NITRIC ACID ( from Neo-Lat. nitrnm, nitre, natron, Lat. nit rain, from Gk. p(-rpop, nit rat, Xtrpop, litron, natron, of Semitic origin, cf. tleb. nctcr, natron, from natur, to loose), 11N11„. A powerfully acid compound of hydrogen, nitrogen, and oxygen. Geber, in his Dc hzrentiono Vcri tatis, described a method of preparing the acid by heating potassium nitrate with alum and cop per sulphate, and Glauber was probably the first to prepare it by the action of free sulphuric acid upon saltpetre (the acid was long known as 'spiritus nitri fIllnans Ulauberi'). In 1669 Mayow described nitric acid as containing two com ponents, one from the air and one from the earth. In 1776 Lavoisier demonstrated that one of its constituents was oxygen, and in 1783 Cavendish showed the composition of the acid by synthesiz ing it from oxygen and nitrogen in the presence of water. However, the true composition of its molecule was not recognized until long after wards, when chemists had realized that acids in general were compounds necessarily containing hydrogen. (Sec CHEMISTRY.) Nitric acid does not occur in a free state in nature, but after thunder storms traces of it are found in rain water, and according to Boussingault. amounts of nitric acid up to 0.66 mg. to the liter have been found in the rain falling on the Alps. It occurs largely, however, combined in the form of alkaline nitrates. in Chile and elsewhere, the formation of the nitrates being supposed to originate in the putrefaction of nitrogenous or ganic matters: the latter are assumed to he con verted into ammonia, and this to be oxidized in presence of the hydroxide of potassium, sodium, or calcium, into the corresponding nitrate. Nitric acid may be made by the action of strong sul phuric acid on the nitrate of sodium or potas sium, the former being generally employed on a commercial scale on account of its cheapness. Cast iron retorts are charged with about 670 pounds of dry sodium nitrate, about 530 pounds of strong sulphuric acid are added, and heat is applied. The volatile nitric aeid, nn forming, passes into a series of large bottles provided with inlet and outlet tubes bottles') and containing small amounts of water. Nitric aeid is, however. not the only product, a certain amount of peroxide of nitrogen being formed at the Caine time. Some of this dissolves in the nitric acid, imparting to it a more or less intense red coloration. Another portion of the peroxide is caused to come into contact with moist air, in a tower attached to the last of the large hot tles. and thus this portion of the peroxide is converted into nitric acid. Sodium sulphate re mains as a by-product of the process in the east iron retorts. The acid product generally contains about 55 per cent. of nitric acid, the rest being water and small amounts of chlorine, iodic acid, oxide of iron, snlphurie acid. sodium sulphate, and peroxide of nitrogen, the non-volatile of these impurities being carried over mechanically during the process of distillation. „Most of the impurities niay be readily gotten rid of by a second distillation, the tirst portion of the distil late and a small last portion containing nearly all the impurities, while the large intermediate portion is practically pure. although it still con
tains a large percentage of water and a small amount of nitrogen peroxide. A third distilla tion, this time after mixing the liquid with an equal volume of oimeentrated sulphuric acid, yields a nitric acid of over 99.5 per cent. strength. To free this from peroxide of nitrogen, it is gently warmed. then removed from the source of heat, and a current of dry air is passed through it until the temperature has been re duced to that of the surroundings. Thus nitric acid is purified for use in chemical laboratories. For many purposes in the arts, however, the acid need be neither very strong nor very pure. Pure acid is a colorless liquid with a specific gravity of 1.53 at ordinary temperatures. The pure aeid, as well as its strong aqueous solutions, decomposes slowly under the influence of light, with formation of water, oxygen, and peroxide of nitrogen. the latter coloring the acid yellow. A similar decomposition, only more rapid, is effected by heat, at temperatures almve SG° C. ( ISi° F.). A given amount of nitric acid may be decomposed entirely by sealing it up in a glass tube and raising the temperature to 200° C. (500° F.). Under reduced pressures nitric acid may be distilled Without decomposition; thus, it may he entirely freed from nitrous acid by sev eral distillations under a pressure of 15 milli meters of mercury, at the temperature of 45° C. ( 113° F.).
Nitric acid is used in large quantities in chem ical laboratories, in both analytical mid syn thetic work. Its uses in the manufactures are very extensive indeed. It is used in the manufac ture of explosives, of coal-tar colors, and of commercial nitrates, including those of silver. lead, iron, aluminum, barium. and strontium. A mixture of nitric and hydrochloric acid. is known as aqua sepia Nitric acid is one of the most powerful oxidizing agents and in a concen trated state readily oxidizes sulphur, phosphorus, carbon, most metals, and many organic sub stances. The oxidation of turpentine, for ex ample, when added to strong nitric :mid. is rapid enough to be accompanied by an evolution of light and of heat of a very high temperature. The following may he mentioned among the physiological fleets of nitric acid: the strong acid is a powerful canst ie. skin IOW and causing erosions and ulcers; even a 10 per cent. solution in water will cause smelling if applied to the skin. In highly dilute form the acid is sometimes administered internally. the effect being an improved appetite and nn of urine. If contimied. however, the administration of the amid mill cause the glI111, to turn sponoy and to bleed. and will loosen the teeth. Whether given in ternally or employed in the form of baths, nitric acid will further cause dyspepsia. foul breath, headaches, debility. etc. It has been suceessfully given in eases of intermittent fever, to allay thirst in diabetes. and for a variety of other pur poses. Externally it is used as an ingredient of gargles, in the treatment of chilblains, for the removal of warts, etc. Aqua regia is sometimes used for the same purposes as nitric acid alone. For a convenient method of detecting aitrie acid, see below under Nitrates.