Nitrate is confirmed by ring test. The brown colour of the ring is due to formation of :

Nitrate is confirmed by ring test. The brown colour of the ring is due to the formation of nitroso ferrous sulphate.

The brown ring is produced when ${\mathrm{NO}}_3^{-}$ is passed through  ${\mathrm{FeSO}}_4$ (neutral), followed by a few drops of  concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$