Experiment #2 – Part 1
Adam Capriola
10/17/07
“Acid/Base Extraction”
Dr. Murray
Lab Partner: David Lemon
I. Introduction
A. Objective
The purpose of this experiment is to separate a prepared mixture of benzoic acid, 4nitroaniline, and naphthalene by the technique of extraction. The compounds will be extracted
on the basis of the solubility properties of the acids, bases, and their salts. The given unknown
sample will be dissolved with dichloromethane. Hydrochloric acid will then be added and
extraction of p-nitroaniline will be performed in two steps: mixing and separation. The extracted
p-nitroaniline will then be isolated by adding aqueous sodium hydroxide which will turn the
solution basic which will cause the p-nitroaniline to precipitate. The benzoic acid will be
extracted by adding sodium hydroxide to the dichloromethane solution and using the process of
separation. The benzoic acid will then be isolated using aqueous hydrochloric acid to turn the
solution acidic which will make the benzoic acid precipitate. These two precipitates will be
collected using vacuum filtration. The 4-nitroaniline will be recrystallized using boiling water,
and will then be weighed and measured for its melting point. The benzoic acid will also be
recrystallized using boiling water, and will then be weighed and measured for its melting point.
Anhydrous sodium sulfate will be added to the left over dichloromethane solution. The mixture
will react and then be filtered, and the solid residue of naphthalene will be weighed.
B. Materials and Safety
Chemical Name
Molecular
Formula
Molecular
Weight
(g/mol)
Liquid
Solid
Hydrochloric
Acid
Sodium
Hydroxide
Sodium Sulfate
Benzoic Acid
HCl
36.46
b.p.
ºC
48
NaOH
40.0
1390
Na2SO4
C7H6O2
142.04
122.12
--249.2 1.2659
241
122.4
Naphthalene
C10H8
128.17
217.9 1.0253
80.2
Dichloromethane CH2Cl2
84.93
39
1.3255
-96.7
4-Nitroaniline
138.12
260
1.437
146149
C6H6N2O2
Solubility
Potential
Hazards
Density m.p.
g/mL
ºC
-−26
H2O
Corrosive
2.130
318.4
H2O
H2O
H2O 2;
EtOH 3;
eth 3; ace
3
H2O 2;
EtOH 3;
eth 3; ace
4
H2O
Causes
burns
Irritant
Safe, found
in foods
H2O
Carcinogen
Slightly
toxic
Toxic
C. Experimental Procedure
A prepared mixture of unknown relative amounts benzoic acid, 4-nitroaniline, and
naphthalene will first be obtained. The weight of the mixture will be taken and recorded. The
sample will then be transferred to a 15 mL glass centrifuge tube. 3.0 mL of dichloromethane
will then be added to the tube and the mixture will be dissolved by shaking the tube gently.
Next, 1.5 mL of 6 M aqueous hydrochloric acid will be added to the centrifuge tube. The
mixture will be mixed by capping the tube and shaking it by hand to thoroughly mix the two
phases. The centrifuge tube will then be vented by loosening the cap, and then it will be clamped
to allow the two phases to separate. The dichloromethane layer will be separated using a pipet
with bulb on top. The dichloromethane layer will be sucked into the pipet and transferred to a
reaction tube. The left over aqueous layer will be transferred into a 20 x 80 mm vial labeled
“aqueous acid extracts” in the same manner using a new pipet. The dichloromethane layer will
then be transferred back to the centrifuge tube using a pipet. This technique will be repeated by
adding another 1.5 mL of 6 M aqueous hydrochloric acid to the centrifuge tube as before.
Next, 1.5 mL of cold 3 M sodium hydroxide solution will be added to the centrifuge tube
with dichloromethane. The same procedure will be followed as above for mixing and separating,
except this time the aqueous layer will be transferred to a 20 x 80 mm vial labeled “aqueous
hydroxide extracts”.
After that, 12 M aqueous hydrochloric acid will be added to the 20 x 80 mm vial labeled
“aqueous hydroxide extracts” dropwise until the solution is acidic. The vial will be placed in an
ice bath while this takes place. Next, 6 M aqueous sodium hydroxide will be added dropwise to
the vial labeled “aqueous acid extracts” until the solution is basic. This will also take place in an
ice bath. The solid formed by these steps will be separately filtered via vacuum filtration using a
25 mL filter flask. The precipitates, benzoic acid and p-nitroaniline, will be transferred to filter
paper and allowed to dry, then will be weighed. If specified, 4-nitroaniline and benzoic acid will
be recrystallized from boiling water. These samples would then be dried, weighed, and have
their melting points determined.
The dichloromethane solution will then be dried by adding about 0.3 g of anhydrous
sodium sulfate to the centrifuge tube. The tube will be briefly shaken and allowed to stand for 5
minutes with occasional swirling. Finally, the dichloromethane solution will be filtered or
decanted into a tared reaction tube under the hood. The weight of the solid residue in the
reaction tube will be determined and the tube will be stoppered. If specified, the solid residue
will be purified by column chromatography.
II. Experiment and Results
A. Data
A sample comprised of a mixture of unknown proportions of benzoic acid, 4-nitroaniline
(p-nitroaniline), and naphthalene weighing 0.292 g was obtained. The sample was transferred
into a 15 mL glass centrifuge tube along with 3.0 mL of dichloromethane. The mixture was
dissolved by gently shaking the tube. Once the mixture was completely dissolved, 1.5 mL of 6
M aqueous hydrochloric acid was added to the centrifuge tube. The tube was capped and shaken
to thoroughly mix the two phases. The tube was then uncapped and clamped vertically to a ring
stand. A pipet was then used to take a sample of the upper layer of the mixture, which was then
put into a separate clean centrifuge tube. An equal amount of water was then put into this test
tube to confirm that the upper layer was the aqueous layer. The contents in the second tube were
then transferred back into the original tube using the pipet.
The lower layer of the mixture, the dichloromethane layer, was then transferred using the
pipet into a clean centrifuge tube. The remaining aqueous layer was transferred using the pipet
into a separate clean tube labeled “aqueous acid extracts”. The dichloromethane layer was then
transferred back to the original centrifuge tube again using the pipet. Another 1.5 mL of 6 M
aqueous hydrochloric acid was added to the tube with dichloromethane and the process of
separating the two layers was performed for a second time. The aqueous layer was transferred to
the same test tube as before containing aqueous acid extracts.
Next, 1.5 mL of cold 3 M sodium hydroxide solution was added to the centrifuge tube
containing dichloromethane. The tube was capped and shaken to thoroughly mix the two phases,
just as before. The dichloromethane layer was again separated from the aqueous layer, and this
time the aqueous solution was transferred into a clean centrifuge tube labeled “aqueous
hydroxide extracts”. This process was repeated a second time by adding an additional 1.5 mL of
cold 3 M sodium hydroxide solution to the dichloromethane solution.
After that, about 0.3 g of anhydrous sodium sulfate was added to the left over
dichloromethane layer remaining in the centrifuge tube. The tube was shaken allowed to stand
with occasional swirling. While the tube was allowed to stand, the tube containing aqueous
hydroxide extracts was placed into an ice bath and 12 M aqueous hydrochloric acid was added
dropwise to the until the solution turned acidic. A precipitate, benzoic acid, formed which was
filtered out via vacuum filtration. The benzoic acid was allowed to air dry for a week. It was
then weighed at 0.074 g and its melting point was about 119 ºC.
The tube labeled “aqueous acid extracts” was also placed in an ice bath and 6 M aqueous
sodium hydroxide was added to the solution until the solution turned basic. A precipitate, pnitroaniline, formed which was also filtered via vacuum filtration. The p-nitroaniline was
allowed to air dry for a week. The resulting p-nitroaniline was weighed at 0.040 g and its
melting point was about 139 ºC. The dichloromethane solution which was allowed to stand was
decanted into a clean test tube. The solvent was then removed under a stream of nitrogen in the
hood. The solid residue, naphthalene, was allowed to air dry for a week. The naphthalene was
weighed at 0.035 g and its melting point was 76 ºC.
Sample Weight (g)
Weight of Benzoic Acid (g)
Melting Point of Benzoic Acid (ºC)
Weight of p-nitroaniline (g)
Melting Point of p-nitroaniline (ºC)
Weight of Naphthalene (g)
Melting Point of Naphthalene (ºC)
III. Conclusions
0.292
0.074
119
0.040
139
0.035 g
76
The compounds extracted seem to be slightly impure. The melting points for benzoic
acid, p-nitroaniline, and naphthalene that were found are all slightly lower than the melting
points I looked up beforehand. There are not huge discrepancies in the numbers, so this leads me
to believe the compounds extracted are the expected compounds, they are just not completely
pure, which is to be expected.
Error in this experiment could have come from a few different sources. One step that
could have led to error was during separation. It was hard to extract only the dichloromethane
layer, so I am sure some of the aqueous layer was picked up with it. This means that the
resulting aqueous solution, whether it be acid or hydroxide extracts, would yield slightly less
benzoic acid and p-nitroaniline than expected. If the error were to happen the other way around
and some of the dichloromethane layer was picked up with the aqueous extracts, which means
less naphthalene would have resulted than expected. It was impossible to be precise with the
separation, so that means there was going to be error in the final weight either way.
If the 3 M sodium hydroxide added during separation was not cold, I think the benzoic
acid would not have been as soluble in the aqueous solution and therefore less benzoic acid
would have resulted than expected. A couple other obvious spots where error occurred were
during vacuum filtration and decanting of the dichloromethane. Some filtrate could have been
lost if it were too close to the edge of the filter paper. During the decanting of dichloromethane,
some of the liquid may not have made it would of the tube. Lastly, I know it was hard to get all
of the naphthalene out of tube to weigh it, so the weight of naphthalene recovered is low.