Question

The brown ring test is performed for the qualitative detection of :

• A. bromides
• B. iodides
• C. nitrates
• D. nitrite

Answer 1

The correct option is C nitrates

A common nitrate test, known as the brown ring test can be performed by adding $FeS{O}_4$ to a solution of a nitrate, then slowly adding concentrated $H_{2}S{O}_4$ 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. Note that the presence of nitrite ions will interfere with this test.

The overall reaction is the reduction of the nitrate ion by iron(II) which is oxidised to iron(III) and formation of a nitrosonium complex where nitric oxide is oxidised to $N{O}^{+}$.

$N{O}_3^{-}+3F{e}^{2+}+4H^{+}\to 3F{e}^{3+}+NO+2H_{2}O$

$\left[Fe\right(H_{2}O{)}_6{]}^{2+}+NO\to \left[Fe\right(H_{2}O{)}_5\left(NO\right){]}^{2+}+H_{2}O$