CBSE Class 12-science Answered

Freshly prepared ferrous sulphate is used in brown ring test because FeSO₄ upon exposure to air, oxidizes to form a corrosive brown-yellow coating of basic ferric sulphate, which is an adduct of ferric oxide and ferric sulphate and thus the brown ring will not be obtained. Oxygen in the air oxidizes iron (II) to iron (III) quite rapidly, in solution. Using a freshly prepared solution makes sure that the solutions contains iron (II) ions needed for the test and not iron (III) ions

12FeSO₄ + 3O₂ ? 4Fe₂(SO₄)₃ + 2Fe₂O₃