Question

In the detection of nitrogen of an organic compound by Lassaigne's test, Prussian blue colour is obtained. This is due to the formation of which of the following complexes?

• A. $\mathrm{Fe}{}_2[\mathrm{Fe}(\mathrm{CN}){}_6]$
• B. $\mathrm{Fe}{}_4[\mathrm{Fe}(\mathrm{CN}){}_6]{}_3$
• C. $\mathrm{Fe}{}_3[\mathrm{Fe}(\mathrm{CN}){}_6]{}_4$
• D. $\mathrm{Na}{}_4[\mathrm{Fe}(\mathrm{CN}){}_6]$

Answer 1

Answer: (B)

In Lassaigne's test for detection of nitrogen in organic compounds Prussian blue colour is obtained due to the formation of iron (III) hexacyanoferrate (II) complex. The reaction is as follows:

$6C{N}^{-}+F{e}^{2+}\to \underset{\text{Hexacyanoferrate}}{[Fe(CN{)}_6{]}^{4-}}$

$3[Fe(CN{)}_6{]}^{4-}+4F{e}^{3+}\stackrel{x\mathrm{H}{}_2\mathrm{O}}{\to }\underset{\text{Iron (III) hexacyanoferrate (II) (Prussian blue)}}{F{e}_4[Fe(CN{)}_6{]}_3\cdot x{H}_{2}O}$