Question

Prussian blue colour is obtained by mixing together an aqueous solution of $\text{F}{{\text{e}}^{\text{3+}}}$ salt with:
(A) Ferricyanide
(B) Ferrocyanide
(C) Hydrogen cyanide
(D) Sodium cyanide

Answer 1

Prussian blue is dark blue pigmented complex compound of iron which is formed by the oxidation process. In Prussian blue there are two different iron atoms present in different oxidation states.

Complete answer:
Potassium ferricyanide reacts with ferrous ions in the acidic medium and forms an insoluble blue pigmented complex compound. In this reaction firstly iron salt in +2 oxidation state (ferrous salt) reacts with potassium ferrocyanide and forms a water insoluble white colour compound known as Berlin white which on oxidises on the later step into a blue coloured complex. This blue colour of complex is due to the absorption of some appropriate wavelength of light for electron transfer.
$\begin{aligned} & \text{3}{{\text{K}}_{\text{4}}}\text{ }\\!\\![\\!\\!\text{ Fe(CN}{{\text{)}}_{\text{6}}}\text{ }\\!\\!]\\!\\!\text{ }\,\,\text{+}\,\text{4FeC}{{\text{l}}_{\text{3}}}\,\,\,\,\to \,\,\,\text{F}{{\text{e}}_{\text{4}}}{{\text{ }\\!\\![\\!\\!\text{ Fe(CN}{{\text{)}}_{\text{6}}}\text{ }\\!\\!]\\!\\!\text{ }}_{\text{3}}}\,\text{+12KCl} \\\ & \text{potasium}\,\text{ferrocynide}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\text{prussian}\,\text{blue} \\\ \end{aligned}$

So, option (B) is the correct option.

Additional information:
The intense colour of Prussian blue salt complex from the transfer of their valence electron (inter valence electron transfer) in between ferrous ion and ferric ion present in and outside the coordination sphere respectively.
The IUPAC name of Prussian is iron (III) hexacyanoferrate (II).

Note:
Coordination compounds are coloured and the reason for the colour in complexes is due to d-d transition of the electron from ground state to excited state.
The iron atom, Fe (III) which is present outside the coordination sphere is paramagnetic in nature while the ferrous ion which is present inside the coordination sphere is diamagnetic in nature due to the absence of unpaired electrons.