Question

The prussian blue colour obtained in the Lassaigne's test for nitrogen is due to formation of:

• A. Iron $(II) $ hexa cyano ferrate $ (II)$
• B. Iron$(III)$ hexa cyano ferrate $ (II)$
• C. Iron $ (III)$ hexa cyano ferrate $(III)$
• D. Iron $(II)$ hexa cyano ferrate $(III)$

Answer 1

Answer: (B)

In Lassaigne's test for nitrogen in sodium extract, add fresh and saturated solution of $FeSO _{4}$ followed by $NaOH$. Now boil and cool the content. Then, add dil $H _{2} SO _{4}$ and $FeCl _{3}$ solution. Formation of Prussian blue precipitate shows the presence of nitrogen.

$FeSO _{4}+2 NaOH \rightarrow Fe ( OH )_{2} \downarrow+ Na _{2} SO _{4}$

$Fe ( OH )_{2}+\underset {\text{From sodium extract}}{6 NaCN }\rightarrow Na _{4}\left[ Fe ( CN )_{6}\right]+2 NaOH$



$2 Na _{4}\left[ Fe ( CN )_{6}\right]+4 FeCl _{3} \rightarrow \underset {\text{Iron (III) hexa cyano ferrate (II)}}{Fe _{4}\left[ Fe ( CN )_{6}\right]_{3}+12 NaC}$