Chemical test for ferrous ions

I have a reference sheet for inorganic analysis which says that the confirmatory test for ferrous ions is as follows:



> 
> Make a solution of the given salt (whose cation is to identified) and add potassium ferrocyanide [ potassium hexacyanidoferrate(II) ] solution to it.
> If a dark blue precipitate is formed, then the presence of ferrous ions in the given salt is confirmed.
> 
> 
> 


What is the chemical composition of this dark blue precipitate? Or is there a flaw in the test that I have specified? If it is flawed, then what is the appropriate test?




The test is better suited for $\ce{Fe^3+}$.
$$\ce{Fe^3+ + [Fe(CN)6]^4- -> Fe4[Fe(CN)6]3}$$


The compound $\ce{Fe4[Fe(CN)6]3}$ has a specific name. Its called [Prussian Blue](https://en.wikipedia.org/wiki/Prussian_blue).


If you perform the test for $\ce{Fe^2+}$, the precipitate won't be that dark. First *partial* conversion of $\ce{Fe^2+}$ to $\ce{Fe^3+}$ takes place. Then the above reaction takes place to give the prussian blue precipitate.


There are specific tests for $\ce{Fe^2+}$ as well. You can use [Dipyridyl](https://en.wikipedia.org/wiki/2,2%27-Bipyridine) to get a red colouration which is due to the formation of $\ce{[Fe(dipy)]^2+}$.
Or, you can use [Dimethylglyoxime](https://en.wikipedia.org/wiki/Dimethylglyoxime) to get red colouration again which is due to $\ce{[Fe(DMG)2]}$.


*Reference*


JD Lee, Inorganic Chemistry (IITJEE Edition), page 278.


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