Sometimes the solubility of an organic compound is quite similar to that of its impurities. In such a case it is not possible to purify an organic compound by simple crystallization. The crystals thus obtained are not pure. These contain impurities as well. However, these can be purified by fractional crystallization.
Fractional crystallization is done by first preparing a hot, concentrated solution. When the solution is cooled, the first drop of crystals is obtained. The crystals thus obtained contain more of one of the components which has a lower solubility and less of the component with a higher solubility. On the other hand, the filtrate contains less of components with a lesser solubility and more of components with a higher solubility.
The crystals thus obtained are crystallized again. The crystals now obtained will be less impure as more impurities will be passed on to the mother liquor. The crystallization is repeated a number of times till the crystals are free from impurities.
Similarly the filtrate, i.e. the mother liquor, is crystallized again and again. Each crystallization helps remove some more of the compound which is less soluble. Finally the mother liquor contains a pure compound, which has a slightly higher solubility.