Nitric Acid, Or Hydric Nitrate, the most important compound of oxygen and nitrogen, formed by the union of nitric anhydride or anhydrous nitric acid (see Nitrogen) and water. It was formerly called aqua fortis, and was known to the alchemists; but its composition was first determined by Cavendish in 1785. "When nitrogen is mixed with about 12 times its volume of hydrogen, and a jet of the mixed gases is burned in the air or in oxygen, the water produced will be found to contain a small quantity of nitric acid; and it was his experiments on the formation of water that led Cavendish to the discovery of the acid. If a number of electric sparks are passed between two points over moistened litmus paper, a red spot will be produced on the paper from the action of nitric acid which has been formed by the combination of atmospheric nitrogen and oxygen in the presence of watery vapor; and during a thunder storm the acid is produced in a similar manner in quantity sufficient to be detected by delicate tests. The formula of nitric acid is HN03, and according to modern theory it is a compound of hydrogen with a radicle called nitrion (N03), and is regarded as a salt of hydrogen.
The nitrion is produced by the union of water with nitric anhydride (H2O + N306=HaNa08 or HN03), and is the form in which nitric acid is converted when it unites with a metal to form a nitrate. The production of nitric anhydride (N2 06) was effected by Deville by passing a current of dry chlorine gas slowly over crystals of dry nitrate of silver, the salt being first raised to about 203° F. till decomposition commences, and then lowered to about 140°, the operation being conducted with the greatest care. The chlorine displaces the nitrion of the nitrate of silver (AgN03), chloride of silver (AgCl) is formed, and the nitrion breaks up into nitric anhydride and oxygen, the latter escaping (2NO3-=N205 + 0). The receiver being surrounded by a freezing mixture, the anhydride condenses into brilliant colorless crystals derived from the right rhombic prism, melting at 85° and boiling at 113°, with decomposition. This theory of nitric acid has not the apparent simplicity of the older views, which regarded the acid as a monohydrate of pentoxide of nitrogen, or HO,K05, and the metallic nitrate as a compound of N06, with the oxide of the metal. - Manufacture. Nitric acid is obtained for chemical purposes from one of the alkaline nitrates.
When potassic nitrate is heated in a retort with strong sulphuric acid (H2SO)4, double decomposition takes place, bisulphate of potash (hydric potassic sulphate) and nitric acid being formed, as shown in the following equation: KN03 + H2S04=HN03 + KH,S04. The bisulphate remains in the retort, while the nitric acid distils over and may be condensed in a receiver. In preparing small quantities, equal weights of nitre and oil of vitriol are placed in a glass retort, and the distillation takes place as represented in the figure; the retort, a, containing the materials, and the Liebig's condenser, 5, effecting the cooling while the product is on its way to the receiver, c, which is placed in a shallow vessel containing cold water or ice. During the process red fumes appear in the retort, in consequence of the conversion of a part of the acid into some of the lower oxides of nitrogen, and a powerfully corrosive yellow liquid condenses in the receiver. On the large scale, large cylindrical iron retorts, lined with fire clay above the level of the fluid mass, and placed horizontally, are employed, instead of the small glass ones, and a series of large earthen Woulf's bottles replace the ordinary receiver, convenient arrangements being provided for the introduction of the materials.
It is usual to employ nitrate of soda in place of nitrate of potash on account of its cheapness, and also to use a smaller proportion of sulphuric acid. In this case, instead of bisulphate (KH,S04), there remains in the retort the normal sulphate (K2 S04), but a greater degree of heat is required to expel the last portions of acid. - Properties. The acid obtained in the manner described has a reddish yellow color, in consequence of the presence of some of the lower oxides of nitrogen. It may be freed of these by redistillation with an equal bulk of oil of vitriol, and passing a current of dry air through the liquid, which should be gently warmed and protected from the light. But the acid is so unstable, from its disposition to part with its oxygen, that it soon becomes partially decomposed. When pure it is a limpid, fuming, colorless, powerfully corrosive liquid, having a specific gravity of 1-53 at 59°, of 1.559 at 32°, freezing at - 67° F., and boiling at 187°. The boiling point from its commencement rises, owing to decomposition, until it reaches 250°, at which point the distillation goes on.
The acid contains a larger proportion of water, the composition being 2HN03,3h20, but appears to be a hydrate of considerable stability, having a specific gravity of 1 '424. A weaker acid when distilled parts with a portion of its water till it arrives at this density, and a stronger acid becomes reduced to the same, so that an acid of the above density can be continuously distilled for an indefinite time; but by varying the pressure Eoscoe found that the density and consequently the proportion of water varied. The following table, abbreviated from Kolb, shows the specific gravity and percentage of hydric nitrate (HN0)3 contained in acids of different strength, at 32° and 59° F.:
Laboratory Apparatus for Nitric Acid.
HNO3 in 100 parts by weight.
The nitric acid of commerce is generally contaminated with a variety of foreign matters, such as sulphuric acid, chlorine, and oxide of iron, which are easily detected by the usual tests. It varies in strength, often containing more than 50 per cent, of water. In consequence of its disposition to part with oxygen, nitric acid is extensively employed as an oxidizing agent. If the strong acid is dropped on hot pulverized charcoal, combustion will be produced. If it is mixed with oil of vitriol and poured upon oil of turpentine, the latter will burst into a flame. It parts with its oxygen to phosphorus with explosive violence. _ It rapidly corrodes organic substances, particularly animal tissues, and when slightly diluted stains the skin, wool, feathers, and albuminous bodies a bright yellow. It acts with great en-ergyupon the more oxidizable metals; gold, platinum, rhodium, and iridium alone resist its oxidizing power. Its action is most energetic when its specific gravity is between 1.35 and 1.25, and the presence of nitrous acid increases its power. The action of nitric acid upon metals is not so simple as that of sulphuric acid, whose components are held together with stronger affinities.
According to one view, when sulphuric acid unites with a metal, the latter first becomes oxidized by combining with the oxygen of the water, while the hydrogen is set free. Another and later opinion is that the metal simply displaces the hydrogen from its union with the sulphion, S04, thus: HSO4 + M = MSO4 + II. When nitric acid is employed, the metal in the same way displaces the hydrogen from its union with the nitrion (N03), but hydrogen is not evolved, because it instantly unites with oxygen derived from decomposing acid or its lower oxides. The mode of action varies with the metal acted upon and other circumstances. When silver is dissolved in the cold in an excess of diluted nitric acid, nitrous acid (HN02) is formed, and there is no evolution of gas. If the solutionis warm, the action is more violent, and nitric oxide (N02) is disengaged, or a similar effect may be obtained by employing a more oxidizable metal, as copper or mercury. When the acid is pretty strong (sp. gr. 1.4), peroxide of nitrogen is disengaged, and by raising the temperature free nitrogen is also evolved, the acid undergoing complete deoxidation.
A metal having a stronger affinity for oxygen, as zinc, will cause the evolution of nitrous oxide; and when the acid is concentrated ammonia is one of the products, and may be found combined with the excess of acid. The principal uses of nitric acid are in the manufacture of nitrates of the metals and of sulphuric acid, where it completes the process of oxidation by furnishing an addition equivalent to the sulphurous acid. (See Sulphuric Acid).