USEFUL PROCEDURAL GOODIES
For Single and Double Unknown Analyses
RECRYSTALLIZATION (to purify your desired product)
This technique takes advantage of differences in solubility to separate desired crystals from contaminant crystals in a solid.
Crystals are dissolved in a suitable hot solvent, then cooled. As the solution cools, only the pure crystals crash out, while the
contaminants remain dissolved. The key to success is having a hot clear solution that you can cool slowly (keeping it
covered) so that crystals can form slowly while excluding impurities. Filtration can then be used to separate the pure crystals
from the impurities left in the filtrate.
The criteria for a suitable solvent choice are:
 When heated, the solvent dissolves everything (all product and all impurities)
 When the solution cools, the impurities stay dissolved, but the desired product becomes insoluble and crashes out
Sometimes one solvent will meet both of these criteria, but sometimes there isn’t one good choice of solvent. In this case,
mixing two miscible solvents with different solubility properties can accomplish this (see #5 below). Often ethanol is a good
choice of solvent to begin with, but consult the protocol you are using.
Follow these steps to recrystallize your product:
1.
Work in a beaker: Choose beaker size so that it will be ~1/2 full or less after all solvent has been added. Keep the
beaker covered with a watch glass unless you are actively working inside it.
2.
Dissolve your crystals in a small amount of a suitable solvent: Put your crude solid into the beaker and dissolve
in a suitable solvent as you heat the beaker (see above for suitable solvent choice). Use the smallest volume of
solvent that, when at its boiling point, is able to completely dissolve the solid (use trial and error). Sometimes there
is insoluble matter that does not disappear even when you add a little more solvent. Insoluble material needs to be
removed by hot filtration – see instructions for Hot Filtration.
3.
Check your solvent choice with this test: After you have a clear, hot solution, pour about 1 mL of the hot solution
into a test tube. Swirl the tube vigorously to cool its contents fast (if no crystals form, cool in ice).


If crystals do form in the tube, this is a
good solvent. Swirl the tube contents, return
them to the beaker, and cover it. Reheat if
necessary to get a clear solution, then cool
slowly, eventually on ice (go to #4 below).
If no crystals form in the tube, either you have used
too much solvent, or this solvent won’t work. You
may need a mixed solvent system (go to #5 below).
4.
Scratch quickly and cool slowly. While stirring/scratching the solution vigorously with a glass stir rod, allow it to
cool slowly to room temperature and then on ice until crystals form. You are done with this process when it appears
that no more crystals will form.
5.
Mixed-solvent recrystallization. Your crystals may be very soluble in one solvent, in which case they may not
crash back out. They may be insoluble in another solvent, and you won’t be able to dissolve them at all. If there is
not one solvent that meets the criteria for a suitable solvent, employ a mixed-solvent system. Usually, an alcoholwater mixed system will work (but the solvents must be miscible). First, dissolve the crystals in a small amount of
warm alcohol (ethanol or methanol – check your specific protocol). Add water slowly to the solution and mix to see
if cloudiness forms. If it becomes cloudy, stir and scratch gently. If crystals form, return the tube contents to the
beaker, add the appropriate amount of water, cover, reheat as needed to get a clear solution, and cool as above in #4.
The goal is to find a balance between the two solvents by adjusting their proportions little by little.
6.
Still not getting crystals? Return the contents of the tube to the beaker, keep it uncovered and set your glass stirring
rod inside, boil the solution down to ½ its current volume, and then start again at #4.
7.
Filter your crystals on a Hirsch funnel and let them air-dry for at least a day before taking a melting point.
-1-
USEFUL PROCEDURAL GOODIES
For Single and Double Unknown Analyses
HOT FILTRATION (to remove insoluble material from a recrystallization in progress)
This technique only needs to be performed if there are insoluble contaminants in your solution even when the solution is heated to
clearness. Look at the setup in the figure below. The idea is to create a prime environment for isolating all the soluble product and
impurities, which get extracted into the solvent below, from the insoluble impurities, which get left behind in the filter up top. Once
the soluble product and soluble impurities are extracted into the solvent below, you can take this solution and carry out
recrystallization as normal. The reason why only a thin layer of alcohol solvent should be used in the bottom of the first beaker is that
you end up diluting your product more than you might like to through this hot filtration, so it will be more difficult to coax crystals to
form in recrystallization in this less concentrated environment.
Follow these steps to hot-filter your solution.
1.
Assemble the apparatus shown below: Use a 400 ml beaker and stemless funnel. Fold the largest piece of filter paper you
can find into a cone (fold in half, and then in half again), tear off one of the top corners (not the tip!) to avoid suction of the
paper against the glass walls, and insert the filter paper into the funnel. Cover this setup with a large watch glass.
2.
Add solvent: Fill the beaker with a thin layer of alcohol solvent. Bring beaker solvent to a slow boil and allow vapors to heat
up all glassware. At the same time, heat the suspension you wish to filter (your dissolved product solution).
3.
Add your suspension: The glassware of the setup is warm enough when the boiling solvent begins to drop condensate off
the tip of the funnel. At this point, uncover the beaker and pour as much of your suspension as possible into the funnel.
4.
Wait until solution has passed through the filter: Cover again and continue to the boil very gently (but don’t let it go dry).
The desired material is in the filtrate. Discard the filter paper and insoluble impurities you captured there.
5.
Recrystallize your product: It’s already dissolved and heated, so just scratch vigorously while cooling slowly to see if
crystals form.
6.
Not getting crystals? You’ve added solvent volume to the system, so this is likely to occur. Boil the solution down to ½
volume, then scratch vigorously while cooling slowly.
Pour in hot
suspension
Keep
covered
with
watch
glass!
Hot Plate (gentle heating)
Setup for hot filtration
2
Boiling
solvent
(thin
layer)