Professor Dave here, let’s talk about recrystallization.
After learning a variety of separation techniques, we will need to look at just one technique
for the purification of solids.
Sometimes a chemical reaction will occur such that a solid preciptate will form, which we
can then filter out of solution.
But it is inevitable that this solid will contain impurities.
When the lattice structure is forming during the precipitation, ions and other particles
that do not belong in the lattice will get trapped inside and disrupt the pattern of
the lattice.
There is a solution to this problem, and it is called recrystallization, so let’s see
how that works now.
Let’s say we have our impure solid that we wish to recrystallize.
The way this works is that we will choose a very particular solvent in which to dissolve
this solid.
The solid must have a very low solubility in this solvent at room temperature, but a
very high solubility in this solvent at very high temperatures, which makes sense given
that solubilities will tend to increase as temperature increases, given the entropic
favorability of dissolution.
So we will choose a solvent carefully, and we will heat it up to boiling or near boiling.
This will then be added to the impure solid, which has been placed in a beaker.
We add more solvent, heating and stirring the contents of the beaker as necessary until
all of the solid is dissolved, being sure to add only as much as is necessary for dissolution
to occur.
At this point, the impurities which were locked in the lattice are liberated, and can now
float around in solution.
Once dissolution is complete, we can then begin the cooling process.
This will be done by first removing any heat source and allowing the beaker to come down
to room temperature.
Then the beaker can be placed in an ice bath, whereby the cooler surroundings will draw
heat away from the contents of the beaker.
Crystals should begin to form in the beaker, which will be a much purer version of the
solid, with the impurities having been removed and which now are extremely dilute in solution.
If crystal formation is slow or difficult to initiate, we can use a seed crystal, whereby
we add a little bit of the solid that is supposed to form, which provides a surface around which
ions in solution can coordinate, and this will kick start the crystallization process.
This can also sometimes be done simply by scratching the side of the beaker with a stirring
rod, as this will liberate a tiny amount of glass that can also provide seeding behavior.
Once crystallization is complete, we can use some filter paper on a Buchner funnel, and
dump the contents of the beaker onto the filter paper.
Utilizing a vacuum, the solvent comes through the funnel and the crystals will sit on top
slowly drying, and after sufficient time, we should have crystals that are nice and
dry and pure.
We can test their purity by performing melting point analysis and comparing the melting point
range to tabulated values for pure compounds.
A narrow melting point range very close to the tabulated value indicates a high degree
of purity, whereas a wider range or a less accurate range indicates that impurities still
persist.
So recrystallization is quite a simple technique.
It really just consists of dissolving a solid in hot solvent, and then allowing things to
cool so that it crystallizes again, but this time with very little of the initial impurity
within the lattice.
The trick is to choose the correct solvent that will allow this to proceed properly,
since we need the solid to be relatively insoluble in this solvent at room temperature, yet soluble
at higher temperatures.
In addition, the ideal situation involves an impurity that is soluble at room temperature,
so that it remains in solution even when it cools, and has no opportunity to precipitate
along with the desired product.
Or similarly, the impurity could remain insoluble even at the higher temperature, so that once
the solid dissolves, we can simply remove the solid impurity by filtration.
This selection process often simply involves trial and error, prior experience involving
solvents of differing polarities, or a prediction made on the basis of tabulated solubility
data, if such data exists for the compound that is intended for recrystallization.
Whatever the case may be, as long as the solid is able to dissolve and then recrystallize,
the resulting crystals will certainly be of a higher purity than they were before, as
it is inevitable that some proportion of the impurities will not be able to find themselves
within the lattice as it reforms, so recrystallization is a powerful technique for the purification of solids.