In the ring test of NO3– ion, Fe2+ ion reduces nitrate ion to nitric oxide, which
combines with Fe2+ (aq) ion to form brown complex. Write the reactions involved in the formation of brown ring.
+ 3Fe2+ + 4H+ NO + 3Fe2+ 2H2O
[Fe(H2O)6]SO4 + NO [Fe(H2O)5NO]SO4
Thus the complex formed is brown in colour.