There are several criteria for purity of organic compounds. Out of these which one is considered best?

1. Mixed melting point
2. Melting point
3. Microscopic examination
4. Colour

Mixed melting point is considered to be the best criteria for purity of a substance. The purified sample is mixed with a small quantity of pure compound and melting point of mixture is determined. If melting point of mixture is same as that of the pure compound, the sample compound is pure, otherwise it requires further purification.