RP 10: Pure organic solid: preparation & purity

Hazards

Method

1. Make the crude solidAQA's exemplar: aspirin. Warm salicylic acid (2-hydroxybenzoic acid) with ethanoic anhydride and a few drops of concentrated phosphoric acid catalyst in a ~60 °C water bath for 15 minutes, then pour into cold water to destroy the excess anhydride and crash out the crude product.
2. Filter under reduced pressureCollect the crystals on a Büchner funnel with the pump running: seat the damp filter paper, pour, rinse with a little ice-cold water, and let the airflow pull the cake dry. Faster than gravity filtration and gives a drier product.
3. RecrystalliseDissolve the crude solid in the minimum volume of HOT solvent, filter hot if there are insoluble specks, then cool slowly to room temperature and finally in ice. Pure crystals grow while the impurities — present in much smaller amounts — stay dissolved.
4. Wash and dryFilter again under reduced pressure, wash with a splash of ice-cold solvent, and dry between filter papers or in a desiccator/low oven.
5. Test purity: melting pointPack a few millimetres of dry product into a capillary and heat slowly in the melting-point apparatus near the expected value. Pure compounds melt sharply within ~1–2 °C of the data-book value; impurities lower AND broaden the range.
6. The liquid half of RP10For a pure organic liquid: distil, wash in a separating funnel, dry over an anhydrous salt and redistil, collecting over a narrow boiling range — every skill from RP5 applies verbatim.

Mistakes examiners see every year

What to expect

Pure aspirin melts sharply at 135 °C (data-book); the crude product melts low and wide until recrystallised. A typical student yield after one recrystallisation is 50–60% (indicative) — report both yield and melting range.