NITRIC ACID is the most important of the five compounds which oxygen forms with nitrogen (q.v.). Until 1S49 it was only known in the hydrated form (the aquafortis of the older chemists), but in that year Devine showed that anhydrous nitric acid, or I 'nitric anhydride (NO.), might be obtained in transparent colorless crystals by the action of perfectly dry chlorine gas on well-dried crystals of nitrate of silver, the reaction being • exhibited in the equation: Nitrate of silver. Chlorine. Chloride of silver. Nitric anhydride. Oxygen.
AgO,NO Cl = AgCI + NO + 0.
It is a very unstable compound, and sometimes explodes spontaneously. It dissolves in water with evolution of much heat, and forms hydrated nitric acid.
Hydrated nitric acid (symb. HO,N05, equiv. 63, sp. gr. 1.521), when perfectly pure, is a colorless, limpid, fuming, powerfully caustic fluid, possessing an intensely acid reac tion, as own by its action on litmus. It boils at 184°, and freezes at abont-40°. It parts very readily with a portion of its'oxygen to most of the metals, and hence is much used in the laboratory as an oxidizing agent. Its mode of action on the metals requires a few remarks. In order that a metal should unite with nitric. or any other acid, it is necessary that it should be in the form of an oxide, This oxidation is, however, effected at the same time that the metal and nitric acid are brought in contact, by one portion of the latter becoming decomposed and converting the metal into an oxide, while the remaining portion combines with the oxide thus formed, to produce a nitrate. The exact nature of the decomposition varies in the case of different metals.
Nitric acid, whether in the concentrated or in a more dilute form, acts. energetically on organic matters. As examples of such actions, we may refer to its power of decolor izing indigo; of staining the skin and all albuminous tissues of a bright-yellow color; of coagulating fluid albumen; and of converting cotton fiber into an explosive substance.
The monohydrated acid (II0,N05) is by no means a stable compound. If it be exposed to the action of light it is decomposed into hyponitric acid (NO,) (the peroxide of nitrogen of Graham) and oxygen; and mere distillation produces a similar effect. When it is mixed with water it emits a sensible amount of heat, owing to the formation of a much more stable hydrate, ITO,NO., 3Aq, which distils at 250° without change, and is unaffected by exposure to light. Its specific gravity is 1.424; and it is found that a weaker acid when heated parts with its water, and a stronger acid with its acid, till each arrives at this density. The existence of this hydrate has, however, been recently called in question by Roscoe.
The so-called funthignitric acid is merely a mixture of the pure acid with hyponitric acid.
Nitric acid does not occur naturally in a free state; but it is found tolerably abundant in combination with potash, soda, lime, and magnesia; and after thunder-storms traces of it, in combination with ammonia, are found in rain water. It may be formed in
small quantity by passing a series of electric sparks through a mixture of its component gases in the presence of water, which is a mere imitation, on a small scale, of the mode in which it is produced in the atmosphere by a storm. It is usually prepared in the laboratory by the application of heat to a mixture of equal weights of powdered niter (nitrate of potash) and oil of vitriol (hydrated sulphuric acid) placed in a retort. A com bination of sulphuric acid and potash remains in the retort. while the nitric acid distils over, and is condensed in the receiver, which is kept cool by the application of a wet cloth. The reaction is explained, by the equation: Niter. Sulphur.lc acid. Nitric acid. Bisulphate of potash.
KO,N0c, ± 2(110,804 .1,i10,110,2S03.
During distillation red fumes appear, arising from the decomposition of a portion of the nitric acid and a formation of some of the lower oxides of nitrogen. Iu this operation equivalents of oil of vitriol arc taken for one of niter, these being the proportions found by experience to be most suitable. If they are taken, equivalent for equivalent, a very impure red fuming acid is the result. In the manufacture of nitric acid on the large scale, the glass retort is replaced by a cast-iron cylinder coated with fire-clay, and the receiver by a series of earthen condensing vessels connected by tubes; and nitrate of soda, found native in Peru, is substituted for niter, in consequence of its being a cheaper salt, and of its containing 9 per cent more nitric acid.
.Nitric acid combines with baseS to form nitrates, some of which, as those of potash. soda, oxide of ammonium, silver, etc., are anhydrous, while others combine with a cer tain number (often six) equivalents of water of crystallization. Most of them are solu ble in water. crystallizable, and readily fusible by heat; and at an elevated temperature they are all decomposed, usually leaving only the oxide of the metal. If paper be soaked in a solution of a nitrate, allowed to dry, and ignited, it burns in the smoldering mode characteristic of This is, however, shared by a few other salts.
The tests for this acid when it is present in small quantities are less satisfactory than those for the other ordinary mineral acids. All its compounds are so soluble that no precipitant for this acid is known. The best method for its detection is mixing the fluid to be tested with a little concentrated sulpauric acid, and then pouring a strong solution of protosulphate of iron upon it, so as to form a separate layer. If much nitric acid be present, a black color is produced; if only a small quantity is present the liquid becomes reddish-brown or purple; the dark color being due to the formation of nitric oxide by the deoxidizing action of a portion of the iron salt on the nitric acid.
The applications of this acid in the arts, in manufactures. and in chemical processes are very extensive.