In synthetic chemistry, a precipitation (or reprecipitation) can be carried out as a first crude purification step prior to further purification by for example recrystallization. The crude material, which can be either an oil or a solid, is dissolved in a small amount of a good solvent and this solution is then dropped into a larger amount of a poor solvent to cause instant precipitation. Small impurities are easily trapped in the precipitated material, but if there is a significant difference in solubility between the desired product and the present impurities, this process can be an efficient way to quickly remove relatively large amounts of impurities with minimal loss of material.

Things You Will Need

Impure chemical compound
Two suitable organic solvents
Suitable chemical glassware

Step 1

First, you will need to identify both a good solvent that can easily dissolve your crude material at room temperature and a poor solvent that will not dissolve your product but hopefully some of your impurities. Bear in mind that these two solvents should be miscible. Some examples of useful solvent pairs with quite different properties are dichloromethane/methanol, toluene/methanol, alcohols/water and tetrahydrofuran/water.

Step 2

Dissolve your crude material in a small amount of the good solvent at room temperature. Any insoluble impurities can be filtered off at this point. Transfer your solution to a dropping funnel or similar as this will make it easy to add this solution drop wise to the poor solvent.

Step 3

Add your solution drop wise to a large amount of poor solvent. A ratio of at least 1:5 or 1:10 between the good solvent and the poor solvent is typically used. Stir the solution during addition to ensure proper mixing of the solvents.

Step 4

The crude material should precipitate immediately upon addition. If the good solvent is more volatile than the poor solvent, slow evaporation will facilitate complete precipitation as the concentration of the poor solvent slowly increases. Isolate the crude material by suction filtration on a Buchner funnel and wash it on the filter with a small amount of the poor solvent.

Tips & Warnings

Chemistry experiments should only be done under the supervision of trained chemists in proper chemistry labs using extracted fume hoods.

Always use appropriate personal protective equipment such as safety glasses, lab coats and disposable gloves.