Ncert Exercises & Solutions

Question 12.37:
In the Lassaigne’s test for nitrogen in an organic compound, the Prussian blue colour is
obtained due to the formation of:
(a) Na $_{4}$ [Fe(CN) $_{6}$ ] (b) Fe $_{4}$ [Fe(CN) $_{6}$ ]3 (c) Fe $_{2}$ [Fe(CN) $_{6}$ ] (d) Fe3[Fe(CN) $_{6}$ ] $_{4}$

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 the process, sodium
cyanide first reacts with iron (II) sulphate and forms sodium hexacyanoferrate (II). Then,
on heating with sulphuric acid, some iron (II) gets oxidised to form iron (III)
hexacyanoferrate (II), which is Prussian blue in colour. The chemical equations involved
in the reaction can be represented as

$6 C N^{-} + F e^{2 +} \to {[F e {(C N)}_{0}]}^{2 -}$
$3 {[F e {(C N)}_{6}]}^{4 -} + 4 F e^{3 +} \overset{{}_{3}H_{2} O}{\to} f e_{4} {[F e {(C N)}_{6}]}_{3} x H_{2} O$
$P r u s s i a n b l u e$
Hence, the Prussian blue colour is due to the formation of Fe $_{4}$ [Fe(CN) $_{6}$ ] $_{3}$ .

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