Mandatory Experiment 7.1
Recrystallisation of benzoic acid and determination of its
melting point
Student Material
(a) Recrystallisation
Theory
An impure solid may be purified by first dissolving it in the minimum quantity of a
boiling solvent. Insoluble impurities are then removed by rapid filtration of the hot
mixture. The filtrate is next allowed to cool slowly. Since the solubility of the solid
decreases as the temperature falls, crystals will form. The soluble impurities mostly
remain dissolved in the solvent as the crystal structure “re-grows” in a regular pattern.
Chemicals and Apparatus
Benzoic acid (1 g sample to be purified)
Deionised water
Ice-cold deionised water
Electronic balance
100 cm3 beakers
250 cm3 beaker
250 cm3 conical flask
Dropping pipette
Oven set at 100 0C
Filter funnel
Filter paper
Clock glass
Hirsch funnel or small Buchner funnel and filter papers to fit
Small Buchner flask
Rubber adaptor to connect Hirsch or Buchner funnel to neck of Buchner flask
Clamp and boss head
Retort stand
Filter pump (or vacuum pump)
Spatula
Hotplate/magnetic stirrer or Bunsen burner, tripod and gauze
Magnetic pellet
Tongs
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Heat resistant mat
Dry cloths
Safety glasses
Procedure
NB: Wear your safety glasses.
1. Boil about 100 cm3 deionised water in a 250 cm3 beaker and keep the water
just at its boiling point.
2. Weigh out about 1 g of the benzoic acid sample in a 100 cm3 beaker.
3. Flute a filter paper – your teacher will show you how to do this. Place the
fluted filter paper in the funnel, and secure it by wetting it.
4. Prepare for hot filtration by standing a 250 cm3 conical flask containing about
20 cm3 of boiling water on a hotplate or over a low Bunsen flame. Sit the
funnel with fluted filter paper in the neck of the flask.
5. Using a dropping pipette, add 25 cm3 of boiling water to the beaker containing
the sample, with constant stirring. Heat the mixture gently to keep the solution
near its boiling point. Any remaining crystals should be dissolved by adding
the minimum extra amount necessary of boiling water.
6. When ready to filter, empty the conical flask, replace the funnel quickly, and
stand the flask on a heat resistant mat. Working quickly but carefully, pour the
boiling solution through the filter paper (Fig.1) in small portions. (It may be
necessary to reheat the beaker periodically.) If any crystals form in the filter
paper, add a little boiling water to dissolve them.
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Fig. 1
7. Empty the filtrate into a warm 100 cm3 beaker. Heat the filtrate until traces of
crystals begin to appear on the sides of the beaker. You now have the
minimum amount of solvent required. Place the beaker on a heat resistant mat
and allow the filtrate to cool to room temperature.
8. To recover the crystals, carefully filter the recrystallised mixture, by vacuum
filtration if possible. If some crystals remain in the beaker, transfer the filtered
liquid back to the beaker and filter again. Repeat this procedure until all the
crystals have been transferred to the funnel.
9. Wash with 2 cm3 of ice cold deionised water and dry the crystals using
vacuum filtration for ten minutes.
10. Using the spatula, remove the crystals from the funnel and transfer them to a
previously weighed dry clock glass. Dry the crystals by placing the clock
glass in an oven set at 80 0C, or on top of a radiator for 24 hours.
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(b) Melting point determination
Theory
One of the physical properties of a pure substance is its melting point, or, more correctly,
its melting point range. The melting point range of a substance is the narrow band of
temperatures between the temperature at which melting begins and the temperature at
which all the solid has liquefied. The solid state and the liquid state of the substance coexist across this small temperature range.
The melting point range of an impure substance is lower and broader than that recorded
for a pure sample of the same substance. Comparing the melting point ranges of the
benzoic acid sample before and after recrystallisation with the melting point range of
pure (Analar) benzoic acid will therefore indicate the success of the recrystallisation
process in improving the sample purity. It will also indicate whether it is as pure as an
Analar sample.
Chemicals and Apparatus
Impure sample of benzoic acid
Purified sample of benzoic acid
Sample of pure (Analar) benzoic acid
Melting point tubes
Mortar and pestle
Rubber band or narrow ring of tubing
Bunsen burner or hotplate
Thiele tube containing liquid paraffin or oil bath containing liquid paraffin or boiling
tube containing liquid paraffin or aluminium block with hole drilled for thermometer or
alternative M.P. apparatus.
Thermometer (preferably -10  150 0C)
Heat-resistant gloves
(i) Melting point determination using an oil bath
1. Grind the sample in a mortar. Fill a melting point tube to a depth of 3 -5 mm
with the crystals as follows: Dip the open end in the crystal sample so that
some becomes stuck in the top of the capillary tube. Drop the capillary tube
closed end downwards through a thick walled glass tube held vertically. This
will dislodge the crystals from the mouth of the tube and they will fall to the
closed end.
2. Using a rubber band (or ring of rubber tubing) attach the capillary tube to the
thermometer so that the sample is near the bulb. Place it in the Thiele tube
(Fig.2) (or other suitable apparatus, e.g. as in Fig.3) containing liquid paraffin.
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Fig. 2
5
Fig. 3
Heat gently with a hot plate or Bunsen burner. Observe the melting point range i.e.
the temperature at which liquid first appears and the lowest temperature at which no
solid remains.
.
(ii) Melting point determination using an aluminium block
1. Sprinkle a thin layer of the substance to be melted on the surface of a clean
aluminium block into which a thermometer has been placed (Fig. 4).
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Fig. 4
2. Heat the block slowly using a Bunsen burner or on a hot plate.
3. Observe the melting point range i.e. the temperature at which liquid first
appears and the temperature at which no solid remains.
.
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Results
0
Melting point of impure benzoic acid
=
C
Melting point of purified benzoic acid
=
0
C
Melting point of pure (ANALAR) benzoic acid
.
=
0
C
Questions relating to the experiments
1. Why is it important to use the minimum amount of hot solvent at the
crystallisation stage?
2. What action would you take if some of the crystals formed on the filter paper or in
the leg of the funnel during the hot filtration?
3. Suggest a way of obtaining a second crop of crystals from the solution obtained
after filtering off the crystals. Would these be as pure as the first crop? How
would you confirm this?
4. What would you expect a forensic scientist to observe about the melting point if
two samples of the same substance (both equally pure) were mixed and the
melting point recorded?
5. What would you expect the scientist to observe about the melting point if pure
samples of two different substances with the same melting point were mixed and
the melting point recorded?
6. Two samples have melting points 115 –118 0C and 120 – 121 0C. Could they be
the same substance? How would you check your answer?
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Teacher Material
Recrystallisation

To flute a filter paper:
1. Fold the paper in half.
2. Open and turn the paper through 90º before folding in half again.
3. Open and turn the paper through 45º before folding in half again.
4. Open and turn the paper through 90º before folding in half again the open filter paper is now divided into eighths.
5. Turn the open filter paper over and repeat the process of dividing
the paper into eighths, using the spaces between the creases
already created.
6. The paper should now be divided into sixteenths, with every
second crease formed by folding the paper in opposite directions.
7. Turn over the filter paper, and gather the folds.

A filter funnel with a short leg, or a stemless filter funnel, should be used
if available. This prevents the problem of crystal formation in the leg of
the funnel.

Using a solvent in which the solute has good solubility in the hot solvent
and very poor solubility at lower temperatures will produce the maximum
yield of crystals. Water is a very suitable solvent for the recrystallisation
of benzoic acid.

Adding a drop of dilute hydrochloric acid to the solution before the
crystals are filtered and dried can improve the yield of benzoic acid.

The yield can also be improved by cooling the solution below room
temperature before the crystals are filtered from the solution.

If crystals do not appear, gently scratch the bottom of the flask to initiate
crystallisation. Alternatively, evaporate off some of the solvent and allow
to cool again.

Water is the most suitable solvent for the recrystallisation of benzoic acid.
For other solids, a different solvent may be required. Choosing the most
suitable solvent for a particular solid so as to maximise the yield in the
recrystallisation process is a matter of trial and error using small samples.
Sometimes mixtures of two miscible solvents prove the most effective.

It is advisable to have a supply of boiling water available at the start of the
experiment. This should enable students to finish the experiment in a
shorter time.
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Melting point determination

Using a Micro Bunsen for heating improves the accuracy of each of the
methods described.

The aluminium block method gives less accurate results than any of the oil
bath methods. It is, however, very convenient in the school situation in that
melting point tubes are not required, and melting of the sample may be
observed more easily.

A third method of measuring melting point involves placing the melting point
tube in the well of an electrical melting point apparatus and observing the
melting through the viewer.

To make melting point tubes from capillary tubing, proceed as follows.
Make 10 cm lengths of capillary tubing by etching with a glass cutter at
10 cm intervals, and snapping. Rotate one end in the Bunsen flame so that the
end seals. Polish the open end with the flame, but be careful that this end does
not start to close.
Extension Work

Demonstration of a supersaturated solution and seeding using sodium ethanoate,
CH3COONa.3H2O. This substance dissolves in its own water of crystallisation upon
heating. The solution upon heating forms a supersaturated solution that will
crystallise very quickly when seeded with a small crystal of sodium ethanoate to form
a solid again. The sample is stored in a Pyrex bottle that can be stoppered tightly. A
few drops of water may have to be added from time to time to keep it hydrated fully.

The use of decolourising charcoal can be demonstrated by contaminating the benzoic
acid with a little food dye. Before carrying out the hot filtration 1 - 2 g of activated
charcoal is added to the hot solution. As well as removing all solid impurities,
filtration removes coloured soluble impurities that have been adsorbed onto the
charcoal.
Preparation of Reagents
 The sample of benzoic acid to be purified can be contaminated with a very small
amount of copper(II) oxide.
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Quantities needed per working group
2 - 3 g sample of benzoic acid to be purified
0.1 g sample of pure (Analar) benzoic acid
2 l deionised water
Safety considerations

Safety glasses must be worn.

The use of heat-resistant gloves is recommended.

Hair should be tied back if necessary.

The usual precautions when handling glassware for an experiment should be
observed. Additional caution is needed when handling hot glassware. Dry cloths
should be used. Capillary tubing heated in a Bunsen flame is particularly
hazardous as it is still extremely hot even when it is not glowing. Fire polish the
cut ends of the capillary tubing. Dispose carefully of glass splinters from the
glass-cutting step.

Exercise care with the hot solvent to avoid scalds from the boiling water or
steam. Be careful when heating the liquid paraffin bath. Avoid using old oil, as it
tends to be discoloured, and very smoky on heating. Do not heat to very high
temperatures because it could ignite.

Avoid burns from hotplates.

Clamp apparatus in place.
Chemical hazard notes
Benzoic acid may be irritating to the skin and eyes. Its vapour is very irritating to lungs.
Disposal of wastes
Add solid waste and residual liquid paraffin to excess sand and place in refuse bin. Dilute
liquid wastes with water and flush to foul water drain.
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Specimen Results
Thiele tube method
Melting point of impure benzoic acid
= 118 - 121 0C
Melting point of purified benzoic acid
= 121 - 123 0C
Melting point of pure (ANALAR) benzoic acid
= 121 - 123 0C
Aluminium block method
Melting point of impure benzoic acid
= 116 - 118 0C
Melting point of purified benzoic acid
= 118 - 119 0C
Melting point of pure (ANALAR) benzoic acid
= 118 - 119 0C
Suggested Solutions to Student Questions
1. Why is it important to use the minimum amount of hot solvent at the crystallisation
stage?
It is necessary to use hot solvent in order to produce a solution that will be supersaturated
at room temperature. On cooling, the substance recrystallises from solution. The
minimum amount of solvent is used because this reduces the quantity of the substance
that can remain dissolved when the hot filtered solution is allowed to cool. The yield of
recovered crystals would be very low if too much solvent were used.
2. What action could you take if some of the crystals formed on the filter paper or in the
leg of the funnel during the hot filtration?
Boiling water would have to be dropped on to the crystals using a dropping pipette to
dissolve them. The filtrate would then contain more than the minimum quantity of boiling
solvent to keep the substance dissolved. In order to avoid low yields due to the substance
remaining dissolved after cooling, it would be necessary to boil off most of the extra
solvent before allowing the filtered solution to cool.
3. Suggest a way of obtaining a second crop of crystals from the solution obtained after
filtering off the crystals. Would these be as pure as the first crop? How would you
confirm this?
Heat the filtered solution to boiling. Allow its volume to reduce by half. Do not boil to
dryness. Allow the solution to cool slowly to room temperature and place it in an ice12
water bath to cool further. A small second crop of crystals is obtained, and may be
filtered off with a Hirsch funnel and dried.
This crop of crystals will not be as pure as the first since the soluble impurities are now
more concentrated in the partially evaporated mother liquor and will be incorporated into
the crystalline structure of the second batch of crystals more than they were in to the first.
4. What would you expect a forensic scientist to observe about the melting point if two
samples of the same substance both equally pure were mixed and the melting point
recorded?
Since these samples are chemically identical and of equal purity, there will be no
lowering of melting point when they are mixed.
5. What would you expect the scientist to observe about the melting point if pure samples
of two different substances with the same melting point were mixed and the melting point
recorded?
The two substances when mixed will act as impurities present in one another and so a
lowering and broadening of the original melting point will be observed.
This procedure is used to test whether two substances with the same melting point are in
fact the same substance or not.
6. Two samples have melting points 115 - l18 0C and 120 – 121 0C. Could they be the
same substance? How would you check your answer?
Yes, they could be the same substance. The first one is lower and broader than the second
and this difference could be due to impurities. Purify the first sample by recrystallisation
and record its melting point again. If it increases and narrows to the same melting range
as the second sample they could be the same substance. Carry out a mixed melting point
test as referred to in Question 5 to check.
Industrial, Environmental & Social Links
Industrial

Recrystallisation is widely used in the bulk chemical and pharmaceutical
industries to purify products.

Recrystallisation is used in sugar production to obtain sugar crystals from syrupy
sugar solutions. Seeding is carried out to speed up the process. Several crops are
obtained from the mother liquor, the white sugar crystals first and darker sugar from
later crops. Activated charcoal is also used to remove coloured impurities.

Salt is extracted from sea water by allowing the water to evaporate in large pools. The
salt crystallises when this is done.
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