SolutionsCrystallization
1. The ideal solvent
Which conditions must a good solvent meet to crystallize a compound?
- Dissolve a lot of the compound when hot and little when cold.
- Not react with the compound.
- Dissolve the impurities very well (they stay in the mother liquor) or not at all (they are removed by hot filtration).
- Be volatile enough for the crystals to dry easily.
2. Fast or slow
Why is it best to cool the hot solution slowly?
Because slow growth gives larger, purer crystals: the molecules have time to take their places in the lattice in an orderly way and the impurities stay in solution. Sudden cooling gives small crystals that trap impurities.
3. How much is recovered?
The solubility of benzoic acid in water is 6.8 g/100 mL at 95 °C and 0.34 g/100 mL at 25 °C. If you crystallize 5.0 g from the minimum amount of water at 95 °C and cool to 25 °C, what is the most you can recover?
Minimum volume: 5.0 × 100 / 6.8 = 74 mL. At 25 °C, 0.34 × 0.74 = 0.25 g stays dissolved, so at most 4.75 g (95 %) is recovered.