Lassaigne's test is used to detect the presence of N,S,Cl,Br,I in organic compounds using sodium fusion extract. In the case of nitrogen, Prussian blue colour is obtained due to the formation of Fe4[Fe(CN)6]3. Reactions involved in this are as follows Na+
C+N
C&N from organic compund
→NaCN
FeSO4+NaOH→Fe(OH)2+Na2SO4
Fe(OH)2+
2NaCN
Obtained from sodium extract
→Fe(CN)2+2NaOH
Fe(CN)2+4NaCN→Na4[Fe(CN)6]
3Na4[Fe(CN)6]+4FeCl3→
Fe4[Fe(CN)6]3
Ferric ferrocyanide (Prussian blue coloured compound)