When nitrate salt is treated with dil HCl and freshly prepared solution of FeSO4 and some conc. H2SO4 is added by the sides of test tube, a brown ring of ferrous nitrososulphate is formed. It is called ring test of NO3−ion. 6FeSO4+2HNO3+3H2SO4⟶3Fe2(SO4)3 +2‌NO+4H2O [Fe(H2O)6]SO4+NO⟶[Fe(H2O)5‌NO2‌SO4. +H2O ‌ or ‌ [Fe(H2O)5‌NO]2+