The sodium fusion test, or Lassaigne’s test, is used in elemental analysis for the qualitative determination of the presence of foreign elements, namely halogens, nitrogen, and sulphur, in an organic compound. It was developed by J. L. Lassaigne. The compound is heated with sodium metal to convert the elements present in the organic compound into the water-soluble salts of sodium. Sodium is fused with the organic compound and then the fused mass is extracted with water. The extract is filtered and the filtrate is called sodium extract or Lassaigne’s extract. Nitrogen, if present in the compound, is converted into NaCN in Lassaigne’s extract. Halogens form a salt of sodium, such as NaCl, NaBr or NaI, while sulphur forms sodium sulphide (Na S) on fusion of the organic compound with sodium.
This video explains how to detect the presence of nitrogen, sulphur, chlorine, bromine and iodine in organic compounds by Lassaigne’s test.