Question

Nitrogen detection in an organic compound is carried out by Lassaigne's test. The blue colour formed corresponds to which of the following formulae?

• A. $F{e}_3\left[Fe\right(CN{)}_6{]}_2$
• B. $F{e}_4\left[Fe\right(CN{)}_6{]}_3$
• C. $F{e}_4\left[Fe\right(CN{)}_6{]}_2$
• D. $F{e}_3\left[Fe\right(CN{)}_6{]}_3$

Answer 1

The correct option is B $F{e}_4\left[Fe\right(CN{)}_6{]}_3$

In Lassaigne's test for nitrogen, the formation of Prussian blue colour confirms the presence of nitrogen.
This Prussian blue colour is due to the formation of iron(III)hexacyanoferrate(II), $F{e}_4\left[Fe\right(CN{)}_6{]}_3$, which is also known as ferriferrocyanide.
$3Na\left[Fe\right(CN{)}_6]+4FeC{l}_3\to F{e}_4[Fe(CN{)}_6{]}_3+12NaCl$
Hence, option (b) is the correct answer.