Recrystallizations are commonly performed in organic chemistry experiments to purify solids based on their solubility. Solid crystals are dissolved in a solvent. When the solution is cooled, crystals of pure product form and the impurities remain dissolved in the solvent. Multiple recrystallizations may be performed to obtain a purer product:
Steps to Recrystallization:
1. Choose a suitable solvent: Place a small amount of crystals in several test tubes. To each test tube, add a different solvent. Common recrystallization solvents include methanol, ethanol, cyclohexane, benzene, acetone, and water. The solvent should have low solubility at low temperatures and high solubility at high temperatures. The crystals should not dissolve in the solvent at room temperature, but should dissolve upon heating.
2. Transfer the crystals to an Erlenmeyer flask. The use of an Erlenmeyer flask is important because the solution will be heated. The small neck of an Erlenmeyer flask prevents the small amounts of solvent added from evaporating off. The flask size is dependent on the amount of crystals being purified. Choose a flask 2-3 times larger than the amount of solvent you think you will add.
3. Dissolve the crystals: Add a small amount of solvent to the crystals and slowly heat on a hot plate or in a hot water bath. Once the solution is boiling, use a pasteur pippete to add small amounts of solvent incrementally until the crystals are dissolved. Do not dissolve the crystals below the boiling point temperature of the solution or too much solvent will be used. Use as little solvent as possible to dissolve the crystals. Some impurities are insoluble, so do not continue adding solvent to dissolve a few solid particles.
4. Removing solid impurities by gravity filtration: Add 2 mL of recrystallization solvent in a clean Erlenmeyer flask. Place a fluted filter in a short-stemmed funnel and put the funnel in the Erlenmeyer flask. Heat the Erlenmeyer flask and pour the hot recrystallization solution through the funnel. To avoid crystallization, keep the unfiltered solution hot and add it to the funnel in small quantities. If the filtered solution in the Erlenmeyer crystallizes, redissolve the solid and allow it to cool slowly, according to the directions in step 6.
5. Removing color impurities with activated charcoal: If the crude crystals are colorless and the recrystallization solution is a dark color, color impurities can be removed with activated charcoal. Add 50 mg of activated charcoal to the hot solution. Heat the solution to just below the boiling point for a few minutes. Filter out the charcoal according to the gravity filtration instructions in step 4.
6. Cool the solution: The slower the solution is cooled, the slower the crystals will form and the purer they will be. Take the flask off the heat allow it cool to room temperature. Once the solution is room temperature, place it in an ice bath or refrigerator. If no crystals form the solution may be supersaturated. Scratch the inside of the flask just below the surface of the liquid with a glass stirring rod to induce crystallization. Scratching the flask increases the surface area of the glass and provides a rough surface for the crystals to form. Crystallization can also be induced by “seeding”. A small amount of crude crystals is added to the cool solution. If crystals still fail to form, too much solvent may have been added. Boil off excess solvent and cool the solution again.
7. Collect the pure crystals: Chill a washing solvent. If the solvent is not chilled, the crystals may dissolve and decrease the percent yield. Use vacuum filtration. Pour the solution of liquid and crystals into filter. Wash the recrystallization flask with the chilled solvent and pour into the filter. The crystals can be dried by aspiration or air drying.