The blue colouration obtained from the Lassaigne's test of nitrogen is due to the formation of

Step-by-step solution

In Lassaigne's test for nitrogen, the organic compound is fused with metallic sodium to convert nitrogen into sodium cyanide, NaCN . The extract is then treated with ferrous sulphate, FeSO ₄ , which reacts with NaCN to form sodium ferrocyanide: Fe ²⁺ + 6 CN ⁻ [ Fe ( CN )₆]⁴⁻ . On adding ferric chloride, FeCl ₃ , some Fe ²⁺ ions are oxidized to Fe ³⁺ ions. These Fe ³⁺ ions react with the ferrocyanide ions to form ferric ferrocyanide, which is Prussian blue in colour. The chemical reaction is: 3[ Fe ( CN )₆]⁴⁻ + 4 Fe