Standard Recrystallization Technique
In the recrystallization of acetanilide, your solution is treated with decolorizing carbon (powdered
charcoal), which must then be removed by hot filtration. When a hot filtration is being performed, extra
solvent is added to discourage your product from crystallizing out in the funnel during the hot filtration.
It’s not often that you have to remove impurities that are less soluble than your target compound
(think: charcoal, sawdust, cat hair….). Usually the traces of impurities are soluble in the chosen
solvent; in this case the procedure for recrystallization simplifies to:
Dissolve the solid in the minimum amount of hot solvent, then cool slowly
Even though a written procedure may give a specific amount of solvent to use for a recrystallization,
these values usually are only an indication of how much solvent may be required. In actuality, you will
be using the minimum amount of solvent required to do the job.
A recrystallization solvent is chosen such that the solubility of your compound is high at the boiling
point, but low at room temperature. You usually start by adding just enough cold solvent to cover the
bottom of the container being used (e.g. an Erlenmeyer flask). This solvent protects your solid from
the heat as it’s applied (e.g. by using a hotplate) because, with agitation, the internal temperature
shouldn’t exceed the boiling point of your solvent. Otherwise, your solid can “scorch” or thermally
decompose as the flask is heated. A stir bar or boiling stone is used to ensure even boiling and to
prevent “bumping” (explosive boiling).
Once the mixture reaches a boil, small amounts of solvent are added portionwise. Typically, you’ll use
a Pasteur pipette to add the solvent down the sides of the vessel, although a squirt bottle can be used
for distilled water. This method of addition serves two purposes: any solid clinging to the sides of the
vessel are washed back down into the bottom, and the added solvent has a chance to warm up
before it hits the surface of your solution.
Written procedures often pre-heat the solvent to be added, but on the scale your recrystallizations are
performed on this normally isn’t required. When the new solvent is initially added to the flask, the
boiling action will typically subside because of the temporary plunge in temperature. You may also
see some solute come out of solution (e.g cloudiness where the solvent entered the vessel). Once the
solvent resumes boiling, this material will have re-dissolved, and you are ready to add more solvent. It
should not take long to resume boiling, and solvent should be constantly added at a good pace.
Otherwise, if old solvent is boiling away faster than you are adding new solvent, you are going
backwards! This is a common problem with solvents that are more volatile than water.
You are looking for the point where the last traces of solid or cloudiness just barely disappear. At that
point, you have a saturated solution, and you should then remove the vessel from the heat and allow
it to cool slowly and undisturbed. As the temperature decreases, so does your compound’s solubility,
and it usually crystallizes from solution.
Many of the common errors students make involve the underlined words in the procedure above.
Minimum amount of solvent: Your compound’s solubility at room temperature is not zero. After
filtration, the mother liquor is saturated in your compound (in the “real world”, this solvent is often
concentrated and refiltered to collect second, third, fourth etc. “crops” of crystals and maximize yield).
If you used too much solvent, you will get less product crystallizing out later. The more solvent you
use, the more product you lose.
Getting the solvent hot enough: Usually “hot” means “boiling”. Especially in the case of water,
sometimes the solvent gets hot and steamy but doesn’t quite reach a boil. That means you haven’t
maximized the solvent’s ability to dissolve solute; you end up using more solvent, and losing more
product in the mother liquor.
Cooling: On occasion, a student performs a hot filtration instead by mistake, then wonders why
nothing was filtered out. You need to cool the solution at least to room temperature to maximize the
amount of solid that crystallizes out. Sometimes a procedure directs you to go further and chill the
mixture in an ice bath, but that can have an unfavorable side effect, which leads to…
Slow and undisturbed cooling: The growing crystals act as a template, and selectively incorporate
molecules of itself into the crystal lattice. The more rapidly these crystals grow, the less selective they
are, and the more impurities get trapped inside the growing crystal.
Recrystallization acts as a method of purification because the crystals exclude impurities, which
remain in the mother liquor. After filtration, the impurities are carried away in the filtrate.
Rapid cooling leads to rapid, less selective crystal formation. So does mechanical shock. For
example, sometimes after cooling crystals haven’t formed, and a “supersaturated solution” results.
Until a “seed crystal” forms, the molecules don’t have a template and may not find a way to come out
of solution. A trick that can be used to get crystals to grow out of such a supersaturated solution is to
scratch the sides or bottom of the flask with a spatula or glass rod. The vibrations trigger seed
formation, and the material rapidly crystallizes out.
Unless necessary, such mechanical shock (scratching, scraping, picking the flask up, putting it down)
is best avoided. You will get the largest, purest crystals if you have the fewest seed crystals
generated and the slowest recrystallization feasible.