Turnbull’s blue is:
A. Ferricyanide
B. Ferrous ferrocyanide
C. Ferrocyanide
D. Ferrous ferricyanide

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Hint: It is precipitated when a ferrous salt is reacted with a ferricyanide. The result is used as a pigment in paints and inks.

Complete answer:
Turnbull’s blue is a result of a reaction between a ferrous ($F{e^{2 + }}$) with ferricyanide. It is often confused with Prussian blue and it was also found that the structures of both the compounds are identical. But the precipitation process of individual compounds created very small differences in the colour and their texture. Ferrous salt reacts with potassium ferricyanide to form a blue precipitate which is called Turnbull's blue. The reaction is as follows:

$3F{e^{2 + }} + 2{K_3}[Fe{(CN)_6}] \to F{e_3}{[Fe{(CN)_6}]_2} \downarrow $

The final product is our blue precipitate.
So, the correct answer is “Option D”.

Note: Prussian blue is formed when ferric salt reacts with ferrocyanide. Here we see both oxidation states of iron but only +2 oxidation states of iron are observed to form coordination complexes with cyanide. And the other +3 oxidation state iron ions form the net coordination compound.