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Question

A brown ring is formed in the ring test for NO3-,i.e. nitrate ion. Why the brown ring is formed?


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Solution

  • When a freshly prepared solution of FeSO4is added to a solution containing NO3-ions, it leads to the formation of a brown-coloured complex. This is known as the brown ring test of nitrate.
    NO3-(aq)+3Fe2+(aq)+4H+(aq)→NO(g)+3Fe3+(aq)+2H2O(l)
    FeH2O62+(aq)+NO(g)→Fe(H2O)5(NO)2+(aq)+H2O(l)Brownring
  • The Fe2+ involved in the reaction reduces nitrates to nitric oxide which then react with hydrated ferrous ion to give a brown coloured complex, i.e., nitroso ferrous sulphate.
  • This brown ring test can be used to detect the presence of nitrate ions.

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