Explain the brown ring test for nitrate ions.

Explanation

The brown ring test for nitrates depends on the ability of `Fe^(2+)` to reduce nitrates to nitric oxide (NO) that 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 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)^(-) + 3Fe^(2+) + 4H^(+) to NO + Fe^(3+) + 2H_(2)O`
`[Fe(H_(2)O)_(6)]^(2+) + NO to underset("Brown complex")([Fe(H_(2)O)_(5).(NO)]^(2+) + H_(2)O)`