Purification of an organic solid

How to separate aspirin from reaction mixture after it is formed?

Vacuum filtration using a Buchner funnel

What are the impurities present in the filtered aspirin?

• Ethanoic acid - this is the by product

• Unreacted ethanoic anhydride (excess added)

• Unreacted 2-hydroxybenzoic acid

• Concentrated H₂SO₄ catalyst (or any catalyst used for the manufacture)

What is the process of purifying aspirin called?

Recrystallisation

Describe the process of recrystallisation

1. Dissolve the impure solid in the minimum amount of hot solvent

2. Filter the solution whilst hot

3. Allow the solution to cool and crystallise

4. Filter the crystals under reduced pressure (vacuum filtration)

5. Wash with cold solvent and dry

Why is the minimum amount of hot solvent used?

To form a saturated solution of the aspirin

Why must the solution be filtered whilst hot?

To remove insoluble impurities (the aspirin is still dissolved in the hot solution)

Why is the solid washed with cold solvent?

To remove soluble impurities (cold solution prevents the aspirin redissolving)

What physical property is used to check the purity of the aspirin?

Melting Point

What two ways could the result show the product is impure?

1. Melting point is lower than data book value

2. Melts over a range of values rather than sharply at the melting point

How to decide what solvent to use for recrystallisation?

Solid is soluble when solvent is hot and insoluble when solvent is cold

What is the difference in appearance between crude and pure aspirin?

Pure aspirin will have larger crystals, that are more needle-like and whiter (also shinier)