Recrystallisation of Benzoic Acid & Determination of its Melting Point

Recrystallisation of a Sample of Benzoic Acid and Determination of its Melting Point

Image From "Leaving Certificate Rapid Revision Chemistry."

By using a the Buchner funnel and flask to rapidly filter the solution, with the aid of a suction pump, there is a lower possibility of the substance crystallizing out on the filter paper or funnel.

If crystallization is slow to take place, either of the following steps can be taken to speed it up:

(a) Evaporate off some of the solvent and allow the solution to cool again, or 

(b) "seed" the solution by adding a crystal of the pure compound into the solution.

The crystals can be dried by pressing them between two dry sheets of filter paper and then placing them in a desiccator.

Melting Point Determination

The crystals are placed in a capillary tube and then heated in an aluminium block. The temperature at which the crystals melt is noted. The process is then repeated and the average value is taken as the melting point.

The melting point can tell us whether the substance if pure or just slightly impure. If the substance melts sharply at 122 degrees Celsius,  then we know that it is pure. But if the sample melts over prolonged temperatures then we know that the substance is slightly impure and it will melt at lower temperatures that the pure sample.

By Niall Fahey & Stephen Garavan Class 5A1