Question

Intense blue colour of prussian blue arises as a result of:
(A) Electron transfer between Fe (II) and Fe (I)
(B) Electron transfer between Fe (II) and Fe (III)
(C) d - d transition of an electron
(D) Ligand metal charge transfer

Answer 1

The property of imparting colour is also shown by compounds which have mixed valence. The electrons absorb light of certain wavelengths resulting in intervalence charge transfer taking place due to which colour is imparted.
Complete step by step answer:
In organic qualitative analysis, we have often done Lasseigne’s test which is used for detection of halogens, nitrogen and sulphur. I want to refresh the reaction involved in the detection of nitrogen for better understanding of this concept:
Step 1: Nitrogen present in the organic compounds reacts with sodium metal to form sodium cyanide.
$Na + C + N \to NaCN$
Step 2: The sodium cyanide obtained above is added to ferrous sulphate to form sodium ferrocyanide.
$NaCN + FeS{O_4} \to N{a_4}[Fe{(CN)_6}]$
Step 3: Ferric ions generated during the process react with sodium ferrocyanide (orange-red colour) to form Prussian blue precipitate of ferric ferrocyanide.
$N{a_4}[Fe{(CN)_6}] + F{e^{3 + }} \to F{e_4}{[Fe{(CN)_6}]_3}$
In the above reaction, we can see ferric ferrocyanide, which contains Both Fe(II) and Fe(III) due to which it is said to have mixed valence. There will be an intervalence charge transfer that takes place when it is exposed to a certain wavelength in visible light.

Thus, option (B) is the correct answer.

Note: In step 3, the orange-red colour of sodium ferrocyanide absorbs wavelength around 680nm and imparted Prussian blue colour. More than saying the electron transfer between Fe (II) and Fe (I), it is reduced that sounds exact. So don’t confuse option A and B. This blue pigment is widely used in paints and crayons.