What Is the Purpose of Recrystallization?

Recrystallization purifies chemical compounds in solvents. It works best when there is only a small quantity of impurities in the compound.

Recrystallization, also called fractional crystallization, is a process that involves dissolving an impure compound in a solvent at a high temperature to create a concentrated solution and then cooling the solution slowly. The drop in temperature decreases the solubility of both the chemical compound and the impurities it contains. The pure portion of the compound crystallizes, leaving behind the impurities in the solution. Filtering out the pure crystals separates them from any impurities, and repeating the recrystallization process creates an even purer compound.

For the recrystallization process to be successful, there must be only a small quantity of impurities in the chemical compound, and they must be soluble. It is possible to predict the outcome of the process by using a solubility curve, which is a graphical representation of how the solubility of a substance in a solvent varies with temperature. For the best outcome, the solubility curve should rise rapidly as the temperature rises.

Performed correctly, the recrystallization procedure is a highly effective way to obtain a pure sample of a compound. However, it can take a long time because the slower the rate of cooling, the larger the pure crystals, which makes filtering easier.