Question

Explain brown ring test with the help of chemical equation.

Answer 1

Brown ring test can be performed by adding Iron (II) sulphate to a solution of a nitrate then slowly adding concentrated $H_{2}S{O}_4$ such that it forms a layer below the aqueous solution. A brown ring will form at the junction of the two layers indicating the presence of nitrate ion.
The overall reaction is the reduction of the nitrate ion by iron (II) which is reduced to iron (I) and formation of nitrosodium complex where nitric oxide is oxidised to $N{O}^{+}$,
$N{O}_2^{-}+3F{e}^{2+}+4H^{+}\to F{e}^{3+}+NO+2H_{2}O$
$\left[Fe\right(H_2{O}_6){]}^{2+}+NO\to \underset{Brownring}{\left[Fe\right(H_{2}O{)}_{5}NO{]}^{2+}}+H_{2}O$