LAB 3: CRYSTALLIZATION
PRE-LAB: Complete the pre-lab assignment BEFORE entering lab. Your instructor
will give you instructions about how it should be handed in.
1) Write a safety section for this experiment. Be sure to include all solvents.
2) Fill in the blanks in the experimental procedure section for Lab 3. (For step 1 in part 1
and part 2 show your work.)
3) What are crystals? How are they different from other types of solids (powders,
amorphous)?
4) For a given solvent, most organic solids are more soluble in hot solvent than in the
same solvent cold. How would a graph look which showed the amount soluble on the x
axis and temperature on the y axis?
5) Suppose a hot solution has the maximum amount of a given solid dissolved in it. What
will happen as that solution cools?
PURPOSE: You will learn why crystallization is done in organic chemistry and how
to perform the necessary tasks to recrystallize an impure sample.
BACKGROUND INFORMATION:
Organic solid substances often have impurities in them which make them less
desirable for their intended purposes. Crystallization is an important technique that can
be used to increase the purity of a solid sample if certain conditions are met. First, the
impurities cannot be a large percentage of the sample and, secondly, the properties of the
impurities should be such that the technique would be useful. An important use of
crystallization in industry is the refining of sugar. Raw cane sugar is put through a series
of crystallization steps which results in commercial products of varying degrees of purity.
The table sugar you use is highly refined sucrose that is obtained from cane sugar.
You will recrystallize benzoic acid in this experiment.
O
C
OH
Benzoic Acid
The full procedure to recrystallize a sample involves the following steps:
Step 1: Find a suitable solvent for the recrystallization process.
Step 2: Dissolve the sample in the minimum amount of solvent at its boiling point.
Step 3: Decolorize the solution (if needed) by using decolorizing carbon.
Step 4: If needed, quickly and carefully filter the hot solution to remove any insoluble
impurities.
Step 5: Slowly cool the hot solution to room temperature and then to 0 degrees C in an
ice bath.
Step 6: Filter the crystallized solid using a suction filtration apparatus.
Step 7: Dry the collected solid.
Step 8: Weigh and determine the melting point of the solid that was obtained and
compare it to the reported melting point for that compound. If the melting point is
low or if the melting range is broad, dry the sample further or recrystallize the
sample again.
Depending on the nature and purity of the starting sample, one or more of the
above steps may be omitted. In order to guide you in this, the following considerations
are made:
a. The ideal solvent is one in which the solid is very soluble at the boiling point
of the solvent and very insoluble at 0 degrees. Generally, the rule "like
dissolves like" is a good starting point to determine suitable solvents.
Solubility data found in reference works may be useful.
b. Sometimes when a suitable solvent cannot be found one can use a mixture of
two miscible solvents to achieve the desired solubility properties. The
mixture can be prepared before the solid is dissolved. Alternatively, the solid
can be dissolved in the solvent in which it is more readily soluble and the
other solvent is added drop wise at an elevated temperature until
precipitation just begins.
c. The solvent should be volatile so that it can be removed from the solid
easily.
d. When dissolving the solid in the chosen solvent, heat the sample in a small
amount of the solvent and add more solvent as needed. Do not rush this
process as the dissolution of some solids may take some time. Generally this
procedure is done in an Erlenmeyer flask or beaker and the heating is done
on a hot plate.
e. A very small amount of decolorizing carbon or Norit can be used to remove
colored impurities from the solution. The carbon adsorbent must be
carefully removed from the hot solution using gravity filtration. If this is not
done carefully, often the final product will be gray due to traces of carbon in
the final crystals.
f. If carbon was used and/or if there are insoluble impurities, the hot solution
must be filtered by gravity to remove these insoluble substances. This
procedure must be done as quickly as possible as some of the desired solid
may crystallize at this time which will adversely affect the process. A wide
bore glass funnel (powder funnel) and fluted filter paper are used in this step.
g. The hot solution is set aside and not disturbed as the crystallization occurs.
If the container is touched, the supersaturated solution may crystallize
quickly giving small crystals (which generally contain impurities). A slow
undisturbed crystallization generally results in larger more pure crystals.
h.
i.
j.
k.
l.
m.
If crystallization does not occur in a reasonable time (as determined by your
Instructor), you can sometimes hasten the process by introducing a seed
crystal or scratching the side of the glass container with a stirring rod.
Final cooling to 0 degrees C is done in an ice water bath to maximize the
amount of solid that will precipitate.
The solid is filtered using suction to remove the solvent and the crystals are
moved around the funnel using a spatula while suction is continued to dry
the crystals as much as possible.
The crystals which are left in the glass container can be removed by using a
spatula or a very small amount of cold solvent. Students generally lose
material at this stage because they use too much solvent which will then
dissolve some of the desired product.
The choice of which size funnel to use is made by determining the amount of
solid obtained and the size of the funnels which are available. Do not use a
funnel that is too small or the solid may spill out of the funnel. Do not use a
funnel that is too large as there will be mechanical loses that will lower your
percent recovery.
The melting point of the final solid will determine the success of your work.
The melting point range (see prior experiment) should be small and the
melting point should be very close to the reported value.
Students often note that there is solid material in the filter flask below the funnel
in which the solid has been collected. They often think the funnel leaked some of the
solid. Recall that the filtrate is a saturated solution of the substance you are trying to
obtain. Also recall the suction generally causes some solvent to evaporate. Hence, the
filtrate becomes supersaturated and solid will precipitate. In the whole crystallization
process, a 100 % recovery is generally not possible due to this phenomenon. Sometimes
in important work a second crop of (less pure) crystals is obtained by further working on
the filtrate.
EXPERIMENTAL PROCEDURE:
Take the melting points of benzoic acid provided and record it in your notebook. The
literature melting points of benzoic acid is ________ºC.
PART 1: Crystallization of benzoic acid. Be sure to accurately record the weights of
materials you used and the observations you made in your notebook.
1) Transfer 1.23 mmol (___________mg) of benzoic acid into a 50 mL Erlenmeyer flask.
Question: What does the sample look like? (See Post-Lab question 1)
2) Recrystallize the sample using water. Add a minimum amount of water to the flask –
The solvent should cover the solid, but the solid should NOT dissolve at room
temperature. You will want to add the solvent in 1-2 mL portions to avoid adding too
much solvent. Approximately 5-7mL should work well. Heat the sample until the solid
is completely dissolved .
Question: How much solvent did it take to dissolve the solid? – Record this in your
notebook.
3) Once the solution is homogenous set the flask aside and do not disturb it. Watch
carefully to see if you can see crystals form.
4) When the flask is no longer too warm to the touch, cool the flask in an ice bath.
5) After the crystals have formed, collect them by vacuum filtration.
Question: What happened to the solvent?
6) Rinse the crystals with a small amount of COLD solvent.
Question: what would happen if you rinsed with lots of solvent or with warm solvent?
7) Allow the crystals to dry thoroughly by pulling air over them during the vacuum
filtration, especially if you used water as your solvent.
8) Weigh the dry solid. – Record this in your notebook.
Question: how would your data (weight, mp) be affected if the solid were still wet?
9) Take the melting point and calculate and report your percent recovery.
10) Dispose the solid in the solid waste container and any leftover solvents in the organic
solvent waste container.
PART 2: Crystallization of benzoic acid in methanol/water. Dissolve _____mmol (150
mg) of benzoic acid in a minimum amount (1-2 mL) of warm methanol. Then add water
drop-wise until precipitation just begins. Then repeat the crystallization process as
described above.
IMPORTANT INFORMATION ABOUT THE REPORT:
Post-Lab Questions:
1) Compare the looks of your crystals with those of other lab groups and with the starting
unknown mixture. Are there similarities? Differences?
2) What are the properties of a good solvent for carrying out a crystallization? What are
the factors that affect how much of a solid will dissolve in a particular solvent?
3) What happened to the impurity that was in the original unknown solid mixture?
Where did it go?
4) In part 3, what was the purpose of adding the water?
5) Report your results in a table (or tables). Include the melting points for each sample
and be sure to show your percent recovery of each sample used. Show the correct
number of significant figures which are allowed by the measuring equipment you used
and show your calculations. Breifly, discuss the reasons why your percent recovery is
the value you obtained. Discuss which steps had to be done carefully and which did not
contribute to the precision of your percent recovery calculations.
5) Write a procedure for this experiment.
END OF EXPERIMENT.
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