Recrystallisation

Recrystallisation is a purification technique for solid compounds that relies on differences in solubility with temperature. The material is dissolved in a hot solvent and allowed to crystallise as the solution cools, ideally yielding pure crystals while impurities remain in solution.

General procedure: place the crude solid in the minimum hot solvent that yields a saturated solution, heat until dissolved, and then filter the hot solution to remove insoluble impurities. Allow the filtrate to cool slowly, promoting crystallisation of the desired compound. Collect the crystals by filtration, wash with a small amount of cold solvent, and dry.

Solvent selection is critical. The ideal solvent dissolves most of the compound at high temperature but tolerably little at room temperature, while impurities either stay dissolved or form easily removable residues. If a single solvent is not suitable, a mixed solvent system or a solvent pair with differing polarities can be used to fine-tune solubility.

Limitations and techniques to improve yield and purity include using seed crystals to initiate crystallisation, scratching the vessel to start crystallisation, or cooling slowly to form larger crystals. There may be losses during filtration and washing, and some compounds form solvates or co-crystals with the solvent. Fractional recrystallisation can separate closely related impurities by repeating the process with different solvents or by gradual concentration changes.

Applications and considerations: recrystallisation is widely used in organic synthesis and pharmaceutical preparation to obtain pure solids. Purity is typically assessed by melting point, as well as spectroscopic methods, while yield reflects the balance between removal of impurities and material loss. Safety and waste disposal depend on the solvents chosen.