NITRIC ACID, the most important of the five compounds formed by oxygen with nitrogen; symbol HNO3. When pure it is a colorless liquid, very strong and disagreeable to the smell, and so acrid that it cannot be safely tasted without being much diluted. It is known in the arts as "aqua fortis," and is commonly obtained by distilling niter (potassium nitrate) or Chile saltpeter (sodium ni trate) with strong sulphuric acid. Ni tric acid contains about 76 per cent. of oxygen, a great part of which it readily gives up to other substances, acting thus as a powerful oxidizer. Thus many met als—such as copper, tin, silver, etc.— when brought into contact with this acid are oxidized at the expense of the acid with the production of lower oxides of nitrogen and an oxygenated metallic salt. Nitric acid, when moderately dilute, acts on organic bodies so as to produce a series of most useful substances, notably acetic, oxalic, and picrid acids, isatin or white indigo. Nitric acid is employed in
etching on steel or copper; as a solvent of tin to form with that metal a mordant for some of the finest dyes; in metal lurgy and assaying; also in medicine, in a diluted state, as a tonic and as a sub stitute for mercurial preparations in syphilis and affections of the liver; and also in form of vapor to destroy conta gion.
Nitric acid is one of the mast impor tant essentials in the manufacture of ex plosives, and thus became of great im portance during the World War. Its manufacture through artificial sources was developed in the various countries. The most important of these was the ex traction of nitrate from the air. See NITRATE.