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Recrystallization
The general set-up for recrystallization is presented: the solid is placed in a flask with a return condenser on top. With a small amount of a solvent the mixture is heated to reflux and further solvent is added in small portions until the suspension becomes a hot saturated solution. The solid is regained by slow cooling shown in time lapse whereas the impurities remain in solution. Subsequently, the solid is separated from the solution by filtration.
Several aspects have to be considered concerning recrystallization: The choice of a suitable solvent determines the grade of purification as the solubility of the desired compound as well as of the impurities depends on it. For certain recrystallizations it might be necessary to repeat these several times to accomplish the desired purity. Nevertheless, it has to be taken into account that a small amount of the target compound remains in solution and is therefore lost. Hence the saturated solution should be cooled below room temperature in order to maximize the yield, especially if only small amounts of the desired compound are used or if it is very expensive.