Question

In Lassaigne's test for nitrogen in an organic compound, the blue colouration is due to the formation of

• A. ferrous ferricyanide
• B. ferric ferrocyanide
• C. potassium ferricyanide
• D. potassium ferrocyanide

Answer 1

Answer: (B)

ferric ferrocyanide

Answer 2

To a part of sodium extract, $FeSO _{4}$ solution is added and the contents are warmed. A few drops of $FeCl _{3}$ solution are then added and resulting solution is acidified with conc. HCl. The appearance of bluish green or Prussian blue colouration confirms the presence of nitrogen. The reactions that occur during this test are as
$2 NaCN + FeSO _{4} \longrightarrow Na _{2} SO _{4}+ Fe ( CN )_{2}$
$Fe ( CN )_{2}+4 NaCN \longrightarrow \underset{\text{Sodium ferrocyanide}}{Na _{4}\left[ Fe ( CN )_{6}\right]}$
$3 Na _{4}\left[ Fe ( CN )_{6}\right]+4 FeCl _{3} \longrightarrow$
$\underset{\text{Ferric ferrocyanide (Prussian blue)}}{Fe _{4}\left[ Fe ( CN )_{6}\right]_{3}}+12 NaCl$