Formula of Brown ring is

2HNO3+ 3H2SO4 + 6FeSO4 â†’ 3Fe2(SO4)3 + 2NO + 4H2O(Remaining) [Fe(H2O)6]SO4 + NO â†’ [Fe(H2O)5(NO)]SO4 (Brown ring)+ H2Oa nitrate salt is taken in a test tube . Freshly prepared ferrous sulphate is added and then conc. H2SO4 is added carefully along the sides of the test tube. A brown ring at this interface  between the sol. and H2SO4 layer indicates the presence of NO3- ion.