Please solve question no.b.

Dear Student,

Freshly prepared ferrous sulphate is used in brown ring test, because if FeSO₄ which is stored or it is the old one (prepared earlier) then it reacts with atmospheric oxygen and gets oxidized to form a corrosive brown-yellow coating of basic ferric sulphate, which is an adduct of ferric oxide and ferric sulphate.

12FeSO₄ + 3O₂ → 4 Fe₂(SO₄)₃ + 2Fe₂O₃

Thus the brown ring which is a brown complex, [Fe(H₂O)₅NO]SO₄ will not be obtained, hence to obtain the brown ring in ring test a freshly prepared ferrous sulphate is used.

Regards