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I read from a source that the aqueous solution of ferrous sulfate ($\ce{FeSO4}$) gives an unstable brown compound on treating with $\ce{NaNO2}$ and dilute $\ce{HCl}$. Well, it is quite understandable that it is the $\ce{HNO2}$ formed in situ that further causes the reaction. But what is the reaction and what is that unstable compound formed?

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Iron(II) reduces nitrite to nitrogen monoxide which forms a brown pentaaquanitrosyliron(II) complex with the iron(II) ions.

$$\ce{NO2^{-} + Fe^{2+} + 2H+ -> NO + Fe^{3+} + H2O}$$

$$\ce{[Fe(H2O)6]^{2+} + NO -> [Fe(H2O)5NO]^{2+} + H2O}$$

Iron(II) can also reduce nitrate to nitrogen monoxide. This is used in the brown ring test to determine the presence of nitrate in aqueous solutions.

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