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Potassium ferrocyanide $(\ce{K4[Fe(CN)6]})$ is used for the qualitative analysis of ferric $(\ce{Fe^3+})$ salts. It precipitates as Prussian blue $(\ce{Fe4[Fe(CN)6]3})$.

How to explain the blue colour of Prussian blue? May the colour be due to the charge transfer between a ligand and the central atom?

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2 Answers 2

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Prussian blue contains both $\ce{Fe(II)}$ and $\ce{Fe(III)}$. Mixed-valence compounds, such as the one in question, absorb certain wavelengths of visible light resulting due to the phenomenon of inter valence charge transfer (IVCT). General Scheme for IVCT is: $$\ce{L_nM^+-bridge-M'L_n + h\nu ->L_nM-bridge-M'+L_n}$$

In this case, orange-red light around $680\ \mathrm{nm}$ in wavelength is absorbed, and the reflected light appears blue as a result.

Additionally, changes colour from blue to colourless upon reduction. This change is caused due reduction of the $\ce{Fe(III)}$ to $\ce{Fe(II)}$, and thereby eliminating the IVCT responsible for Prussian blue's namesake blue colour. This is also why Prussian Blue exhibitis electrochromic behaviour.

For a more detailed description read this and this. Alternatively, you can look it up in a textbook.

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From the wikipedia article of potassium ferrocyanide:

The intense blue color of Prussian blue is associated with the energy of the transfer of electrons from Fe(II) to Fe(III). Many such mixed-valence compounds absorb certain wavelengths of visible light resulting from intervalence charge transfer. In this case, orange-red light around 680 nanometers in wavelength is absorbed, and the reflected light appears blue as a result(...)Prussian Blue is electrochromic—changing from blue to colorless upon reduction. This change is caused by reduction of the Fe(III) to Fe(II) eliminating the intervalence charge transfer that causes Prussian blue's color.

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