Practical investigations
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In the quantitative determination of acetylsalicylic acid in aspirin tablets by back-titration, why is the hydrolysed aspirin sample not allowed to boil dry during simmering?
What is the importance of simmering the aspirin tablet in the NaOH solution for 20 minutes?
Why is acetylsalicylic acid analyzed using back-titration instead of direct acid-base titration?
Igloo writes ...
The reasons why the aspirin sample is not allowed to “boil dry during simmering” are as follows: once the solution becomes saturated with solids during excessive boiling, the solid contents would spit out. This would be dangerous to the observer, since one of the solids is corrosive sodium hydroxide. Secondly, solid would be lost outside the beaker, so this would lead to inaccurate results when the back titration is subsequently carried out.
The aspirin tablet is boiled in the NaOH solution for 20 minutes in order to ensure that the hydrolysis reaction is complete. Covalent bonds in the aspirin (acetylsalicylic acid) are being broken and this process takes time – it’s not instantaneous.
A back titration gives the most accurate results since in a direct titration with alkali there is always a risk of the ester group being hydrilysed at the same time as the carboxylic acid group is being neutralised. This would lead to an overestimate of the amount of acetylsalicylic acid present. If a direct titration is carried out, it is normally carried out at low temperatures (e.g. in an ice-bath) in order to minimise ester group hydrolysis.
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updated: 22 July 2007
