Page:The New International Encyclopædia 1st ed. v. 14.djvu/680

* NITRIC ACID. 580 NITEIC ACID. these impurities being carried over mechanically during the process of ilistiUation. ilost of the impurities may 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 internuulialc 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 vohmie of concentrated sulphuric acid, yields a nitric acid of over i)9.5 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 puritied for use in chemical laboratories. For nuiny purposes in the arts, however, the acid need be neither very strong nor ver_v |)urc. Pure nitric acid is a colorless liquid with a specilie gravity of 1.53 at ordinary temperatures. The pure acid, as well as its strong aqueous solutions, decomposes slowly under the inlluence of light, with formation of water, oxygen, and peroxide of nitrogen, the latter coloring the acid yellow. A similar decomposition, only more rapid, is cfl'eeted by heat, at temperatures above 86° C. (187° 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 di.stilled without decomposition; thus, it ma}' be entirely frec<l from nitrous acid by sev- eral distillations under a pressure of 15 milli- meters of nU'rcurA", at the temperature of 45° C. (11,S° F.). Nitric acid is used in large quantities in chem- ical laboratories, in botli analytical and s^ti- 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 acids is known as aqua regia (q.v. ). 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 acid, is rapid enough to be accompanied by an evolution of light and of heat of a very high temperature. The following may be mentioned among the pht//iiolo(iiciil efforts of nitric acid; the strong acid is a jiowerful caustic, staining the skin yel- low and causing erosions and ulcers: even a 10 per cent, solution in water will cause swelling if applied to the skin. In highly dihite form the acid is sometimes administered internally, the effect being an improved appetite and an increased secretion of urine. If continued, however, the administration of the acid will cause the gums to turn spong>' and to blceil, and will loosen the teeth. Whether given in- ternally or employed in the form of baths, nitric neid will furllier cause dyspepsia, foul breath. headaches, debility, etc. It has been successfully given in cases 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 nitric acid, see below luider yitralcs. NnnATE.'S. The salts of nitric acid, i.e. the compounds resulting from the substitution of metals for the hydrogen of nitric acid, are termed nitrates. The most important of these is nitnite of sillier, AgNO;,, which is extensively used in photography and in medicine. It is made by dis- solving metallic silver in gently heated 30 per cent, nitric acid, the pure salt crystallizing out on cooling. It may be piu-ilicd by fusing, when the nitrates of other metals that are likely to be l)rescnt are reduced and rendered insoluble, so that pure silver nitrate may be <lissolved out of the fused mass on cooling. Silver nitrate is very soluble in water and dissolves in four times its weight of cold alcohol. It forms colorless tabular erstals of the rhombic svstem. It melts at 198° C. (388.4° F.) without decomiwsition ; but if heated to redness it is reduced to metallic silver. Fused silver nitrate, usually in sticks rendered firm by the addition of a little ])otas- sium nitrate, or silver chloride, is used as a cauterizing agent, being commonly known as liDKir cinixlic. When a|)plied to the skin, silver nitrate combines with albumin to form an in- soluble white albuminate of silver. When used, the sticks are moistcncil with water and rulibed over the surface of the skin. The nitrate is thus used in (liphtheria, croup, chronic ulceration and simple inllamniation of the larynx and trachea, simple chronic laryngitis, whooping cough, vari- ous fiuuis of neuralgia, in various diseases of the eye, in erysipelas, for burns, etc. The stain left by it on the skin may be removed by the use of potassium cyanide. Internally nitrate of silver is sometimes given in certain diseases of the stomach and the heart and in briuichitis. and it has been found useful in certain diseases of the nervous system; for example, in paralysis and ep- ile])sy. In cases of jioisoning by silver nitrate, the jiaticnt should swallow a large anio it of a solu- tion of common salt, which transforms the nitrate into the insoluble and harmless silver chloride, and induces vomiting: the solution of salt should be followed by moderate amounts of milk. Nitrates of Icail, iron, and aluminiun are used in dyeing and e:lico printing, f.rail ni- Iriifc. PbtNO^).. may be obt.iined by dissolving metallic lead, or oxide or carbonate of lead, in dilute nitric acid, the salt crystallizing out on slow evaporation. It is sohible in water, but is insoluble in strong alcohol. It is sometimes used in medicine, esi)eeially as an application for sore nipples. Xilnitc of iron, or ferric nitrate. Fe.(NO,)„. is likewise sometimes used in medi- cine, its solution being prep:ired by the action of nitric acid on freshly (ibt;iincil moist ferric hy- droxiilc. liariiim nitrate. Ba(XO,),. and xtron- liiim nitrate, Sr(NO,)j, are used in making fire- works. The nitrates of potassium and sodium will be found described under S.m.tpktke. Nitric acid and its salts may be most readily detected in solutions by the use of the alkaloid brucine: a little brucine in .solution and a few ilrops of strong sulphuric acid are added to the liquiil under examination, which turns pink if it eon- t;uns nitric acid or a nitrate. The nitrates also include a number of organic sibslances. the most important of which are nitroglycerin and •jun- cotton (qq.v.). See also Nitkooen.