The brown ring test

By zungu January 12, 2019 chemistry

All nitrates undergo the same reaction with iron (II) sulphate and concentrated sulphuric acid and this reaction becomes a test for the soluble nitrates, the brown ring test. This test is carried out by crushing a little potassium nitrate, putting it in a test tube, adding water to a depth of about 2 cm and then shaking the test tube to dissolve the potassium nitrate (note that any metal nitrate could have been used instead of potassium nitrate).

This is followed by adding a little sulphuric acid and then two or three crystals of iron (II) sulphate, which have also been crushed. The contents are shaken to dissolve them. Finally, the test tube is held in a slanting position and a slow continuous stream of concentrated sulphuric acid is poured down the side of the test tube. The acid forms a separate layer underneath the aqueous layer and, at the junction of the two, a brown ring will be seen. This brown ring is the characteristic test for all soluble nitrates.

Explanation
The concentrated sulphuric acid and the nitrate react to produce nitric acid:
KNO_3(s) + H₂SO_4(aq) → KHSO_4(aq) + HNO_3(aq)

The nitric acid formed is reduced by some of the iron (II) sulphate to nitrogen monoxide, NO:
6FeSO_4(s)+2HNO_3(aq)+3H₂SO_4(aq)→ 3Fe₂(SO₄)_3(aq)+4H₂O_(l)+2NO_(g)

The nitrogen monoxide produced then reacts with some of the remaining iron (II) sulphate to form a dark brown complex, FeSO₄.NO, which appears as a ring.
FeSO_4(aq) + NO_(g) → FeSO₄.NO_(aq)

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