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EXP-2
CHEM 233L
PURIFICATION OF IMPURE BENZOIC ACID
Introduction:
Recrystallization and liquid-liquid extraction are the two important purification
techniques. In this experiment, these two techniques were used to purify benzoic acid from
a three-component mixture of carboxylic acid, amine (a base), and neutral compound. The
sample used in this experiment contained benzoic acid, p-nitroaniline and NaCl. After
separating benzoic from the mixture, the melting point and IR of pure product were taken
and compared with the actual values. Also, the percent recovery of benzoic acid of each
technique was calculated. Then, in term of percentage recovery and purity of final product,
determine which technique (recrystallization or extraction) was referable.
Table of Physical Properties:
Compound Name
Molecular
Weight
(grams)
122.1232
Melting
point
(oC)
122.4
Boiling
point
(oC)
249
Density
(g/ml)
Solubility
Precautions
1.08
Slightly
soluble
0.34g/100
mL
May be harmful if
swallowed. May
cause irritation.
Keep in closed
container if not in
use.
138.1256
146
332
1.437
0.08g/100
mL
May cause
mutations, eye
and skin
irritation, and
trouble breathing,
etc. Handle
extreme caution
58.44277
801
1413
2.165
10g/100
O
OH
Benzoic acid
White powder
NH2
O2N
Para-nitroaniline
Yellow crystal with
a pungent, faint
ammonia-like odor
NaCl
White crystals
H
mL at 20oC
84.9328
39.8
-96.7
1.3255
Slightly
soluble
Cl
H
Cl
Dichclromethane
Weakly polar
________________________________________________________________________
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Safety Notes:
 Benzoic acid may be harmful and cause irritation. Keep it in closed container if
not in use.
 Para-nitroaniline may cause mutations, eyes and skin irritation, and trouble
breathing  Handle it extreme caution

Procedure:
EXP 2A: RECRYSTALLZATION OF IMPURE BENZOIC ACID FROM WATER
1. Heat the hot plate to 110-130 oC. Place 100 mL of water in a 250 mL beaker and
bring to a boil.
2. Place approximately 1.0 g of impure benzoic acid in a 125 ml flask and add
approximately 25 mL of hot water. If your sample appears to have an unwanted
color, it can be removed later with charcoal.
3. Heat the mixture on a hot plate until the water boils. If necessary, add small
quantities of hot water with a Pasteur pipet until all the white benzoic acid was
dissolved into solution. If you see particles that do not seem to dissolve even with
additional hot water, don’t worry; they will be removed in the filtration step.
4. Remove the flask from the hot plate and allow to cool down about 10 oC before
adding decolorizing charcoal. Decolorizing charcoal is usually added to remove
colored impurities. The amount of the charcoal added depends on the size of your
material. Between 5-10% of the weight is typical amount.
5. Place the solution containing the charcoal back on the hot plate and boil for 5
minutes.
6. Remove any solid insoluble impurity by using the technique of gravity filtration.
The hot benzoic acid was filtered using a short stem funnel and a 125mL flask to
collect the filtrate. Notice that the filtrate should be kept hot to prevent early
crystallization. Keep both flasks on the hot plate to keep hot at all times. The
funnel should not overflow to prevent contaminating your filtrate with charcoal. If
this occurs, the hot filtration will be repeated a second time.
7. Chill the filtered benzoic acid solution in an ice bath. Then filter the crystals on a
Buchner funnel and wash the flask and crystals with a small amount of ice-cold
water (in separate beaker, equal parts ice and water were mixed, about 25 mL
each).
8. Next, dry the crystals on the funnel by using the vacuum to pull air over them for
at least 5 minutes. If the crystals still have a yellow impurity, recrystallize the
crystals from a minimum amount of hot water once again. It is not necessary to
use charcoal.
9. Dry the crystals overnight.
10. The melting point and IR was determined on next day of class.
EXP-2B: PURIFICATION OF IMPURE BENZOIC ACID BY EXTRACTION:
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1. Weigh 500 mg of the chemical mixture containing benzoic acid, p-nitroaniline
and NaCl.
2. Add about 5 mL of water to a separatory funnel (make certain the stopcork works
properly, is tight but able to be turned, and that is does not leak).
3. Add about 15 mL of CH2Cl2 . Two phases are observed. Even though the liquids
are each colorless, dichloromethane, which is denser than water, will go to the
bottom. The dividing line between two liquids should be seen.
4. Add the mixture to the liquid contents of the funnel. Insert a ground-glass stopper
into the top of separatory funnel and mix well. You should be able to observe that
all of the crystals dissolve and disappear from view. At this time, the NaCl has
dissolved into the water, making an aqueous salt solution. The benzoic acid and pnitroaniline was dissolved in CH2Cl2.
5. Extract the CH2Cl2 twice with 20 mL of water in order to remove the NaCl. To
extract effectively, shake the separatory funnel to thoroughly mix the two phases.
6. Periodically, with the stopper firmly in place (keep a finger on the stopper at all
times), turn the separatory funnel upside down (with the tip near the stopcock
pointing upward).
7. Open the stopcock to relieve any pressure (you will probably hear a hiss or sound
of escaping gas. The water phase will be found on top. The organic phase is at the
bottom.
8. Collect the organic phase in a 125 mL flask and collect the aqueous phase in a
beaker. Do not discard any of the solution until you have completed the
experiment.
9. Return the organic phase into the separatory funnel.
10. Add about 10 mL of the 10% NaOH solution to make the upper aqueous phase
basic (excess OH- ion; turn red litmus blue) and neutralize the benzoic acid
(which was dissolved in the organic phase) and the benzoate ion was converted,
which is charged and now will partition into the aqueous phase.
11. Mix the content of the separatory funnel for a couple minutes and vent 3-4 times;
the aqueous phase from the upper organic phase should be ready.
12. Place the separatory funnel in the O-ring clamp and let the two phases separate
completely. When the interface is apparent, remove the ground-glass stopper and
open the stopcock gently.
13. Collect the aqueous sodium benzoate in a 125 mL flask.
14. Add enough 6 M HCl to the aqueous phase to react with the sodium benzoate to
convert it back into benzoic acid, which is mostly insoluble in water.
15. When enough acid was added, test the liquid by the blue litmus paper to verify
acidity (blue litmus paper turns red)
16. When the liquid was known to be slightly acidic, collect the recrystallized benzoic
acid.
Reference:
www.chemfinder.com
http://ptcl.chem.ox.ac.uk/MSDS
EXP-2B: PURIFICATION OF IMPURE BENZOIC ACID BY EXTRACTION:
Questions: Answer the following questions.
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1) Two techniques were used in this experiment.
a. Recrystalization
b. Extraction.