Hi,
Recrystallization is a laboratory technique used to purify
solids based on their different solubilities. A small amount of
solvent is added to a flask containing an impure solid. The
contents of the flask are heated until the solid dissolves. Next,
the solution is cooled. A more pure solid precipitates, leaving
impurities dissolved in the solvent. Vacuum filtration is used to
isolate the crystals. The waste solution is discarded.
There are five major steps in the recrystallization process:
dissolving the solute in the solvent, performing a gravity
filtration, if necessary, obtaining crystals of the solute,
collecting the solute crystals by vacuum filtration, and, finally,
drying the resulting crystals.
- Dissolving the solute in the solvent
- Add a small portion of boiling solvent to the beaker that
contains the impure sample and a boiling chip.
- Heat the beaker containing the solute and continue adding
boiling solvent incrementally until all of the solute has been
dissolved. If additional solvent can be added with no appreciable
change in the amount of solute present, the particulate matter is
probably insoluble impurities.
- Hot Gravity Filtration
- This step is optional if there is no visible particulate matter
and the solution is the expected color (most organic compounds are
white or light yellow)
- If the solution is not the expected color, remove the boiling
solution from the heat and allow it to cool to beneath the boiling
point of the solvent. Add a small amount of activated carbon (about
the size of a pea) and mix the solution. If too much activated
carbon is used, excessive loss of the desired product will result.
Boil the solution containing the activated carbon for 5 to 10
minutes. A filter aid will need to be placed in the filter paper to
remove the carbon in the following steps.
- Flute a piece of filter paper and place it inside of a stemless
funnel. A funnel with a stem is prone to premature
recrystallization inside the stem because the filtrate can cool as
it passes through the stem. At these cooler temperatures, crystals
are likely to form.
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- Heat a beaker that contains some of your recrystallization
solvent. Place the funnel and filter paper assembly in the beaker
so that the rising vapors from the boiling solvent can heat the
funnel and filter paper. Having the set up heated before filtration
will prevent crystals from forming on the paper and in the funnel
(see Figure 1 below).
Figure 1. Hot gravity filtration. Keeping the set up hot
prevents crystals from forming prematurely.
- Keeping the solution very hot so the solute stays dissolved,
pour the solution through the funnel and filter paper assembly. As
the filtrate begins to accumulate, heat the receptacle beaker; the
resulting vapors will help to prevent any crystallization in the
funnel or on the filter paper.
- If the funnel was properly heated before filtration, all of the
solution will have passed through and no crystals will have formed
on the paper or in the funnel. If crystals have formed, pouring a
small amount of boiling solvent through the funnel will dissolve
these. If the solution is still discolored after using activated
carbon and filtering, either the color is from the compound and
will not go away or you need to repeat the step with the addition
of activated carbon.
- The solution should be allowed to cool slowly to room
temperature. Gradual cooling is conducive to the formation of
large, well-defined crystals.
- Vacuum Filtration
(see Filtering Techniques, remembering these additional
points)
- Agitate the crystals with a fire polished glass-stirring rod
before pouring the mother-liquor along with the crystals through
the Buchner funnel. Apply the maximum amount of suction possible
using the aspirator.
- Some crystals may have been left behind in the beaker; there
are two ways to effect a quantitative transfer of all of this
material. Either use a portion of the filtrate to rinse the beaker
or use a rubber policeman on the end of your stirring rod to scrape
the remaining crystals into the Buchner funnel.
- When the crystals have been collected and washed, allow the
aspirator to run for several minutes so that the crystals have an
opportunity to dry.
- Drying the Crystals
- When the crystals have been dried as much as possible in the
Buchner funnel, use a scoopula to remove them to a beaker or
crystallizing dish. This will ensure that the crystals are not
contaminated by filter paper fibers as they dry.
- After removing all the crystals from the filter paper, remove
the filter paper and scrape any remaining crystals from the
funnel.
- Spreading the crystals out in a beaker or a crystallizing dish
will provide for the most efficient drying as the crystals will
have a maximum of exposed surface area.
- When the crystals are dried, the purity of the sample can be
measured by performing a melting point determination.
- What to do if crystals don't form
If crystals don't form upon slow cooling of the solution to room
temperature there are a variety of procedures you can perform to
stimulate their growth. First, the solution should be cooled in an
ice bath. Slow cooling of the solution leads to slow formation of
crystals and the slower crystals form, the more pure they are. Rate
of crystallization slows as temperature decreases so cooling with
an ice bath should only be used until crystals begin to form; after
they do, the solution should be allowed to warm to room temperature
so crystal formation occurs more slowly. If no crystals form even
after the solution has been cooled in an ice bath, take a fire
polished stirring rod and etch (scratch) the glass of your beaker.
The small pieces of glass that are etched off of the beaker serve
as nuclei for crystal formation. If crystals still do not form,
take a small amount of your solution and spread it on a watch
glass. After the solvent evaporates, the crystals that are left
behind can serve as seeds for further crystallization. Both these
methods of nucleation (i.e. etching and seed crystals) cause very
rapid crystallization, which can lead to the formation of impure
crystals.
Crystals will not form if there is a large excess of solvent. If
no crystals form with the methods already discussed, a portion of
the solvent may need to be removed. This can be accomplished by
heating the solution for a period of time in order to evaporate
some solvent. The new, concentrated solution, should be cooled, and
the previously mentioned methods to stimulate crystallization
should again be attempted.
Another potential problem in recrystallization is that the
solute sometimes comes out of solution in the form of an impure oil
instead of forming purified crystals. This usually happens when the
boiling point of the solvent is higher than the melting point of
the compound, but this is not the only scenario in which this
problem presents itself. If this begins to happen, cooling the
solution will not stimulate crystallization, it will make the
problem worse. If an oil begins to form, heat the solution until
the oil portion dissolves and let the whole solution cool. As the
oil begins to form again, stir the solution vigorously to break up
the oil. The tiny beads of oil that result from this shaking may
act as the nuclei for new crystal formation.