(a) Explain with the help of a balanced equation the brown ring test for nitric acid.
(b) Why is freshly prepared ferrous sulphate solution used for testing the nitrate radical in the brown ring test? [5 MARKS]

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Solution

a) 3 Marks
b) 2 Marks

(a) 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 $F e S O_{4} . N O$ (nitroso ferrous sulphate).
$6 F e S O_{4} + 3 H_{2} S O_{4} + 2 H N O_{3} \to 3 F e_{2} (S O_{4})_{3} + 4 H_{2} O + 2 N O$
$F e S O_{4} + N O \to F e S O_{4} . N O N i t r o s o f e r r o u s s u l p h a t e (a b r o w n c o m p o u n d)$
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.

(b) A freshly prepared ferrous sulphate is used in the test because ferrous sulphate on exposure to the atmosphere is oxidised to ferric sulphate and the test will not be positive with ferric sulphate.

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

(b) Why is freshly pepared ferrous sulphate solution used for testing the nitrate radical in the brown ring test ?

In brown ring test, a freshly prepared ferrous sulphate solution is used because:

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