Q48Short answer
In the ring test of NO3 ion, Fe ion reduces nitrate ion to nitric oxide, which 2+ combines with Fe (aq) ion to form brown complex. Write the reactions involved in the formation of brown ring.
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NO3 + 3Fe + 4H → NO + 3Fe + 2H2O 2+ 2+ [Fe(H2O)6] + NO → [Fe(H2O)5(NO)] + H2O (brown complex)