Question

Detection of nitrogen by Lassaigne's test .

Answer 1

Lassaigne's Test :: It is a general test for the detection of halogens, nitrogen and sulphur in an organic compound. These elements are covalently bonded to the organic compounds. In order to detect them, these have to be converted into their ionic forms. This is done by fusing the organic compound with sodium metal. The ionic compounds formed during the fusion are extracted in aqueous solution and can be detected by simple chemical tests. The extract is called sodium fusion extract or Lassaigne's extract.
Test for Nitrogen:

The carbon and nitrogen present in the organic compound on fusion with sodium metal gives sodium cyanide (NaCN) soluble in water. This is converted in to sodium ferrocyanide by the addition of sufficient quantity of ferrous sulphate. Ferric ions generated during the process react with ferrocyanide to form prussian blue precipitate of ferric ferrocyanide.

Na + C + N → NaCN

6NaCN + FeSO₄ → Na₄[Fe(CN)₆] + Na₂SO₄

Sodium ferrocyanide

Na₄[Fe(CN)₆] + Fe³⁺ → Fe₄[Fe(CN)₆]₃

Ferric ferrocyanide