In the brown ring test for the nitrate ion, the brown colour is due to

Options

(a) [Fe(H₂O)₅NO]²⁺
(b) [Fe(H₂O)₅NO]³⁺
(c) [Fe(H₂O)₆]⁺
(d) [Fe(H₂O)₅NO₂]²⁺

Correct Answer:

[Fe(H₂O)₅NO]²⁺

Explanation:

When an aqueous solution of the nitrate salt is treated with freashly prepared solution of ferrous sulphate and concentrated H₂SO₄, a brown ring is formed on account of the formation of complex [Fe(H₂O)₅NO]SO₄ at the junction of two liquids. NaNO₃ + H₂SO₄ → NaHSO₄ + HNO₃. 6FeSO₄ + 2HNO₃ + 3H₂SO₄ → 3Fe₂(SO₄)₃ + 4H₂O + 2NO. [Fe(H₂O)₆]SO₄.H₂O + NO → [Fe(H₂O)₅NO]SO₄ + 2H₂O(brown ring).