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Q.
In the brown ring test for the nitrate ion, the brown colour is due to
Solution:
When an aqueous solution of the nitrate salt is treated with freshly prepared solution of ferrous sulphate and conc. $H _{2} SO _{4}$, a brown ring is formed on account of the formation of complex $\left[ Fe \left( H _{2} O \right)_{5} NO \right] SO _{4}$ at the junction of two liquids.
$NaNO _{3}+ H _{2} SO _{4} \rightarrow NaHSO _{4}+ HNO _{3}$
$6 FeSO _{4}+2 HNO _{3}+3 H _{2} SO _{4} \rightarrow 3 Fe _{2}\left( SO _{4}\right)_{3}+4 H _{2} O +2 NO$
$\left[ Fe \left( H _{2} O \right)_{6}\right] SO _{4} \cdot H _{2} O + NO \rightarrow[ Fe \underset{\text{Brown ring}}{\left( H _{2} O \right)_{5} NO }] SO _{4}+2 H _{2} O$