Question

Give reasons for the following. Give the chemical equations wherever required.

A brown ring is observed when a freshly prepared ferrous sulphate solution is added to a nitrate ion solution followed by conc. H₂SO₄.

Answer 1

Nitrate ions are reduced by Fe²⁺ in the presence of concentrated sulphuric acid to nitric oxide (NO), which then coordinates with unreacted Fe²⁺ ions to form a brown-coloured nitroso-iron(II) complex at the liquid junction.

\[\ce{NO^-_3 + 3Fe^{2+} + 4H+ -> NO + 3Fe^{3+} + 2H2O}\]

\[\ce{[Fe(H2O)6]^{2+} + NO -> [Fe(H2O)5(NO)]^{2+} (Brown ring) + H2O}\]