Once all of the sample is dissolved from heating, the flask
is placed in an icebath (or refrigerator) to cool it down.
See Figure 5. Often, it's desirable to allow the solution
to cool to room temperature in open air before placing it
in the icebath, as slow cooling often leads to purer crystals.
|Figure 5. Cooling the solution.
cooling for some time, crystals should form on the bottom
of the flask, as shown in Figure 6. You can often aid crystallization
in stubborn cases by scratching the inside of the flask with
a glass rod at the air-solvent junction. This scratching flecks
off small shards of glass that form a surface upon which the
solid can crystallize. Sometimes, the solution can be "seeded"
by adding a small seed crystal of pure sample to the cold
solution. This seed crystal also forms a surface for the solid
to crystallize upon. If no crystals form upon cooling, you
may have used too much solvent, and should evaporate some
of the solvent and try again.
|Figure 6. Fifteen minutes later...
Once the solid has crystallized, you then filter
the suspension to obtain the pure solid. This is typically
done via vacuum filtration (Figure 7). Once the solid has
been filtered, it is sometimes washed with additional quantities
of chilled solvent. If you wash the sample, make sure not
to overdo it because the added solvent will dissolve some
of your sample (this is why it's important to add cold solvent
to wash the sample).
have the product, it should be dried. Much of the drying can
be done by aspirating the product in the vacuum filtration
apparatus for several minutes before removing it. The purified
solid can be dried in open air or under a vacuum. If you need
a small amount of dry sample (to do a melting point analysis,
for example), you can rub a small amount of the solid on a
shard of porous porcelain, or between two sheets of filter
paper. In difficult cases, it may be necessary to perform
more than one recrystallization to completely purify the sample.
to Laboratory Help