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Question

Explain with the help of a balanced equation the brown ring test for nitric acid.

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Solution

Brown ring test: Take an aqueous solution of any metal nitrate in a test-tube and add an equal quantity of freshly prepared solution of iron(II) sulphate and pour slowly a few drops of concentrated sulphuric acid along the side of the tilted test-tube as shown in figure.

A brown ring will appear at the junction of the two liquids. The composition of the brown ring is FeSO4.NO (nitroso ferrous sulphate).
6FeSO4+3H2SO4+2HNO33Fe2(SO4)3+4H2O+2NO
FeSO4+NOFeSO4.NONitroso ferrous sulphate (a brown compound)
It is formed by reduction of nitric acid to nitrogen monoxide by ferrous sulphate, which then dissolves in the rest of ferrous sulphate solution producing the brown ring. The ring formed is very unstable and disappears on shaking or gentle warming.

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