Principle of recrystallisation
Recrystallisation is used to purify an impure solid. The required compound is taken into solution while hot and forms crystals as the solution cools, leaving soluble impurities in the remaining liquid.
Choose a solvent in which the product becomes much more soluble on heating than it is when cold. The solvent should also be easy to remove and chemically unreactive towards the product.
| Property of the solvent | Why it is useful |
|---|---|
| Product has much greater solubility when hot | The solid can enter solution during heating and then crystallise as the mixture cools. |
| Soluble impurities remain in solution | They remain in the liquid rather than being collected with the crystals. |
| Volatile | The solvent can be removed readily from the recovered solid. |
| Chemically unreactive towards the product | Purification does not alter the compound being isolated. |
Recrystallisation sequence
Dissolve
Add a minimum volume of hot solvent to the impure solid until the required product dissolves.
Remove insoluble material
If insoluble impurities remain, filter the solution while it is hot. Keeping the filtration apparatus warm reduces crystallisation during filtration.
Crystallise
Allow the filtrate to cool so that crystals form. Further cooling in ice can increase the amount of solid recovered.
The minimum quantity of solvent is important because product that remains dissolved after cooling is not recovered as crystals and therefore lowers the yield.
Slow cooling can produce larger, well-defined crystals. If crystals do not begin to form readily, agitating the vessel or scratching its inner surface with a glass rod can initiate crystallisation.
Hot filtration
Use a warmed funnel containing fluted filter paper and carry out the filtration quickly. A short stem helps reduce the chance of crystals forming inside the funnel and causing a blockage.
Exam Tip: When explaining why only a minimum volume of hot solvent is used, link it to yield: using more solvent leaves more product dissolved after cooling and therefore reduces the amount recovered.