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] + 4Fe3+ \(\overset{xH_2O}{\rightarrow}\) 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.