Question

Why does adding a citric acid solution cause the aspirin crystals to reform?

Answer 1

Aspirin structure, we can see has carboxyl group. In unionized state in solid or crystalline form. But when aspirin crystals are added to water the COOH group ionizes a proton. This gives anion of aspirin which dissolved in water. This is reversible reaction and proton may occasionally recombine to anion and regenerates aspirin. But as the proton and anion are solvated well by water molecule the equilibrium of this solvation reaction lies far more on anionic solvated form.

But the citric acid which is capable of donating protons in an aqueous medium increases proton concentration in the solution containing aspirin anion. Then according to Le-Chateliar’s principle the equilibrium shifts on the side that consumes the added protons i.e. on the side of aspirin and hence the recrystallization of aspirin takes place.

Hence on addition of citric acid aspirin crystals reforms.