The brown ring test is performed byputting iron(II) sulfate to a solution that contains nitrate ions. After that, slowly and carefully add concentrated sulphuric acid without disturbing the solution so that a sulphuric acid layer is formed under the aqueous solution. If nitrate is present, a brown ring consisting of [Fe(H2O)5NO]SO4 will form at the junction of the two layers.

Concentrated sulphuric acid is added to a mixture of Iron(ii)sulphate and a suspected nitrate solution, the acid sinks to the bottom. This is because sulphuric acid is denser than the solution. This reaction is exothermic, a brown ring is formed at the junction of the two layers. The brown ring is due to the formation of iron(ii) suplhate - nitrogen (ii) Complex, FeSO4.NO or Nitroso iron(ii) complex. There is no place for NO2, so please correct.