Acid-Base TitrationAcid-Base TitrationAn Analysis of Aspirin An Analysis of Aspirin

TabletsTablets

Acid-Base TitrationAcid-Base TitrationAn Analysis of Aspirin An Analysis of Aspirin

TabletsTablets

Limitation in this experiment: Assume that all the acidic

ingredients come from acetyl-salicylic 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 acid3 tablets of Disprin aspirin (1.44g)5 capsules of Fuse aspirin (0.72g)Phenolphthalein (act as indicator)

Apparatus list:burette x 2pipette x 2conical flask x 6volumetric flask x 2

beaker x 4measuring cylinder x 1funnel x 2

Experimental setup 1(Hydrolysis of aspirin)

Procedure 1Hydrolysis of aspirin:

1.Record the weight of aspirin tablets2.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):

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.

Procedure 2Back titration with hydrochloric

acid:

Data and results:

Brand name: DisprinNo. of tablets used: 3

Weight: 1.44gAverage volume of hydrochloric acid : 14.73cm3

Brands name: Fuse No. of capsules used: 5

Weight: 0.72gAverage 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

well-shaken.• 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

No. of moles of HCL used=14.73/1000*0.10375=0.00153molInitial no. of mole of NaOH=1*25/1000=0.025molNo 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

times0.008*10=0.08mol / tabletCost of aspirin per mole=13/(0.08*24)=$6.78/mol

BRAND 2

No of moles of HCL used.=17.85/1000*0.10375=0.00185Initial no. of mole of NaOH=1*25/1000=0.025molNo 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 times0.0046*10=0.046mol / capsuleCost of aspirin per mole=10/(0.046*10)=$21.8/mol

Conclusion:Conclusion:Conclusion:Conclusion:

Disprin is the best buyDisprin is the best buy

Reference:Reference:Reference:Reference:Chemistry in contextChemistry in context

(G.C Hill J.S Holman Nelson)(G.C Hill J.S Holman Nelson)

Websites:Websites:http://capital2.capital.edu/faculty/wbeckthttp://capital2.capital.edu/faculty/wbecktel/PAsprinL.htmel/PAsprinL.htm

http://chemlabs.uoregon.edu/Classes/Exthttp://chemlabs.uoregon.edu/Classes/Exton/CH229/AspAnalysis/AspAnalysis.pdfon/CH229/AspAnalysis/AspAnalysis.pdf

http://web.esf.edu/dljohnson/fch380net/lhttp://web.esf.edu/dljohnson/fch380net/labs/seventh.htmabs/seventh.htm

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