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:
- Adding
an appropriate solvent to the impure material
- Heating
the suspension until everything dissolves
- Cooling
the solution to allow the product to crystallize
- Isolating
the purified solid by vacuum filtration
- Drying
the purified solid.
Figure 1. Flasks work well |
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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.
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.
Performing
steps 3-5
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