A brown ring is formed in the ring test for ${\mathrm{NO}}_{3}^{-}$ ion. It is due to the formation of-

Hint: The IUPAC name of the compound formed in brown ring is PentaaquanitrosylIron(I)Sulphate.

The brown ring complex compound is given as
[Fe(H2O)5NO]SO4
The complex ion is [Fe(H2O)5NO]2+
Oxidation numbers of H2O is zero and NO is +1
Oxidation state of Fe is +1.

The IUPAC name of the compound is PentaaquanitrosylIron(I ) Sulphate.