Mixed melting point determination

You can confirm the identity of a compound by determining a mixed melting point. If you prepare a mixture of your unknown chemical and the one you suspect it may be and measure the melting point of the mixture then there are two possible results:

1. The melting point of the mixture is the same as the pure compound, which means that the unknown compound and the known compound are the same.
2. The melting point of the mixture is lower than either of the two pure components and the melting range is large. This is because the two compounds are different with the result that one is an impurity in the other.

For example, both benzoic acid and mandelic acid are white crystalline solids which melt at 121°C. However a 1: 1 mixture of the two compounds begins to melt at about 80°C.

The usefulness of mixed melting points is limited in that you must have some idea of the chemical nature of your unknown compound and a sample of the suspected compound must be available.