Experiment #1
Adam Capriola
9/13/07
“Recrystallization and Melting Point Determination”
Dr. Murray
Lab Partner: David Lemon
I. Introduction
A. Objective
The purpose of first part of this experiment is to first recrystallize impure acetanilide that
is contaminated with dye methylene blue. Decolorizing charcoal will be used to purify the
sample. The starting and ending weights of the acetanilide will be recorded to determine
percentage of acetanilide recovered. Also, the melting points of the impure acetanilide and
recrystallized acetanilide will then be determined using a Melt-Temp device. During the second
part of the experiment, the melting point range of pure naphthalene will be measured using a
Mel-Temp device. The will calibrate the machine, and then an unknown compound will be
identified by discovering its melting range using the technique of mixture-melting points.
During the final part of the experiment, the microscale recrystallization of 50 mg of impure
trans-1,2-dibenzoylethylene will take place using ethanol to dilute the solvent and the Craig tube
technique for recrystallization. The starting and ending weights of the trans-1,2dibenzoylethylene will be recorded to determine percentage recovered. Also, the melting points
of the impure trans-1,2-dibenzoylethylene and recrystallized trans-1,2-dibenzoylethylene will be
determined using a Melt-Temp device.
B. Materials and Safety
Chemical Name
Molecular
Formula
Molecular
Weight
(g/mol)
Liquid
Solid
Acetanilide
C8H9NO
135.17
b.p. Density m.p. ºC
ºC g/mL
304 1.21
114.3
Naphthalene
C10H8
128.17
218 0.9900
trans-1,2C16H12O2
dibenzoylethylene
Ethanol
C2H5OH
236.27
79-82
108.00111.00
79
0.8129
Solubility
Potential
Hazards
Slightly
Slightly
Soluable
toxic
Insoluable Flammable,
possible
carcinogen
Slightly
toxic
Soluable
Slightly
toxic,
flammable
C. Experimental Procedure
The first part of the experiment is the recrystallization of impure acetanilide. To
begin, 2.5 grams of impure acetanilide will be obtained from the teacher’s assistant. Of that
sample, 0.1 grams will be saved for a melting point determination. The remainder will be
weighed and placed into a 250 mL Erlenmeyer flask with 50 mL of water and several boiling
stones. The mixture is to be heated until the acetanilide dissolves. The heat will then be taken
away and 25 mL of cold water and about 2.5 grams of decolorizing carbon pellets will be added
to the mixture. The mixture will then be brought to a boil and boiled gently for one to two
minutes. An additional 0.5 grams of decolorizing carbon pellets can be added if the blue color
has not been completely removed. The solution should be boiled for another five minutes and
this process can be repeated until the blue color is completely removed. The solution should
then be filtered through a fluted filter into a warmed 125 mL Erlenmeyer flask. The original
Erlenmeyer and filter paper will then be rinsed with an additional 10 mL of hot water. The 125
mL Erlenmeyer flask with the mixture is then to be cooled in an ice bath. The crystals will then
be collected using vacuum filtration and a Buchner funnel. The crystals are then to air-dry on a
watch glass for several days. Then the weights and melting points of the purified and impure
samples will be determined using a Mel-Temp apparatus.
The second part of the experiment is melting point determination. Two melting
point capillaries with samples of pure naphthalene will be tested with a Mel-Temp device to
determine their melting point. An unknown will then be distributed. The unknown will be tested
using the Mel-Temp device, first heating at a rate of 10-20 ºC to locate the approximate melting
range, and then it will be heated at about 1-2 ºC to determine the exact melting point. To
determine the identity of the unknown, the two known compounds with similar melting points
will be obtained and be made into mixtures of equal amounts of unknown and known compound.
The melting range of these mixtures will be determined to find the identity of the compound.
The last part of the experiment is the recrystallization of trans-1,2dibenzoylethylene. First, a sample of 50 mg of impure trans-1,2-dibenzoylethylene is to be
obtained. The sample will then be transferred to a Craig tube. Next, 0.5 mL of 95% ethanol will
be added using a pipet pump and 2 mL graduated pipet along with a wood boiling stick. The
mixture will be heated in a hot sand bath until the solvent begins to boil. At this time, additional
ethanol will be adding dropwise until the solid completely dissolves. Then, the Craig tube is to
be removed from the heat and allowed to slowly cool to room temperature. Once it is room
temperature, the Craig tube will cool in an ice bath and then placed in a centrifuge to collect the
crystals. The crystals are to then air dry, and the starting and ending weights are to be measured.
II. Experiment and Results
A. Data
First, a 2.443 g sample of impure acetanilide was obtained along with a second sample
weighing about 0.1 g, which was later used for a melting point determination. The 2.443 g
sample of impure acetanilide was placed into a 250 mL Erlenmeyer flask along with about 50
mL of water, and several boiling stones. The impure acetanilide was a greyish, brownish, flakey
powder. When mixed with the water, the water turned a bright transparent blue color. This
mixture was then heated in the Erlenmeyer flask on a hot plate until the acetanilide dissolved.
After the acetanilide dissolved, the Erlenmeyer flask was removed from the heat and 25 mL of
cold water and about 2.5 g of decolorizing carbon pellets were added to the flask. The mixture
was then brought to a boil and was boiled for about 2 minutes. The mixture produced a black
residue and most of the the blue color disappeared after the decolorizing pellets were added and
it was brought to a boil. About an additional 0.5 g of decolorizing pellets were added and the
mixture was boiled for another couple minutes to remove any more traces of blue color. The
mixture was then filtered through a fluted filter in a stemless funnel into a warmed 125 mL
Erlenmeyer flask. The original Erlenmeyer flask was rinsed with hot water to aid in transfering
all of the mixture into the fluted filter. As the mixture was filtered, crystals began to form in the
filtrate in the 125 mL Erlenmeyer flask. This flask was then put into an ice bath to cool for about
5 minutes. The acetanilide mixture was then filtered via vacuum filtration and a Buchner funnel.
Transparent rectangular shaped crystals were collected in the filter paper. The acetanilide
crystals collected were set aside to dry for a week. The crystals recovered were weighed and the
melting point of the impure acetanilide and pure acetanilide were determined using a Mel-Temp
device. The crystals recovered weighed 0.731 g. The melting point of impure acetanilide was
113 ºC and the pure acetanilide melting point was 115 ºC.
For the second part of the experiment, a sample of pure naphthalene was obtained and
loaded into two cappillaries to determine its melting point using a Mel-Temp device. The
temperature of the Mel-Temp was raised fairly slowly and the first trial resulted in a melting
point of 81.5 ºC. The second trial resulted in a melting point of 81.0 ºC. An unknown sample
labelled “4” was obtained and its melting point was determined using the Mel-Temp. During the
first trial, which was the fast trial, the temperature of the Mel-Temp was raised about 10-20 ºC
per minute and the unknown had a melting point of 104 ºC. During the slow trial, the
temperature of the Mel-Temp was raised about 1-2 ºC per minute and the unknown had a melting
point of 110.5 ºC, which was comparable to m-toluic acid and resorcinol. Two new samples
were made, mixing an even amount of each known compound with unknown “4”. The melting
point of these mixtures were again determined using the Mel-Temp. The melting point for the
unknown mixed with m-toluic acid was 106 ºC, while the melting point for the unknown mixed
with resorcinol was 110 ºC.
For the third part of the experiment, a 0.068 g sample of impure trans-1,2dibenzoylethylene was obtained an put into a Craig tube along with 0.5 mL of 95% ethanol and a
wood boiling stick. The of impure trans-1,2-dibenzoylethylene was a yellow powdery substance
that when combing with the ethanol, changed the color of the liquid to a transparent yellow. The
tube was heated in a hot sand bath. It was covered in sand up to the level of solution present and
it was heated until the solvent began to boil. A few additional drops of 95% ethanol were added
until all the solid dissolved. The Craig tube was then removed from the heat and was allowed to
cool for about five mintues at room temperature and then another few mintues in an ice water
bath. The Craig tube was then put into a centrifuge. The supernatant was discarded and crystals
were taken out of the Craig tube and put onto filter paper using a small spatula. The crystals
were yellow, thin, and long. The crystals were then allowed to dry for a week. The crystals
recovered were weighed and the melting point of the impure trans-1,2-dibenzoylethylene and
pure trans-1,2-dibenzoylethylene were determined using a Mel-Temp device.
Sample Weight Impure Acetanilide (g)
Final Weight Pure Acetanilide (g)
Percent Recovery Acetanilide (%)
Melting Point Impure Acetanilide (ºC)
Melting Point Pure Acetanilide (ºC)
Melting Point Naphthalene, Trial 1 (ºC)
Melting Point Naphthalene, Trial 2 (ºC)
Melting Point Unknown Trial 1, Fast Trial (ºC)
Melting Point Unknown Trial 2, Slow Trial (ºC)
Melting Point Unknown mixed with m-toluic acid (ºC)
Melting Point Unknown mixed with resorcinol (ºC)
Sample Weight Impure trans-1,2-dibenzoylethylene (g)
Final Weight Pure trans-1,2-dibenzoylethylene (g)
Percent Recovery trans-1,2-dibenzoylethylene (%)
Melting Point Pure trans-1,2-dibenzoylethylene (ºC)
2.443
0.731
29.92
113
115
81.5
80.0
104
110.5
106
110
0.068
III. Conclusions
I do not really know how to judge the results from the first part of the experiment. A
percent recovery of 29.92% seems low to me, but that could be close to the expected recovery
value. I would have to know the molecular formula of the impure acetanilide to figure out how
many moles of pure acetanilide were there to begin with, then compare that to the number of
moles of acetanilide recovered. I do know that there were many parts of the experiment where
error could have occurred. For example, if too many decolorizing carbon pellets were added,
they would have absorbed some of the acetanilide along with the impurities. If the mixture of
acetanilide, water, and decolorizing pellets was not boiled long enough, there would still be left
over impurities and some portion of acetanilide would still be impure and would not yield the
expected amount of crystals. If the funnel and filter paper were too cold, some crystals would
begin to form in the filter paper and not make it to the filtrate. The filtrate could not have been
allowed to cool long enough and some crystals might not have formed. Some of the crystals
could have been poured outside of the filter paper and went into the filtrate during vacuum
filtration. There were many steps where error could have occurred, so it is highly unlikely to
recover the actual amount of acetanilide that was present in the impure sample.
The results from the second part of the experiment seemed accurate. The melting point
of the unknown was nearly identical as the melting point of the unknown mixed with resorcinal.
The melting point of the unknown mixed with m-toluic acid was lower than that of pure known,
which makes sense. Impurities usually make substances have lower melting points. This means
the unknown was resorcinal. Error in this part of the experiment could come from using the
Mel-Temp incorrectly or not evenly mixing even amounts of unknown and known samples.
That could cause the temperature readings to be different.
For the third part of the experiment, error could have come from many different parts.
For example, the impure trans-1,2-dibenzoylethylene could have not all dissovled during boiling.
Crystals could start to form on the boiling stick before the Craig tube was centrifuged, which
would have then been lost. The Craig tube could have not been cooled long enough to form all
the crystals. Some crystals could have been poured off along with the supernatant after
centrifugation. Some crystals were not recovered from the Craig tube because they were too
hard to get out. There were many sources of potential error, so again it is highly unlikely to
recover that original amount of trans-1,2-dibenzoylethylene present in the impure sample.