Definition for The Crystallization Procedure
From Biology Forums Dictionary
Recrystallization is a useful purification method for most organic compounds that are solids at room temperature. The selection of a proper solvent is the most critical part of the recrystallization procedure. Organic solids are usually more soluble in hot solvent than in a comparable volume of cold solvent. The process of recrystallization involves dissolution of the solid in an appropriate solvent at elevated temperature and the subsequent reformation of the crystals upon cooling. This way, many impurities will stay in solution and your target compound is purified. However, it is not possible to recover your entire compound following a recrystallization. To maximize your yield, it is very important to only use a minimum amount of hot solvent.
Choosing the appropriate solvent is the most challenging part of the recrystallization. In choosing a solvent, remember that “like dissolves like”, which means that a nonpolar compound will dissolve well in a nonpolar solvent, and a polar compound will dissolve well in a polar solvent. Also note whether a solvent is flammable or not and find its boiling point. This will enable you to select an appropriate heating device. Generally speaking, solvents with boiling points below 100°C should be heated in a water bath (NO OPEN FLAME), above 100°C can be heated directly on a hot plate or you may use a sand bath.
Selecting a solvent is crucial for recrystallization of an organic solvent. An ideal recrystallization solvent should:
- Dissolve the entire compound at the boiling point of the solvent.
- Dissolve very little or none of the compound when the solvent is at room temperature.
- Have different solubilities for the compound and the impurities
- Have a boiling point below the melting point of the compound so that the compound actually dissolves, not melts, in the hot solvent.
- Have a relatively low boiling point.
The best way to be certain that a solvent will be a good recrystallization solvent it to try and see it. Test the solvent on a small amount of the compound. If a solvent doesn’t dissolve the crystals when hot, or it readily dissolves the crystals at room temperature, try another solvent. Remember to always use only a minimum amount of hot solvent.
Once you have selected your solvent you are ready to recrystallize your sample. The process of recrystallization can be broken into the following steps:
- Selection of an appropriate solvent or mixture of solvents (consult table)
- Dissolution of the solute using a minimum amount of hot solvent
- Decoloration of the solution if necessary (with an activated form of carbon)
- Filter: Removal of suspended solids (through filtration of the hot solution)
- Crystallization of the solid from the solution as it cools
- Collecting the purified solid by filtration
- Washing the crystals with an appropriate solvent to remove impurities
- Drying the crystals
Following is a flow chart for the recrystallization procedure:
