Tests for Nitric Acid and the Nitrates

Various tests for nitric acid and nitrates are mentioned below:

Action of heat

When nitric acid or nitrates of metals other than sodium and potassium are heated, reddish brown fumes of nitrogen dioxide are produced.

Heating with charcoal

When the nitrate salts are heated with charcoal powder, the mixture catches fire.

Treating with copper

a) When copper is added to concentrated nitric acid, reddish brown fumes of nitrogen dioxide are produced.

b) On adding copper turnings to any nitrate salt and then adding some concentrated sulphuric acid followed by warming the mixture gives reddish brown fumes of nitrogen dioxide.

Brown ring test

Take about 2-3 ml of dilute nitric acid, and add equal volume of concentrated sulphuric acid. Heat is evolved. Cool well under the tap. Then keeping the test tube as slanting as possible, pour an equal volume of freshly prepared ferrous sulphate solution down the side of the test tube.

A brown ring is formed at the junction of the liquids as shown in figure 6.19.

Instead of nitric acid, aqueous solution of a nitrate salt can also be used. It will react with the sulphuric acid to yield nitric acid. In presence of sulphuric acid, nitric acid oxidizes ferrous sulphate to ferric sulphate and itself gets reduced to nitric oxide.

This nitric oxide reacts with the excess ferrous sulphate to form nitrosoferrous sulphate (FeSO₄.NO). This is the brown ring. It is a very unstable compound. On gentle warming, it decomposes and the brown colour disappears.