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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?

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

Reference

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

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