When the organic sample containing nitrogen and sulphur is strongly heated with sodium metal the ionic compound sodium thiocyanate is formed. Sodium thiocyanate gives blood-red colour with neutral ${\mathrm{FeCl}}_3$.

$$\begin{gathered} \mathrm{Na} +\mathrm{N}+\mathrm{C}+\mathrm{S}\stackrel{}{\to }\mathrm{NaSCN} \\ {\mathrm{SCN}}^{-} + {\mathrm{Fe}}^{3+}\stackrel{}{\to }{\left[\mathrm{Fe}\left(\mathrm{SCN}\right)\right]}^{2+} \end{gathered}$$