Question

A brown ring is formed in the ring test for ${\mathrm{NO}}_3^{-}$,i.e. nitrate ion. Why the brown ring is formed?

Answer 1

• When a freshly prepared solution of ${\mathrm{FeSO}}_4$is added to a solution containing ${\mathrm{NO}}_3^{-}$ions, it leads to the formation of a brown-coloured complex. This is known as the brown ring test of nitrate.
  ${{\mathrm{NO}}_3}^{-}\left(aq\right)+3{\mathrm{Fe}}^{2+}\left(\mathrm{aq}\right)+4{\mathrm{H}}^{+}\left(\mathrm{aq}\right)\to \mathrm{NO}\left(\mathrm{g}\right)+3{\mathrm{Fe}}^{3+}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$
  $$\begin{gathered} {\left[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}\left(aq\right)+\mathrm{NO}\left(\mathrm{g}\right)\to {\left[\mathrm{Fe}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\left(\mathrm{NO}\right)\right]}^{2+}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ \left(\mathrm{Brown}\mathrm{ring}\right) \end{gathered}$$
• The ${\mathrm{Fe}}^{2+}$ 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.