The more soluble a medicine is, the more quickly it passes from the digestive system into the bloodstream after being swallowed

Aspirin is a weak acid, so the solution has a pH greater than 7 at the end-point, and phenolphthalein is a suitable indicator to use.


The worksheet leads students through the calculations, but it may be necessary to explain that not all of a sample of commercial aspirin is likely to dissolve. Students should also understand why phenolphthalein indicator is used in this experiment.


  • Conical flasks (at least one of them must be dry),100 cm3, x 6
  • Filtration apparatus
  • White tile
  • Burette and stand
  • Pipettes, 25 cm3 (or 50 cm3) and 10 cm3
  • Access to a balance reading to 0.01 g


  • Phenolphthalein indicator
  • Aqueous sodium hydroxide, 0.020 mol dm–3

Health, safety and technical notes

  • Read our standard health and safety guidance here
  • Wear eye protection
  • For more information on phenolphthalein indicator, see CLEAPSS Hazcard HC032.
  • Sodium hydroxide can cause severe burns to the skin and is dangerous to the eyes, see CLEAPSS Hazcard HC091a


  1. Weigh accurately about 0.5 g of aspirin into a 100 cm3 conical flask, add exactly 50 cm3 of distilled water (use a pipette or burette) and swirl for 5 minutes. Filter the solution into a dry 100 cm3 conical flask.
  2. Using a 10 cm3 pipette, transfer four separate 10 cm3 samples into 100 cm3 conical flasks.
  3. Add four drops of phenolphthalein to each of the flasks and titrate with 0.020 mol dm–3 aqueous sodium hydroxide until the first permanent pink colouration.
  4. Carry out one rough, and at least two accurate, titrations and record your results in a table.
  5. Find the mean volume, V cm3, of aqueous sodium hydroxide needed to react with the aspirin dissolved in 10 cm3 of solution.


  • Using 0.020 mol dm–3 sodium hydroxide and 10 cm3 samples of dissolved aspirin, the titres should be about 9 cm3 of sodium hydroxide at room temperature based on the solubility of aspirin being 0.33g in 100 cm3 at room temperature.
  • The relative molecular mass of aspirin is 180 g mol–1
  • The solubility varies significantly with temperature and is in the range 0.2–0.4 g/100 cm3 at room temperature.

Further investigation

  • Repeat the activity, but leave the mixture to stand overnight to see whether more of the aspirin dissolves. Hydrolysis of the aspirin is likely to take place. How will this affect the results obtained? Try testing for 2-hydroxybenzoic acid in the solution.
  • Plan and perform an experiment to find the solubility of aspirin at body temperature (37 °C).
  • Find out about the problems of stomach ulcers caused by aspirin.