Question

# In the Lassaigne’s test for the detection of nitrogen in the organic compound, the blue coloured compound formed is:A. Ferric ferricyanideB. Ferrous ferricyanideC. Ferric ferrocyanideD. Ferrous ferrocyanide

Hint – In the Lassaigne’s test for nitrogen in an organic compound, the sodium fusion extract is boiled with iron (II) sulphate and then acidified with sulphuric acid. In this process a Prussian blue colour compound is formed.

We have been asked the name of the blue colour compound formed in the Lassaigne’s test.
So, we discuss the Lassaigne’s test first-
The Lassaigne’s test is done to detect nitrogen in an organic compound.
In this test, first sodium fusion extract is boiled with iron (II) sulphate and then acidified with sulphuric acid.
In this process, sodium cyanide first reacts with iron (II) sulphate and forms sodium hexacyanoferrate (II).
Then, on heating with sulphuric acid, some of the iron (II) gets oxidised to form iron (III) hexacyanoferrate (II), this compound is Prussian blue in colour.
The Lassaigne’s test chemical equation is given below-
$6C{N^ - } + F{e^{2 + }} \to {[Fe{(CN)_6}]^{4 - }}$
Then, ${[Fe{(CN)_6}]^{4 - }}$ reacts with Fe-
$3{[Fe{(CN)_6}]^{4 - }} + 4F{e^{3 + }}\xrightarrow{{x{H_2}O}}F{e_4}{[Fe{(CN)_6}]_3}.x{H_2}O$
Hence, the formation of Prussian blue is due to the formation of $F{e_4}{[Fe{(CN)_6}]_3}$ , ferric ferrocyanide.
So, the correct option is C.

Note – Whenever such types of questions appear, then as mentioned in the solution first we discuss the Lassaigne’s test, about the reactions that occur during the test and then as specifies in the solution blue colour compound is formed which is $F{e_4}{[Fe{(CN)_6}]_3}$ .