Separating a pain-relief tablet – acid–base extraction of aspirin, paracetamol and caffeine
ChemistryOrganic ChemistryAges 16–17Virtual lab
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Next in Organic Chemistry
A step-by-step virtual lab: crush an over-the-counter pain-relief tablet, dissolve it in an organic solvent, filter off the insoluble binder and extract in a separating funnel. NaHCO₃ turns aspirin into its water-soluble salt, NaOH then takes paracetamol, and caffeine stays in the organic layer. Acidify the extracts, filter the solid or extract it back into solvent, dry and weigh each fraction, read its melting range and compare with the label.
Lesson: Acid–base extraction with a separating funnel; percentage recovery; analysing an over-the-counter tablet
What it shows
A pain-relief tablet is a mixture. Aspirin is a carboxylic acid, paracetamol (acetaminophen) is a very weakly acidic phenol, caffeine is a very weak base and the binder does not dissolve. So the weak base NaHCO₃ turns only aspirin into a water-soluble salt, the strong base NaOH is needed for paracetamol, and caffeine stays in the solvent. Acidifying brings the dissolved salt back to the neutral molecule so that it can be recovered. The masses never match the label exactly: some stays dissolved in water, other compounds follow the water layer, and some is lost in transfer. The model uses rounded room-temperature partition ratios and solubilities.
How to use
Choose the Tablet, Number of tablets, Solvent and Solvent volume, press Crush and dissolve, then Filter. In the funnel, choose the Base, press Add and shake, and say which layer is water. Press Finish extraction. Select each Fraction: use Add 0.5 mL HCl until the pH is below 2, then Filter the solid or Extract back; for the others, use Dry and weigh or Dry and evaporate. Compare the table with the label.
Parameters you can change
- Tablet Tablet A (aspirin, paracetamol, caffeine), Tablet B (aspirin, caffeine), Tablet C (paracetamol, caffeine), Tablet X (no label)
- Number of tablets 1–4
- Solvent Ethyl acetate, Dichloromethane
- Solvent volume 20–100 mL
- Base NaHCO₃ 5 %, NaOH 5 %
- Base volume 5–30 mL
Questions to explore
- For tablet A, why is the aspirin fraction lighter when you filter the solid than when you extract it back into solvent?
- Extract tablet A with NaOH first and skip NaHCO₃: how do the mass and melting range of that extract change?
- How does the mass of the filter residue change with dichloromethane instead of ethyl acetate, and why?
