Explain the brown ring test for nitrate ions.

The brown ring test for nitrates depends on the ability of $Fe^{2+}$ to reduce nitrates to nitric oxide $(NO)$,which then reacts with $Fe^{2+}$ to form a brown ring complex.
To a sample containing $NO_{3}^{-}$ ions,a dilute ferrous sulphate solution is added,and then concentrated sulphuric acid is added carefully along the sides of the test tube. $A$ brown ring at the interface between the solution and sulphuric acid layers indicates the presence of nitrate ions in the solution.
$NO_{3}^{-} + 3 Fe^{2+} + 4 H^{+} \rightarrow NO + 3 Fe^{3+} + 2 H_{2}O$
$[Fe(H_{2}O)_{6}]^{2+} + NO \rightarrow [Fe(H_{2}O)_{5}(NO)]^{2+} + H_{2}O$ (Brown complex)