Question

Ferric ion forms a Prussian blue precipitate due to the formation of

• A. $K_4[Fe(CN)_6]$
• B. $K_3[Fe(CN)_6]$
• C. $Fe(CNS)_3$
• D. $Fe_4[Fe(CN)_6]_3$

Answer 1

Answer: (D)

$Fe_4[Fe(CN)_6]_3$

Answer 2

Ferric ion forms a prussian blue precipitate due to the formation of $Fe _{4}\left[ Fe ( CN )_{6}\right]_{3}$. This complex is formed during the determine action of presence of nitrogen in the given sample. In this method, to portion of sodium fusion extract, freshly prepared ferrous sulphate, $FeSO _{4}$ solution is added and warmed. Then about 2 to 3 drops of $FeCl _{3}$ solution are added and acidified with conc. HCl. The appearance of a prussian blue colour indicate the presence of nitrogen.

$FeSO _{4}+2 NaOH \longrightarrow Fe ( OH )_{2}+ Na _{2} SO _{4}$
$6 NaCN + Fe ( OH )_{2} \longrightarrow \underset{\text{Sodium ferrocyanide}}{{Na _{4}\left[ Fe ( CN )_{6}\right]}}+2 NaOH$
$3 Na _{4}\left[ Fe ( CN )_{6}\right]+4 FeCl _{3} \longrightarrow \underset{\text{Ferric ferrocyanide (prussian blue)}}{{Fe _{4}\left[ Fe ( CN )_{6}\right]_{3}}}+12 NaCl$