Nitric Acid

Nitrogen forms a small family of oxoacids — hyponitrous acid ($H_2N_2O_2$), nitrous acid ($HNO_2$), and nitric acid ($HNO_3$) — of which nitric acid is by far the most important.

Preparation

In the laboratory, nitric acid is made by heating potassium or sodium nitrate with concentrated sulphuric acid in a glass retort:

Industrially, it is produced almost entirely by the Ostwald process, which is built around the catalytic oxidation of ammonia by atmospheric oxygen:

The nitric oxide formed here reacts further with oxygen to give nitrogen dioxide:

and the nitrogen dioxide dissolves in water to yield the acid itself:

The $NO$ regenerated in this last step is recycled back into the process. The aqueous acid obtained this way can be concentrated by distillation to about 68% by mass; a further, stronger concentration up to 98% is achieved by dehydrating it with concentrated sulphuric acid.

Properties

Pure nitric acid is a colourless liquid, freezing at 231.4 K and boiling at 355.6 K. The laboratory-grade acid is close to 68% $HNO_3$ by mass, with a density near 1.504 relative to water. Under gas-phase conditions, the $HNO_3$ molecule adopts a planar structure.

In aqueous solution nitric acid behaves as a strong acid, ionising essentially completely to give hydronium and nitrate ions:

Concentrated nitric acid is also a powerful oxidising agent and attacks almost every metal except the noble metals such as gold and platinum. Exactly which reduction product forms depends on the acid's concentration, the temperature, and the nature of the metal being oxidised. Copper, for instance, gives different nitrogen oxides with dilute and concentrated acid:

Zinc shows the same pattern, giving $N_2O$ with the dilute acid and $NO_2$ with the concentrated acid:

A few metals, notably chromium and aluminium, do not dissolve in concentrated nitric acid at all — the acid instead builds up a thin, unreactive passive oxide film on the metal's surface that shields it from further attack.

Concentrated nitric acid can oxidise non-metals and their compounds too: it converts iodine to iodic acid, carbon to carbon dioxide, sulphur to sulphuric acid, and phosphorus to phosphoric acid:

Brown ring test …