For testing an organic compound, Lassaigne's filtrate is to be prepared. To prepare the Lassaigne's filtrate, take a pea-sized and dried fresh piece of sodium and put it in a small ignition tube and heat it till sodium melts, A pinch of the given organic compound (or a drop of it if it is a liquid) is now added to the fused sodium and heated again to red hot. The fusion tube contents are now transferred to a china dish containing 10-15 ml of water. The tube breaks and contents are boiled in the China dish. The solution, after filtration, is called Lassaigne's filtrate.

Test for Nitrogen

Na + C + N \(\overset{\Delta}{\rightarrow}\) NaCN

From organic compound

Now to a part of Lassaigne’s filtrate freshly prepared FeSO₄ is added when the following reactions occur. A small amount of H₂SO₄ is added.

2NaCN + FeSO₄ → Fe(CN)₂ + Na₂SO₄

Fe(CN)₂ + 4NaCN → Na₄[Fe(CN)₆) Sodium hexacyano ferrate (II)

Some of Fe²⁺ ions are oxidised on heating to Fe³⁺ ions.

3Na₄[Fe(CN)₆] + 4Fe^{3+ \(\overset{xH_2O}{\rightarrow}\)} Fe₄[Fe(CN)₆]₃ xH₂O + 12Na⁺ Iron (III) hexacyanoferrate (II)

(Prussian blue)

This Fe³⁺ ion gives Prussian blue colour which confirms the presence of nitrogen in the compound.