Study of Nitric Acid Study of Compounds: Nitric Acid (HNO3)

Nitric acid, HNO3, is a strong monobasic acid (one replaceable hydrogen). The pure acid is colourless, but it slowly decomposes in light and warmth to give nitrogen dioxide, which dissolves and turns it yellow or brown. Dilute nitric acid behaves like a typical acid, but concentrated nitric acid, especially when hot, is one of the most important oxidising agents in the school laboratory.

Nitric acid is prepared by warming potassium nitrate (or sodium nitrate) with concentrated sulphuric acid: KNO3 + H2SO4 gives KHSO4 + HNO3. Concentrated sulphuric acid is chosen because it is less volatile and so displaces the more volatile nitric acid. An all-glass retort is used because the hot acid vapours attack rubber and cork. The temperature is kept below 200 degrees Celsius: above this, the acid decomposes and a hard crust of sulphate sticks to the glass.

As an acid, dilute HNO3 turns blue litmus red, neutralises bases to give nitrates and water, and reacts with carbonates to release carbon dioxide. As an oxidiser, it reacts with metals giving water and an oxide of nitrogen instead of hydrogen: dilute acid tends to give nitric oxide (NO, colourless, turning brown in air), and concentrated acid gives nitrogen dioxide (NO2, reddish-brown). Only very dilute HNO3 with magnesium or manganese gives hydrogen.

To confirm a nitrate, the brown ring test is used. The nitrate solution is mixed with freshly prepared ferrous sulphate solution, then concentrated sulphuric acid is added slowly down the side of the tilted tube so it forms a layer below. A brown ring of the complex [Fe(H2O)5NO]SO4 appears at the junction of the two liquids. Freshly prepared ferrous sulphate is essential, as old solution oxidises to ferric sulphate and no ring forms.