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How To Do a Recrystallization

By Edited Nov 13, 2013 0 0
Crystals

A recry
stallization is a technique widely used in synthetic chemistry for the purification of solid compounds. The technique utilizes the fact that a solvent's ability to dissolve a given compound increases with increasing temperature. Thus, a solid compound with impurities is brought into solution by heating and subsequently forced out of solution by cooling. If the crystallization process that happens during cooling is allowed to happen slowly, then the compound should crystallize in a pure form while the less abundant impurities should stay in solution.

Things You Will Need

Impure chemical compound
One or more suitable organic solvents
Suitable chemical glassware

Step 1

First you need to identify a suitable solvent for your recrystallization. A good solvent will have a fairly high boiling point and your compound should have a low solubility in this solvent at room temperature but a high solubility near the boiling point of the solvent. Some commonly used solvents are hexane, heptane, cyclohexane, chloroform, toluene, water, methanol, ethanol, isopropanol, acetonitrile, ethyl acetate and butyl acetate.

Step 2

Add a small amount of solvent to your solid compound and heat the suspension close to the boiling point. Add more solvent in small portions until all of the solid compound has dissolved. You can do this in an Erlenmeyer flask or a round-bottom flask with a reflux condenser using a water bath, an oil bath or a heat gun as the heat source. Make sure to stir or swirl the solution to avoid excessive bumping at reflux. If the last bit of your solid compound does not dissolve when adding more solvent, it might be an insoluble impurity. You can remove any insoluble impurities at this point by decanting or filtering the warm solution.

Step 3

You should now have a clear solution without solid residues and the next step is to allow the solution to slowly cool back down to room temperature whereupon crystallization should occur. Do this simply by removing the heat source. Be patient and allow plenty of time for crystals to form. If no crystallization is observed you can cool the solution further down using for example an ice bath or you can allow some of the solvent to evaporate, add a seed crystal or scratch the inside of the flask with a spatula to induce crystal formation.

Step 4

When the crystallization has occurred you need to separate the crystals from the solution, which hopefully mostly contains the soluble impurities. This is typically done by suction filtration using a Buchner funnel. You can wash the isolated crystals on the filter with a small amount of cold solvent, but be careful not to redissolve your recrystallized material. Dry the isolated crystals in air or in a vacuum oven.

Step 5

If you cannot find a suitable solvent for recrystallizing your compound, you can use a two-solvent recrystallization approach instead. For this, you use a solvent – one that easily dissolves your solid compound at an elevated temperature – and an anti-solvent – one that does not dissolve your solid compound. Dissolve your solid compound in a small amount of solvent at an elevated temperature and then add small portions of the anti-solvent until the solution turns cloudy and your compound starts to come out of solution again. Add a small amount of solvent to bring everything back into solution and proceed as described for the one-solvent recrystallization above. Some commonly used solvent/anti-solvent pairs are chloroform/ethanol, isopropanol/water and toluene/methanol. Note that the two solvents should be miscible and both should have fairly high boiling points.

You might find that one set of conditions works well for removing one impurity while another set of conditions is better for removing another impurity.

It is not uncommon to do several recrystallizations to purify a compound to a high standard.

You should try to avoid solvents with boiling points higher than the melting point of the solid compound or at least refrain from heating the solution at a temperature higher than the melting point of the solid compound to avoid melting the compound.

Tips & Warnings

Chemistry experiments should only be done under the supervision of trained chemists in proper chemistry labs using extracted fume hoods.

Always use appropriate personal protective equipment such as safety glasses, lab coats and disposable gloves.
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