Acid-Base Titration
An Analysis of Aspirin
Tablets
Limitation in this experiment:
Assume that all the acidic
ingredients come from acetylsalicylic acid(aspirin)
Introduction:
Aspirin is widely used as pain killer and
lower body temperature.
The main constituent of aspirin tablets is
acetyl-salicylic acid(CH3COOC6H4COOH). Aspirin
passes unchanged through the acidic conditions in
the stomach but is hydrolysed to ethanoate ions
and salicylate ions by the alkaline juices in
intestines. Over doses of aspirin may even causes
death.
CH3COOC6H4COOH + 2OH-  CH3COO- + HOC6H4COO- + H2O
Principle:
The object of this experiment is to determine the
percentage of acetyl-salicylic acid in aspirin tablets. A known
amount of standard sodium hydroxide solution is used in
excess to hydrolyses a known mass of aspirin tablets.
CH3COOC6H4COOH + 2NaOH  CH3COONa +HOC6H4COONa +H2O
The unused sodium hydroxide which remains is then
titrated with standard acid. The amount of alkali required for
the hydrolysis can now calculated. The number of moles of
acetylsalicylic acid which have been hydrolysed can be found.
Brands of aspirin:
Brand 1: Disprin ($13 each pack, 24 tablets per pack)
Brand 2: Fuse ($10 each pack, 10 capsules per pack)
Chemicals:
1M sodium hydroxide
0.10375M hydrochloric acid
3 tablets of Disprin aspirin (1.44g)
5 capsules of Fuse aspirin (0.72g)
Phenolphthalein (act as indicator)
Apparatus list:
burette
x2
pipette
x2
conical flask
x6
volumetric flask
x2
beaker
x4
measuring cylinder x 1
funnel
x2
Experimental setup 1
(Hydrolysis of aspirin)
Procedure 1
Hydrolysis of aspirin:
1.Record the weight of aspirin tablets
2.Transfers the crushed aspirin tablets into a clean conical
flask.
3.Pipette 25 cm3 of NaOH in to the aspirin- containing
conical flask.
4.The same volume of distilled water is added to form a
mixture.
5.Leave the mixture to react for 10 minutes.
6.Transfer the mixture to a volumetric flask and make up to
the mark with distilled water.
Experimental setup 2
(Back titration):
Procedure 2
Back titration with hydrochloric
acid:
1. Pipette 25cm3 of hydrolyzed solution into a
conical flask.
2. Phenolphthalein is added as indicator.
3. Back titrate the hydrolyzed solution with
hydrochloric acid.
4. Record the volume of hydrochloric acid.
5. Repeat the above experiment for a least 4
times.
Data and results:
Brand name: Disprin
No. of tablets used: 3
Weight: 1.44g
Average volume of hydrochloric acid : 14.73cm3
Brands name: Fuse
No. of capsules used: 5
Weight: 0.72g
Average volume of hydrochloric acid: 17.85cm3
Observation

Aspirin + NaOH
Phenolphthalein
added

Aspirin + NaOH +
phenolphthalein
Titrate with
hydrochloric acid
and the endpoint is
reached.
Source of errors:
• The aspirin tablets may contain more than one
kind of acid other than aspirin.
• The aspirin may not fully dissolve in the distilled
water.
• The mixture of aspirin solution may not be wellshaken.
• Extra drops of hydrochloric acid may added while
the end-point is reached.
• It was not easy to get a good estimation to 0.05
cm3 from the burette.
Precautions:
• NaOH is corrosive. We should avoid
contacting the solution with our skin.
• Safety goggles must be put on during
heating and titration.
• We should wash our hands after the
experiment
Remarks:
• The pH of the mixture should be
checked by a pH paper before
carrying out the titration.
• Slightly heated the mixture to
ensure all aspirin dissolved.
• We should label the containers of
both sodium hydroxide and
hydrochloric acid as they are
colorless solution.
Improvement:
As some acidic impurities presents in the
aspirin tablets, the accuracy of the above
experiment to determine the concentration
of aspirin is low. However, the
acetylsalicylate ions (ionized aspirin) is an
ester and esters are formed from
combination of an acid and an alcohol. By
reacting the ester with NaOH, alcohol is
formed. Determining the alcohol
concentration by carrying fractional
distillation. Therefore the concentration of
the aspirin can be calculated.
Calculation:
BRAND 1
BRAND 2
No. of moles of HCL used
=14.73/1000*0.10375
=0.00153mol
Initial no. of mole of NaOH
=1*25/1000=0.025mol
No of moles NaOH reacted
=0.025-0.00153
=0.0248mol
∴No. of mole of aspirin reacted
=0.0248mol
∴No. of mole per aspirin tablet
=0.0248/3=0.008mol / tablet
∵the mixture has been diluted 10
times
0.008*10=0.08mol / tablet
Cost of aspirin per mole
=13/(0.08*24)=$6.78/mol
No of moles of HCL used.
=17.85/1000*0.10375
=0.00185
Initial no. of mole of NaOH
=1*25/1000=0.025mol
No of moles reacted
=0.025-0.00185
=0.0232mol
∴No. of mole of aspirin reacted
=0.0232mol
∴No. of cost per aspirin capsule
=0.0232/5=0.0046
∵the mixture has diluted 10 times
0.0046*10=0.046mol / capsule
Cost of aspirin per mole
=10/(0.046*10)=$21.8/mol
Conclusion:
Disprin is the best buy
Reference:
Chemistry in context
(G.C Hill J.S Holman Nelson)
Websites:
http://capital2.capital.edu/faculty/wbeckte
l/PAsprinL.htm
http://chemlabs.uoregon.edu/Classes/Exton
/CH229/AspAnalysis/AspAnalysis.pdf
http://web.esf.edu/dljohnson/fch380net/la
bs/seventh.htm
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