# 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."

Chemically, what is the composition of this dark blue precipitate? Or is there a flaw in the test that I have specified?

If yes, then what is the real test and the chemical formula of the dark blue precipitate?

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.
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 to get a red colouration which is due to the formation of $\ce{[Fe(dipy)]^2+}$. Or, you can use Dimethylglyoxime to get red colouration again which is due to $\ce{[Fe(DMG)2]}$.