Theory

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 Halogen

Halogens present in an organic compound form sodium halide on fusion with sodium metal. Sodium halide extracted with water can be easily identified by adding silver nitrate solution after acidifying with dilute HNO₃.

If chlorine is present:

A white curdy precipitate soluble in ammonium hydroxide solution is formed.

Na + Cl → NaCl

NaCl + AgNO₃ → AgCl + NaNO₃

If bromine is present:

A yellowish white precipitate sparingly soluble in ammonium hydroxide solution is formed.

Na + Br → NaBr

NaBr + AgNO₃ → AgBr + NaNO₃

If iodine is present:

A yellow precipitate insoluble in ammonium hydroxide solution is formed.

Na + I → NaI

NaI + AgNO₃ → AgI + NaNO₃

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 into 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

Na₄[Fe(CN)₆] + Fe³⁺ → Fe₄[Fe(CN)₆]₃

Test for Sulphur

If sulphur is present in the organic compound, sodium fusion will convert it into sodium sulphide. Sulphide ions are readily identified using sodium nitroprusside.

Na + S → Na₂S

Na₂S + Na₂[Fe(CN)₅NO] → Na₄[Fe(CN)₅NOS]

Test for Both Nitrogen and Sulphur

If both nitrogen and sulphur are present in an organic compound, sodium fusion will convert it into sodium thiocyanate, which then reacts with Fe³⁺ to form a blood red complex [Fe(SCN)]²⁺.

Na + C + N + S → NaSCN

Fe³⁺ + 2Na → [Fe(SCN)]²⁺

Note: If the Lassaigne's extract contains excess sodium metal, sodium cyanide and sulphides are formed instead of sodium thiocyanate.

NaSCN + 2Na → NaCN + Na₂S

In such cases, both sulphur and nitrogen must be identified with separate tests.