Problems in recrystallization

There are three common problems encountered during recrystallization:

1. The compound crystallizes in the filter funnel during hot filtration. This is because the solubility of the solute decreases rapidly with temperature and the slight cooling during hot filtration causes precipitation of the solid, even though you are heating the funnel. The answer is to use more than the minimum. amount of solvent and then evaporate off the excess before cooling.
2. The compound does not recrystallize. There are two reasons: you have used too much solvent and you must evaporate off some solvent before cooling, or you have formed a supersaturated solution and you must 'seed' or 'scratch' the solution.
3. The compound precipitates as an oil. This is because compounds with low melting points often come out of a concentrated solution above their melting point. In such cases more solvent should be added and the compound redissolved and cooled so that precipitation is retarded to the temperature at which the crystalline solid comes out of solution. Often 'scratching' the hot solution as it cools can prevent 'oiling out'.