Determination of the Amount of Acetylsalicyclic Acid in One Tablet of Commercial Aspirin

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(I) Object of the experiment

To determine the percentage of acetylsalicyclic acid in one commercial aspirin tablet, using the principle of back titration.

(II) Discussion

Back titration rather than direct titration was used because there was no suitable indicator for acetylsalicyclic acid and sodium hydroxide solution. Therefore, excess amount of NaOH was used to react with acetylsalicyclic acid. The number of moles of unreacted NaOH was determined from titration with hydrochloric acid. Eventually the number of moles of acetylsalicyclic acid and its percentage in the commercial aspirin tablet were deduced.

(III) Procedure

1. The weight of one tablet of commercial aspirin tablet was measured.

2. The aspirin tablet was grinded into powder by mortar and pestle. Then it was rinsed thoroughly with deionized water and poured into a beaker.

(Experimental set up of grinding an aspirin tablet)

3. 25cm3 of 0.2521M NaOH solution, which was in excess so that the equilibrium was driven towards products according to Le Chatelier’s principle, was pipetted to the beaker to hydrolyze acetylsalicyclic acid.

4. The solution was heated using a Bunsen burner for 5 minutes to ensure all acetylsalicyclic acid has undergone hydrolysis and fasten the rate.

(Experimental set up of heating acetylsalicyclic acid with sodium hydroxide)

5. The solution was poured into a 250.0cm3 volumetric flask. Deionized water was used to rinse the conical flask so that all the solution was transferred.

6. 0.1003 M of HCl was diluted tenfold to 0.01003 M. 25cm3 of HCl was pipetted to another 250.0cm3 volumetric flask, then deionized water was added to the volumetric flask until the graduation mark.

7. After the dilution in...

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...ch as random parallax error and the ambiguity in distinguishing between pale pink and colourless.

 Random errors were reduced by taking average of repeated readings. In the titration, the amount of HCl used was almost the same among four runs. Therefore, the random error was minimized.

4. Side reactions might occur between unknown chemicals in the aspirin tablet and NaOH solution. Thus, the amount of NaOH reacted with acetylsalicyclic acid would be overcounted.

 Further exploration of contents of aspirin would be needed to see whether there was any side reaction.

(IX) Conclusion

The commercial aspirin tablet had 93.24% purity. This showed that there were other chemicals like binder was presence in the tablet. Besides, the tablet might not be stored in dry condition, so acetylsalicyclic acid would be hydrolyzed by moisture in the air to form acetic acid.

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