why is freshly  prepraed ferrous sulphate used in the brown ring test?????

Hi Yashika,

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.

12FeSO₄ + 3O₂ → 4Fe₂(SO₄)₃ + 2Fe₂O₃

 

Hope this answers your question.

Best Wishes!