixture out. Spots of ink or plant dye are placed on a pencil line. As the paper is lowered into the solvent, some of the dye spreads up the paper. Adsorption is differential like solubility: some substances are more strongly adsorbed than others.
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. Sign up now. How does chromatography work? We think you would also find it helpful to read our TLC Worksheet.
In more detail:- Chromatography is a method of separating mixtures by using a moving solvent on filter paper. A pencil line is drawn, and spots of ink or plant dye are placed on it. There is a container of solvent, such as water or ethanol. The paper is lowered into the solvent. The solvent travels up through the paper, taking some of the coloured substances with it.
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