The Brown Ring Test is highly sensitive for detecting the Nitrate ($N{O}_3^{-}$) ion.

1. Solution containing nitrate is mixed with fresh Ferrous sulfate ($FeS{O}_4$).
2. Concentrated $H_{2}S{O}_4$ is added carefully along the sides of the test tube.
3. Result: A brown ring forms at the junction of the two liquid layers.

• Chemical Identity: The ring is due to the formation of the complex $[Fe(H_{2}O{)}_{5}NO{]}^{2+}$ (Nitroso-ferrous sulfate). The nitrate is reduced to nitrous oxide ($NO$) by $F{e}^{2+}$, which kemudian coordinates with another $F{e}^{2+}$ ion.