# 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.
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 $$\ce{[Fe(DMG)2]}$$.