Crystallization

Crystallization is a technique used to purify solid compounds. This success of this process is based upon the different solubilities of the desired compound and impurities in a given solvent and also the fact that all compounds are more soluble in a hot solvent than a cold one. In general, the crude product crystals are dissolved in a minimal amount of hot (boiling) solvent. A boiling stick, boiling chip, or magnetic stirbar can be added to ensure smooth boiling. At this stage, any material which does not dissolve can be removed by filtration. The hot solution is allowed to slowly cool and crystals of the desired material form while the impurities remain in the solvent. The crystals are usually collected by vacuum filtration. While this technique does result in some product loss, the resulting crystals are much more pure. This procedure is outlined below for crystallization of benzoic acid from water.
Above on the left, the powdered benzoic acid was added to water and the water was allowed to boil. (Be sure to add a boiling stick or chip to avoid bumping.). On the right just enough water has been added to the flask to dissolve all of the benzoic acid. The solution was allowed to cool to room temperature.
On the above left, the crystals have begun to form. After the crystals and solvent have reached room temperature, the flask was transferred to an ice bath for further cooling. Above right, the crystals and water (to be used during the filtration step) are in the ice bath.
The filtration apparatus is shown above left. Note that the filtration flask is clamped securely. For help with vacuum filtration, click here. Be sure that before adding crystals to the funnel some recrystallizing solvent is added and filtered to ensure that the filter paper is securely seated in the funnel. With the vacuum turned on, pour the crystals into the funnel. Wash the crystals with ice cold solvent and allow to crystals to dry. On the right above is the starting benzoic acid (hexagonal weigh boat), a powdery white substance, and the final crystals (watchglass), shimery white flakes.

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