# Why doesn't N2O give a positive brown ring test?

A common nitrate test, known as the brown ring test can be performed by adding iron(II) sulfate to a solution of a nitrate, then slowly adding concentrated sulfuric acid such that the acid forms a layer below the aqueous solution. A brown ring will form at the junction of the two layers, indicating the presence of the nitrate ion.

I know for sure that certain oxides of nitrogen, such as $$\ce{NO}$$ and $$\ce{NO2}$$ test positively in the brown ring test for nitrates, but I had never found a reliable source stating the same for the other oxides of nitrogen. But one of the questions in a class test I took was:

Which one of the following does not give the Brown Ring test?

1. $$\ce{N2O}$$
2. $$\ce{NO}$$
3. $$\ce{N2O3}$$
4. $$\ce{N2O5}$$

The answer was given as $$\ce{N2O}$$ and I am unable to identify the logic behind the answer. Could it be because the nitrogen compound is reduced in the process of forming the brown complex with iron, while $$\ce{N2O}$$ already has too small an oxidation state to be reduced to $$\ce{NO+}$$?

• You may put it this way, but that's a post factum explanation. Just remember that N2O is relatively inert and doesn't react like that. – Ivan Neretin May 7 at 14:26

\begin{align} \ce{3 [Fe(H2O)6]^2+ + NO3- + 4 H+ &->3 [Fe(H2O)6]^3+ + NO + 2 H2O} \\ \ce{[Fe(H2O)6]^2+ + NO &->3 [Fe(H2O)(NO)]^2+} + H2O\\ \end{align}
\begin{align} \ce{ [Fe(H2O)6]^2+ + NO2- + 2 H+ &-> [Fe(H2O)6]^3+ + NO + H2O} \\ \ce{N2O3 + H2O &-> 2 HNO2}\\ \ce{2 NO2 + H2O &-> HNO2 + HNO3}\\ \ce{N2O5 + H2O &-> 2 HNO3}\\ \end{align}
Therefore, $$\ce{NO}$$ and all higher nitrogen oxides oxidizing $$\ce{Fe^2+}$$, forming $$\ce{NO}$$, react positively in the brown ring test.
$$\ce{N2O}$$ cannot do that, because it cannot form $$\ce{NO}$$ at given conditions.