HNO3 Test - Brown Ring Test
The brown ring test is the standard way to confirm a nitrate ion is present. Freshly dissolved ferrous sulfate is pale green — the colour of Fe²⁺. In the acid medium the concentrated sulphuric acid supplies, nitric acid oxidises some of that Fe²⁺ to Fe³⁺ and is itself reduced to nitric oxide, NO. The brown is not the Fe³⁺ (which is pale yellow): it is the nitroso complex [Fe(H₂O)₅(NO)]²⁺, formed when that NO combines with the Fe²⁺ still in excess. The brown colour is the confirmation. In a real laboratory, the concentrated acid is poured carefully down the side of a tilted tube so it settles beneath the other two as a separate, denser layer, and the reaction happens only where the layers meet — which is why the result is a brown ring rather than a brown tube. This bench mixes reagents straight in rather than layering them, so the same reaction is shown as one colour change through the whole flask instead of a ring at a junction — the chemistry is identical either way.
4 steps · Nitric Acid · Study of Compounds
What you will use
Apparatus
- Flask
Chemicals
- Dilute Nitric Acid
- Ferrous Sulfate
- Concentrated Sulfuric Acid
The steps
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Pour dilute nitric acid into the flask.
Colourless. Nothing about it yet shows a nitrate is there.
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Add freshly prepared ferrous sulfate.
A pale green solution forms as the crystals dissolve — the colour of the Fe²⁺ ion.
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Add concentrated sulphuric acid.
The flask warms; concentrated sulphuric acid releases heat the moment it meets a solution like this.
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Watch the colour of the solution.
A brown colour appears — the nitroso complex [Fe(H₂O)₅(NO)]²⁺, formed as nitric oxide meets the excess Fe²⁺. It confirms a nitrate is present.
What it shows
In the acid medium concentrated sulphuric acid supplies, nitric acid oxidises part of the Fe²⁺ to Fe³⁺ and is reduced to nitric oxide: 6FeSO₄ + 3H₂SO₄ + 2HNO₃ → 3Fe₂(SO₄)₃ + 2NO + 4H₂O. The nitric oxide then binds to the Fe²⁺ left in excess, giving the brown nitroso complex: FeSO₄ + NO + 5H₂O → [Fe(H₂O)₅(NO)]SO₄. That brown — not the Fe³⁺, which is pale yellow — is the confirmatory sign of a nitrate. Careful layering in a real lab keeps that colour change confined to a ring at the junction of the two liquids; this bench shows the identical chemistry as one colour change through the whole flask instead.