Purifying by Recrystallisation
When you make an organic compound it needs to be purified, especially if it is a pharmaceutical chemical. Most organic reactions produce by products even if the reaction is a ‘clean’ one. In the laboratory, this is done by recrystallisation. Find a solvent that dissolves the product more readily at a high temperature rather than at a low temperature, make a hot solution, and allow crystallising on cooling. The crude product may contain impurities which
o are insoluble in the solvent;
o are slightly soluble in the solvent; and
o Dissolve readily in the solvent.
The solvent itself also has to be removed or it acts as an impurity. No residue must be left behind.
Method:
1. Shake your sample with the solvent and warm to dissolve
o The product should dissolves only in hot solvent. Soluble impurities will also dissolve, but there shouldn’t be that many impurities. Insoluble impurities will stay.
2. Filter the solution hot. Use a Buchner funnel and pump and make sure that the solution does not cool too much whilst being filtered.
o Insoluble impurities should stay on the filter paper, soluble impurities and the product should stay in the solution and in the filtrate
3. Allow the solution to cool slowly. If no crystals appear, stir for a few minutes. (If still no crystals appear you have probably added too much solvent!)
o The product will become less soluble as the mixture cools and eventually crystallise. Soluble impurities are less concentrated so they stay in the solution.
4. Filter the solution cold. Using a Buchner funnel and pump. Keep the residue.
o The crystals are separated from the solvent which still contains soluble impurities, leaving the product contaminated only with solvent in which is dissolved a small amount of soluble impurity.
5. Wash the residue with a small amount of cold solvent.
o The Contaminated solvent will pass through the funnel, leaving only product and pure solvent on the filter paper. Warm solvent would dissolve a significant amount of product and it would pass through the filter paper.
6. Dry the product on a watch glass at room temperature.
o Because the solvent is pure it leaves no residue apart from the product.
