Summary of Recrystallization Steps
- Add a small quantity of appropriate solvent to an impure solid.
- Apply heat to dissolve the solid.
- Cool the solution to crystallize the product.
- Use vacuum fitration to isolate and dry the purified solid.
Choose a solvent such that the impure compound has poor solubility at low temperatures, yet is completely soluble at higher temperatures. The point is to fully dissolve the impure substance when it is heated, yet have it crash out of solution upon cooling. Add as small a quantity as possible to fully dissolve the sample. It's better to add too little solvent than too much. More solvent can be added during the heating process, if necessary. The next step is to heat the suspension...
After the solvent has been added to the impure solid, heat the suspension to fully dissolve the sample. Usually a hot water bath or steam bath is used, since these are gentle, controlled heat sources. A hot plate or gas burner is used in some situations. One the sample is dissolved, the solution is cooled to force crystallization of the desired compound...
Slower cooling may lead to a higher purity product, so it's common practice to allow the solution to cool to room temperature before setting the flask in an ice bath or refrigerator.
Crystals usually begin forming on the bottom of the flask. It's possible to aid crystallization by scratching the flask with a glass rod at the air-solvent junction (assuming you are willing to purposely scratch your glassware). The scratch increases the glass surface area, providing a roughened surface on which the solid can crystallize. Another technique is to 'seed' the solution by adding a small crystal of the desired pure solid to the cooled solution. Be sure the solution is cool, or else the crystal could dissolve. If no crystals fall out of solution, it's possible too much solvent was used. Allow some of the solvent to evaporate. If crystals do not spontaneously form, reheat/cool the solution.One crystals have formed, it's time to separate them from the solution...
Crystals of purified solid are isolated by filration. This is usually done with vacuum filtration, sometimes washing the purified solid with chilled solvent. If you wash the product, be sure the solvent is cold, or else you run the risk of dissolving some of the sample. The product may now be dried. Aspiring the product via vacuum filtration should remove much of the solvent. Open-air drying may be used as well. In some cases, the recrystallization may be repeated to further purify the sample.