Carboxylic acids and related compounds
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Why is the direct titration of aspririn against aqueous sodium hydroxide is impossible?
And why is the reaction mixture of aspirin in sodium hydroxide solution heated gently but not strongly?
Igloo replies
Aspirin is an acid and an ester. It reacts with sodum hydroxide in two ways. The alkali neutralises the acid. This happens fast but at the same time it hydrolyses the ester link but much more slowly. So for accurate results it is better to add a known excess of alkali, warm to complete both reactions and then titrate the excess alkali with standard acid (back titration).
Strong heating is hazardous because the alkali is corrosive and might splash out of the reaction mixture, causing potential damage to the surrounding area or to the observer. Alkali seems to a tendency to 'bump' violently on heating.
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Risk assessment
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updated: 28 January 2006
