Chemistry - PRACTICAL ORGANIC CHEMISTRY Question with Solution | TestHub

When nitrogen and sulphur both are present in the organic compound, they combine during fusion to form sodium thiocyanate (sulphocyanide).

Thus, the compounds containing both $\mathrm{N}$ and $\mathrm{S}$ give blood red colour in Lassaigne's test due to the formation of $\mathrm{Fe}{\left(\mathrm{SCN}\right)}_2$. Thus, ${\mathrm{H}}_2\mathrm{N}\left({\mathrm{C}}_6{\mathrm{H}}_4\right){\mathrm{SO}}_3\mathrm{H}$ gives blood red colour in Lassaigne's test of nitrogen.