6.2 Recrystallisation

Recrystallisation is a common separation technique which you will encounter during your degree. In this lesson, you will be introduced to the core concepts and the required knowhow required to perform a successful recrystallisation.

Recrystallisation is a technique used to purify compounds in solvents. It is built on the concept that ideally in a certain solvent your desired compound will not be soluble at room temperature while any impurities will. It takes advantage of the fact that the solubility of most solids increases with rising temperature. Meaning that as temperature increases, the amount of solute that can be dissolved in a solvent also increases.

The aim is to reach an elevated temperature at which both your desired compound and the impurity is dissolved. When everything is dissolved the mixture can be cooled again at which point (hopefully) only your desired compound becomes crystalline again (this is called crashing out) as it is not soluble at lower temperatures while the impurity stays dissolved.