Recrystallisation and Melting point analysis

Recrystallisation

• This is a process that can be used to purify a solid substance. It essentially involves dissolving a substance in an appropriate solvent and then having it come out of the solution in a crystalline form.

The Procedure:

1. Dissolve the impure solid into just the right amount of a warm, approproate solvent (until a saturated solution is formed).
2. If the solution is not clear; filter the solution through a buchner funnel to remove any insoluble impurities.
3. Cool the solution, so that the product recrystallises leaving the smaller amounts of impurities in the solution.
4. Filter the solution to recover the purified product.
5. Wash the solid with small amounts of pure solvent to wash away the solution of impurities still on the solid.
6. Allow the solvent to evaporate.

Melting Point Analysis

• Melting point analysis can be used to determine how pure a substance is.

The Procedure

1. Fill a melting point capillary tube with a small sample of your purified sample. Tap gently on the bench to ensure that the substance goes to the bottom of the tube. (Fill to around 0.5 cm).
2. Insert the capillary tube into the melting point analysis apparatus, and apply heat that will cause the temperature to raise by about 10^oC per minute.
3. Watch carefully for a sign of melting. Record the temperature range over which the substance has melted.

A small range indicates that the substance is pure. If your substance is impure, you will have a solid that melts over a range of temperatures.

A good target to aim for is a substance that will have a range of a few degrees. If the substance is heated too fast, then the melting point will be higher than expected.

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