Lassaigne’s test is used to detect the presence of nitrogen, sulphur and bromine in an organic compound.
It involves following steps :
Step I : Preparation of Sodium Fusion Extract- A small amount of organic substance is fused with small quantity of sodium metal in a fusion tube. The red hot fusion tube is plunged into distilled water and the contents are boiled for a few minutes, then cooled and filtered. The filtrate obtained is called sodium fusion extract or Lassaigne extract. It is usually alkaline.
Step II : Detection of Elements using Sodium Fusion Extract The elements present in the organic compound react with sodium during fusion reaction as follows :
Na + C + N ➝ NaCN (if N is present)
2Na + S ➝ Na2S (if S is present)
Na + S + C + N ➝ NaSCN
(if both N and S are present)
Na + Br ➝ NaBr (if Br is present)
(i) Test for Nitrogen - The sodium fusion extract is boiled with iron (II) sulphate and then acidified with concentrated H2SO4. The formation of Prussian blue colour confirms the presence of nitrogen
(ii) Test for Sulphur- If sulphur is present in the organic compound, sodium fusion will convert it into sodium sulphide. On treating sodium fusion extract with sodium nitroprusside, appearance of violet colour confirms the presence of sulphur.
(iii) Test for both Nitrogen and Sulphur- On treating sodium fusion extract with Fe3+ ions, the formation of blood red colour complex confirms the presence of both nitrogen and sulphur.
(iv) Test for Bromine: Sodium fusion extract is first acidified with nitric acid and then add silver nitrate solution to it. The formation of pale yellow precipitate that is partially soluble in NH4OH confirms the presence of bromine.