A brown ring is formed in the ring test for NO3⊖ ion. It is due to the formation of

The brown ring test is performed for the qualitative detection of nitrates.The brown ring test is performed by putting iron(II) sulphate 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(H2​O)5​NO]SO4​   will form at the junction of the two layers.