Nitric acid, the highest known oxide of nitrogen, may be detected in the atmosphere after thunder-storms, for electricity determines the necessary combination of the gases. United with potash, soda, lime, or ammonia, it forms a nitrate which is found native in efflorescence on the soil of some countries; it occurs, also, in some minerals and in certain plants, e.g., as potash nitrate in pareira root.


Being a volatile acid, it may be prepared from any nitrate (usually a nitrate of potash or soda), by distilling it with the more stable sulphuric acid, when sulphate of potash is formed, and nitric acid being set free, rises with the vapor of water and condenses in the receiver. KNO3 + H1SO4 = HNO3 + KHSO4. Of anhydrous acid, it contains 60 per cent.

The dilute nitric acid contains 6 oz. of the strong acid in 31 oz. of distilled water, or about 1 min. in every 5 min. Heat is developed during its preparation, and condensation of volume occurs.

Characters And Tests

The pure acid, protected from light, remains colorless, but if exposed becomes yellowish in color, from development of orange-colored oxides, mainly N2O4 (nitric peroxide); at a sp. gr. of 1.42 it is a stable compound, boils at 250°, and distils over unchanged; it has a very sour, corrosive taste, and an acrid, suffocating odor; its affinity for water is great, and the white fumes which it emits on exposure are caused by the combination of its invisible vapor with atmospheric moisture forming a cloud of minute drops.

A good test for nitric acid is its action on metallic copper or iron; when undiluted and poured on them, it gives dense red vapors of peroxide of nitrogen and other oxides, but if first diluted, a colorless gas, nitric oxide, NO, is given off, which changes into peroxide, N2O4, and becomes orange-red in color on contact with the air. If the colorless gas, NO, be passed into a solution of protosulphate of iron, it will combine with a portion of it, causing a dark brown color. Morphia and brucia are colored bright red by the acid.

There is no precipitation test for nitric acid, because all neutral nitrates are soluble, but its adulteration with sulphuric or hydrochloric acid is detected by chloride of barium and nitrate of silver respectively.

Nitric acid is a powerful oxidizing agent, and is used in pharmacy to prepare the nitrates of different metals; also for the making of certain organic compounds, as gun-cotton, nitrite of amyl, etc.

Absorption And Elimination

Dilute nitric acid in medicinal doses is diffusible and readily absorbed. In the blood it either combines with alkaline bases forming nitrates, or it circulates in a free state, or loosely joined (invisque) with albumen (Gubler): it cannot be detected free in the blood by analysis. It is eliminated mainly by the urine as nitrate of potash or soda, not as free acid; yet it highly increases the acidity of the secretion by liberating acids weaker than itself (such as uric and lactic acids) from their combinations. From its effects upon the intestinal glandular structure, and from the comparatively small amount passed in the urine, it is probable that some is excreted by the lower bowel.

Physiological Action (External)

Strong nitric acid applied but for a moment, stains organic tissue yellow, and leads to desquamation of the epidermis; if applied firmly and for longer time, it exerts a potent caustic effect, due to abstraction of the water from the tissues and combination of the acid with alkaline bases; xantho-proteic or -picric acid is also formed, and the part becomes yellow.

Dilute solutions exert a stimulant, moderately astringent effect; by continued contact they change most animal and vegetable substances into oxalic, malic, or carbonic acids (H. C. Wood: "Elements," 2d ed., p. 96). Nitric peroxide is an efficient but irritating disinfectant.

Physiological Action (Internal)

For a general statement as to the action of acids on the organism, reference may be made to hydrochloric acid.