Photo: Daniel Case via Wikimedia Commons under a Creative Commons 3.0 license
This experiment can b a companion piece to the preparation of aspirin, or it can be carried out as a standalone activity.
Since it is insoluble in water, aspirin has to be determined by a ‘back titration’ technique. This involves treating a sample of aspirin of accurately known mass with a known amount of sodium hydroxide.
The alkali first catalyses the hydrolysis of the aspirin to ethanoic and salicylic acids and then neutralises these acids. An excess of alkali is used and the amount remaining after reaction is determined by titrating it against a standard solution of sulphuric acid. Subtracting this from the amount added gives the amount used in the reaction
Determination of aspirin – pupil guide
