I was studying qualitative analysis from this book and I found some tests for testing the presence of the cation $\ce{Fe^{++}}$.

There was a test in which we add $\ce{K4[Fe(CN)6]}$ solution to a solution of a ferrous salt which results in the Turnbull blue precipitate. Now according to Wikipedia, the Turnbull blue and Prussian blue are the same compound, ie, ferriferrocyanide $\left(\ce{Fe4[Fe(CN)6]3} \right)$.

But the book says that Turnbull blue is ferroferricyanide $\left(\ce{Fe3[Fe(CN)6]2}\right)$ which kinda makes sense because we are adding ferricyanide to ferrous solution.

I am preparing for an exam in which such questions can be asked SO I just want to settle this once and for all, which is wrong, Wikipedia or the book?

  • $\begingroup$ see also: Hansen, L. D.; Litchman, W. M.; Daub, G. H. Turnbull’s blue and Prussian blue: KFe(III)[Fe(II)(CN)6]. J. Chem. Educ., 1969, 46 (1), 46; dx.doi.org/10.1021/ed046p46 $\endgroup$
    – user7951
    Jan 16, 2016 at 15:23
  • $\begingroup$ @Loong the link is good but what about user104014's answer? Can we say that the equilibrium hugely favours the formation of prussian blue? $\endgroup$
    – najayaz
    Jan 17, 2016 at 6:33
  • $\begingroup$ Update: I have asked "What is the chemical composition of Ferrous Ferrocyanide?" $\endgroup$ Sep 11, 2021 at 5:55

1 Answer 1


Though your textbook is right, Wikipedia isn't completely wrong as in aqueous solution, $\left(\ce{Fe3[Fe(CN)6]2} \right)$ gets converted and is in equilibrium with $\left(\ce{Fe4[Fe(CN)6]3} \right)$ (a redox change) and hence prussian blue and turnbull's blue are identical to look visually. But in a textual way, the naming is given as your textbook says, i.e. $\left(\ce{Fe4[Fe(CN)6]3} \right)$ is Prussian blue whereas $\left(\ce{Fe3[Fe(CN)6]2} \right)$ is Turnbull's blue.


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