How to Perform a Recrystallization

How to Perform a Recrystallization - Introduction

A Buchner funnel may be placed on top of a Buchner flask (filter flask).
A Buchner funnel may be placed on top of a Buchner flask (filter flask) so that a vacuum may be used to separate or dry a sample. Eloy, Wikipedia Commons

Recrystallization is a laboratory technique used to purify solids based on their different solubilities. A small amount of solvent is added to a flask containing an impure solid. The contents of the flask are heated until the solid dissolves. Next, the solution is cooled. ​ The more pure solid precipitates, leaving impurities dissolved in the solvent. Vacuum filtration is used to isolate the crystals. The waste solution is discarded.

Summary of Recrystallization Steps

  1. Add a small quantity of appropriate solvent to an impure solid.
  2. Apply heat to dissolve the solid.
  3. Cool the solution to crystallize the product.
  4. Use vacuum filtration to isolate and dry the purified solid.

Let's take a look at the details of the recrystallization process.

How to Perform a Recrystallization - Add the Solvent

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...

How to Perform a Recrystallization - 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...

How to Perform a Recrystallization - Cool the Solution

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.

Once​ crystals have formed, it's time to separate them from the solution...

How to Perform a Recrystallization - Filter and Dry the Product

Crystals of purified solid are isolated by filtration. 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.