Question

The intense blue colour of prussian blue salts arises from which one of the following?
A.d-d transition
B.Inter valence electron transfer
C.Ligand to metal charge transfer
D.Metal to ligand charge transfer

Answer 1

Hint:Prussian blue is basically ferric ferrocyanide and it is the first modern synthetic pigment. It is produced by oxidation of ferrous ferrocyanide salts. The intense clue colour of prussian is due to the energy associated with the transfer of electrons from Fe II to Fe III.

Complete answer:
Ferric ferrocyanide, also known as berlin blue or prussian blue, is a dark blue pigment. Common histopathology stain used by pathologists is prussian blue. Ferric ferrocyanide is present in some preparation of laundry bluing. Ferric ferrocyanide is extremely insoluble but it also tends to form colloids. Ferric ferrocyanide is also utilized in paints. It is the “blue” in blue prints as well as in the Japanese woodblock prints.
The chemical formula of prussian blue is ${\rm{F}}{{\rm{e}}^{{\rm{III}}}}_{\rm{4}}{\left[ {{\rm{F}}{{\rm{e}}^{{\rm{II}}}}{{\left( {{\rm{CN}}} \right)}_{\rm{6}}}} \right]_{\rm{3}}}$. The oxidation state of the iron outside the coordination sphere of the compound is +3 and the oxidation state of the iron inside the coordination sphere of the compound is +2. This difference between the oxidation states of iron results in the change of colour. The intense clue colour of prussian is due to the energy associated with the transfer of electrons from Fe II to Fe III. This intervalence charge transfer happens due to absorption of certain wavelengths of visible light which in this case, is orange-red light of around 680 nm wavelength and as a cause of this, the light reflected seems blue.

Therefore, the correct answer is option B that is inter valence electron transfer.

Note:
Prussian is one of the most essential medications in the basic health system. It has secured a place in the list of essential medicines according to the world health organisation. When prussian blue is orally taken, it is beneficial as an antidote for certain types of heavy metal poisoning.