Questions on Recrystallization
1. Why, no matter how careful you are, can you never get back 100% of your desired
material in a recrystallization?
2. Where could you lose material in your recrystallization? For each answer tell how likely
it is and how much would be lost … a tiny bit, a substantial amount, a lot.
3. Why do you want the crystallization of the sulfanilamide to proceed slowly?
4. Why do you need to use “ice cold” ethanol to wash your product?
5. What is your theoretical recovery of sulfanilamide? What is your percent recovery?
Why is your recovery higher, or lower, than the theoretical?
6. Compare the melting points of the starting and ending samples of acetanilide. How do
they compare? What does that tell you?
7. If you had an insoluble impurity in your sample, how could you remove it?
Purification of a Crude Solid by Crystallization
Adapted from: Pavia, D.L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. A Small Scale Approach to Organic
Laboratory Techniques; Brooks/Cole Cengage Learning; Belmont, CA, 2011; pp 16‐24.
Introduction
The purpose of this experiment is to introduce the technique of crystallization, a very common
procedure used to purify solids that contain impurities. This technique is based on the fact that both the
solid and the impurities may dissolve in a given solvent, but not to the same extent. Solubility is a
function of the concentration, the temperature and the polarity of the system. These parameters can
be manipulated to vary the solubility of a solid in a particular solvent.
Crystallization involves completely dissolving the crude solid by heating it in a suitable solvent that
dissolves both the solid and any impurities, making a saturated solution. The solution is then cooled
down slowly. The target substance has a decreased solubility at lower temperatures and, upon cooling,
crystals appear in the cooled solution, from which they are separated by filtration. During the filtration
step, the crystals are washed with minimal amounts of the cold crystallization solvent to rinse away the
small amount of impurities remaining. The crystals are ordinarily much purer than the crude solid
because impurities that were originally present in small amounts should remain in solution, and thus
removed by filtration. The purity of the crystallized material can be determined by measuring the
melting point of your sample.
Techniques
To successfully complete this lab, you should familiarize yourself with two important techniques in
Organic Chemistry: Crystallization and Melting Point. The web sites listed below offer a solid
introduction of these techniques, but you may need to search other sites for information, as well (e.g.
Wikipedia, Google search, etc.). Some of the pre‐lab questions can only be answered after you have
spent some time on these sites.
Crystallization Technique:
http://orgchem.colorado.edu/Technique/Procedures/Crystallization/Crystallization.html
Determination of Melting Point Technique:
http://www.chem.ucalgary.ca/courses/351/laboratory/meltingpoint.pdf
In this lab, you will carry out the crystallization of impure sulfanilamide using 95% ethyl alcohol (EtOH) as
solvent. You will determine the percent recovery and assess the purity by measuring its melting point.
Sulfanilamide is one of the sulfa drugs, the first generation of antibiotics to
be used in successfully treating many major diseases such as malaria, H 2N
tuberculosis and leprosy.
O
S NH 2
O
sulfanilamide
The solubility of sulfanilamide in 95% ethyl alcohol is given in the following table:
Temperature (˚C) Solubility (mg/mL)
0
14
20
24
40
46
60
88
78
210
As the table shows, the solubility of sulfanilamide in EtOH increases significantly as the temperature
increases. Therefore, 95% ethyl alcohol is an excellent solvent for crystallization of sulfanilamide. The
ideal solvent for crystallization dissolves a lot of solid at high temperature but barely any at low
temperature. The goal is to dissolve the crude solid in the minimum volume of hot solvent. Therefore,
you must keep the mixture at the boiling point of the ethyl alcohol until the crude solid is completely
dissolved.
After the crystallization of the crude sulfanilamide is performed, you will also calculate the percent
recovery of your sample. The complete recovery of the solid is impossible by crystallization since a
certain amount of compound will remain in solution after crystals have formed. Thus you should have
less than 100% recovery for your sample. If more material is needed, a second crystallization could be
performed from the filtrate to recover a second crop of solid, but it would likely be less pure.
Lastly, you will determine the purity of your crystals by obtaining their melting point. The melting point
of a solid substance is often used to determine purity. The melting point is usually expressed as two
numbers called the melting point range, such as 112–114˚C. The first number is the temperature at
which the substance begins to melt (when liquid is first observed), and the second number is the
temperature at which the sample has completely melted (no solid left). A very pure sample will have a
narrow melting point range that will be close to the literature value. An impure sample will have a lower
melting point, and the range will be larger.
Procedure
1. Weigh 0.75 g of impure sulfanilamide and transfer this to a 25‐mL Erlenmeyer flask. To a second
Erlenmeyer flask, add about 15 mL of 95% ethyl alcohol and a boiling stone. The purpose of the
boiling stone is to promote the smooth boiling of the liquid.
2. Heat the solvent flask on a warm hot plate until the solvent is gently boiling. Since 95% ethyl alcohol
boils at low temperature (78˚C), it evaporates quite rapidly. Setting the temperature of the hotplate
too high will result in too much loss of solvent through evaporation. Do not heat the beaker of
sulfanilamide yet.
3. Before heating the flask containing the sulfanilamide, add enough hot solvent with a Pasteur pipet
to barely cover the crystals. Then heat the flask containing the sulfanilamide until the solvent is
gently boiling. At first this may be difficult to see because so little solvent is present.
4. Add another small portion of solvent (about 0.5 mL), continue to heat the flask, and swirl the flask
frequently. You may swirl the flask while it is on the hotplate, or for more vigorous swirling, remove
it from the hot plate for a few seconds while you swirl it. Remember that the temperature of the
solvent should be at or near the boiling point of 95% ethyl alcohol during this entire process. When
you have swirled the flask for 10‐15 seconds, check to see if the solid has dissolved. If it has not, add
another portion of solvent. Heat the flask again with swirling until the solvent boils. Then swirl the
flask for 10‐15 seconds, frequently returning the flask to hotplate so that the temperature of the
mixture does not drop.
5. Continue repeating the process of adding solvent, heating, and swirling until all the solid has
dissolved completely. Note that it is essential to add just enough solvent to dissolve the solid neither too much nor too little. Because 95% ethyl alcohol is very volatile, you need to perform this
entire procedure fairly rapidly. Otherwise, you may lose solvent nearly as rapidly as you are adding
it and this procedure will take a very long time. The time from the first addition of solvent until the
solid dissolves completely should not be longer than 15‐20 minutes.
6. Remove the flask from the heat and place it on an insulating surface such as a folded paper towel.
The purpose of the insulating surface is to prevent the solution from cooling too rapidly. Cover the
flask with a watch glass or inverted beaker to reduce the rate of cooling and minimize solvent
evaporation. Crystallization should begin by the time the flask has cooled to room temperature. If it
does not occur, gently scratch the inside surface of the flask with a glass rod to induce
crystallization. When it appears that no further crystallization is occurring at room temperature,
place the flask in an ice‐water bath using a beaker. Be sure that the beaker is secured so it would
not tip over in the ice bath.
7. When crystallization is complete, vacuum filter the crystals using a Buchner funnel (this will be
demonstrated in the pre‐lab lecture). Make sure the filtration setup is clamped. Moisten the filter
paper with a small amount of 95% ethyl alcohol and turn on the vacuum (or aspirator) to the fullest
extent. Use a spatula to dislodge the crystals from the bottom of the flask before transferring the
material to the funnel. Swirl the mixture in the flask and pour the mixture into the funnel,
attempting to transfer both crystals and solvent. Therefore, you need to pour the mixture quickly,
before the crystals have completely settled to the bottom of the flask. If some crystals remain, add ~
2 mL of ice‐cold 95% ethyl alcohol, swirl the flask, and transfer the remaining solids.
8. Wash your crystals with approximately 2 mL of ice-cold 95% ethyl alcohol (measured with a plastic
pipette). If necessary, repeat with another 2‐mL portion of ice‐cold alcohol. You should wash the
crystals with a total of about 4 mL of ice‐cold solvent.
9. Continue drying the crystals on the Buchner funnel by suction for about five minutes. Obtain a small
beaker and measure and record its mass in your lab notebook. Then, transfer the crystals into this
labeled, pre-weighed (aka “tared”) beaker for air‐drying. Note the label tape adds mass, so you can
either a. remove the tape later when weighing your sample; or b. weigh the beaker with the label
already attached and indicate this in your notebook. Separate the crystals as much as possible with a
spatula. Cover the beaker with a Kim‐wipe and rubber band and leave in the back of the hood until
the next week.
10. Once dry, weigh the crystals and calculate the percent recovery. Record the appearance of the
crystals (color, shape, size of crystals). Your crystals will most likely look like needles or plates.
11. Determine the melting point of both the pure sulfanilamide and the original impure material.
Remember that the melting point of each sample should be reported as a range. Your instructor will
show you how to prepare a sample for doing a melting point determination and how to use the
melting point apparatus.
Data (to be completed during the lab)
Record the isolated mass and appearance of crystallized sulfanilamide (e.g. 500 mg of white needles),
percent yield (calculate according to: actual mass/theoretical mass x 100%), and the measured melting
point of both the impure and crystallized sulfonamide (indicate the mp as a range).
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