In the brown-ring test for nitrates, the solution is mixed with freshly prepared FeSO₄ solution and conc. H₂SO₄ is poured carefully down the side of the test tube; a brown ring of complex (p) appears

Question

In the brown-ring test for nitrates, the solution is mixed with freshly prepared FeSO₄ solution and conc. H₂SO₄ is poured carefully down the side of the test tube; a brown ring of complex (p) appears at the junction of the two layers, the gas (r) being generated in situ by reduction of the nitrate. The oxidation state of iron and the number of unpaired electrons in the brown-ring complex (p) are respectively :

TestHub · Accepted Answer

(p) = [Fe(H₂O)₅(NO)]SO₄. The ligand is NO⁺ (nitrosonium), so iron is in the +1 oxidation state (3d⁷); the measured magnetic moment (~3.9 BM) corresponds to 3 unpaired electrons, matching Fe(+1) high-spin d⁷ with NO⁺ coordination.

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