Home >> Encyclopedic Dictionary Of Photography >> Formula to Or Film Negative Process >> Nitric Acid

Nitric Acid

silver, nitrate and hydrochloric

NITRIC ACID (Formula, HNO,; molecular weight, 63 ; synonym, aqua fortis). A heavy, colorless liquid prepared by the action of strong sulphuric acid upon potassium or sodium nitrate. The strongest obtainable possesses a specific gravity of about 1.5, but a large proportion of what is now sold as nitric acid contains often as much as from 4o to 5o per cent of water.

It is very easily decomposed, giving off fumes when exposed to the air. Mixed with about four times its volume of hydrochloric acid it forms aqua regia, which possesses the property of dissolving gold and platinum.

Nitric acid acts upon the majority of the inferior metals, and their hydrates or carbonates, producing nitrates. It also acts upon silver and upon various forms of cellulose, such as cotton wool, paper, &c., its action being the same as with the metals. Cotton-wool acted upon by nitric acid forms gun cotton or pyroxyline, which is in reality a nitrate of cellulose, or a mixture of cellulose nitrates.

Nitric acid has a very sharp, penetrating, and obnoxious odor somewhat resembling hydro chloric acid. It is very poisonous, and has a most powerful corrosive action. Commercial

samples sometimes contain traces of hydrochloric acid. For many purposes, for instance in the preparation of silver nitrate, it must be entirely free from this acid. The presence of hydrochloric acid can easily be detected by dropping into a test tube containing some of it a few drops of a silver nitrate solution. If any be present it will combine with the silver to form chloride of silver, and the whole will become turbid. Its uses in photography are chiefly in the manufacture of silver nitrate and pyroxyline; for the latter purpose it must have a specific gravity of at least Nitric acid stains can be removed from the hands or clothes by touching with a solution of potassium permanganate, wash well in dilute hydrochloric acid and then in water.

NITRITES.—The salts of nitrous acid. If solid nitrites be warmed with dilute hydrochloric acid, brownish red fumes will be yielded. Liquids containing them give a white precipitate with silver nitrate, and a brown or black coloration with ferrous sulphate.