In the detection of nitrogen of an organic compound by Lassaigne's test, a prussian blue colour is…

Detection of nitrogen is done with the help of Lassaigne's extract. It is boiled with iron sulphate and then acidified with concentrated with sulphuric acid. The appearance of a prussian blue colour confirms the presence of nitrogen.

$$\begin{gathered} 1. {\mathrm{Fe}}^{2+} + 6{\mathrm{CN}}^{-}\to {\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}^{4-} \\ 2. {\mathrm{F}}^{2+}\to {\mathrm{Fe}}^{3+} + {\mathrm{e}}^{- }\left(\text{In the presence of concentrated} {\mathrm{H}}_2{\mathrm{SO}}_4\right) \\ 3. {\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}^{4-}+4{\mathrm{Fe}}^{3+} \to {\mathrm{Fe}}_4{\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}_3 \\ \left(\text{Prussian Blue}\right) \end{gathered}$$