3. Cooling the solution

Once all of the sample is dissolved from heating, the flask is placed in an icebath (or refrigerator) to cool it down. See Figure 5. Often, it's desirable to allow the solution to cool to room temperature in open air before placing it in the icebath, as slow cooling often leads to purer crystals.

After cooling for some time, crystals should form on the bottom of the flask, as shown in Figure 6. You can often aid crystallization in stubborn cases by scratching the inside of the flask with a glass rod at the air-solvent junction. This scratching flecks off small shards of glass that form a surface upon which the solid can crystallize. Sometimes, the solution can be "seeded" by adding a small seed crystal of pure sample to the cold solution. This seed crystal also forms a surface for the solid to crystallize upon. If no crystals form upon cooling, you may have used too much solvent, and should evaporate some of the solvent and try again.

4. Filtering the solution

Once the solid has crystallized, you then filter the suspension to obtain the pure solid. This is typically done via vacuum filtration (Figure 7). Once the solid has been filtered, it is sometimes washed with additional quantities of chilled solvent. If you wash the sample, make sure not to overdo it because the added solvent will dissolve some of your sample (this is why it's important to add cold solvent to wash the sample).

5. Drying the product

Once you have the product, it should be dried. Much of the drying can be done by aspirating the product in the vacuum filtration apparatus for several minutes before removing it. The purified solid can be dried in open air or under a vacuum. If you need a small amount of dry sample (to do a melting point analysis, for example), you can rub a small amount of the solid on a shard of porous porcelain, or between two sheets of filter paper. In difficult cases, it may be necessary to perform more than one recrystallization to completely purify the sample.

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