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recrystallization guide > laboratory help > home      

Recrystallization Guide

What's the big idea

Recrystallization is an often-used method for purifying solids. Recrystallization works by taking advantage of the different solubility properties of compounds, and allows impurities to be removed from crude solids.

Performing a recrystallization is usually a straightforward task. First, you add a small amount of solvent to the impure compound in a flask, and heat the contents until all the material has dissolved. Then, you cool the solution, allowing crystals of purified solid to crash out of solution, while undesirable impurities stay dissolved in the solvent. You then isolate the pure crystals by vacuum filtration, and discard the waste solution.

Performing a recrystallization

Performing a recrystallization requires five steps:

  1. Adding an appropriate solvent to the impure material
  2. Heating the suspension until everything dissolves
  3. Cooling the solution to allow the product to crystallize
  4. Isolating the purified solid by vacuum filtration
  5. Drying the purified solid.

Figure 1. Flasks work well

1. Adding an appropriate solvent

When choosing a solvent for recrystallization, the impure compound should have poor solubility in the solvent at low temperatures but be completely soluble at high temperatures. This allows the solid to completely dissolve when heated, but crash out of solution when cooled. Ideally, you should add the smallest amount of solvent possible that will still completely dissolve the sample when heated. When adding solvent, it's best to err on the side of adding too little, as you can always add more as you're heating it.

Figure 2. Adding solvent

2. Heating the suspension

Once you have added the solvent to the sample, you need to heat the suspension to dissolve the sample. Heating is probably best done with either a steam bath or a hot water bath, as heating with these sources is controlled and gentle. Alternatively, you could use a hot plate (as shown in Figure 3) or a heat gun.

Figure 3. Click dial to heat suspension.

 Performing steps 3-5
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