Recrystallization

Recrystallization is a purification technique used to remove impurities from solid compounds by dissolving the material in a hot solvent and allowing it to crystallize as it cools, leaving most impurities in the solution. The method relies on differences in solubility between the desired compound and its impurities.

The solvent should dissolve the compound when hot but allow limited solubility at room temperature; impurities are often more soluble in the chosen solvent or in water. Mixed or seed-assisted solvent systems can be used to tune solubility and promote purer crystal formation.

Dissolve the sample in the minimum hot solvent needed to produce a clear solution, then filter while hot to remove insoluble material. Allow the solution to cool slowly, promoting the formation of pure crystals. Once crystallization is complete, collect the crystals by vacuum filtration, wash them with cold solvent to remove adherent impurities, and dry.

Factors affecting yield and purity include solvent choice, the nature of impurities, rate of cooling, temperature control, presence of seed crystals, and the wash solvent. Overheating or using too much solvent can reduce yield, while overly rapid cooling may trap impurities and reduce purity.

Recrystallization is widely used to purify organic compounds, pharmaceuticals, and some inorganic salts. It is not suitable for thermally sensitive substances, and some impurities may co-crystallize with the product. In practice, multiple recrystallizations or alternative methods such as fractional crystallization may be employed to achieve higher purity.