Nitric Acid is the most important of the five compounds which oxygen forms with Nitrogen (q.v.). Until 1849 it was known only in the hydrated form (the aqua fortis of the older chemists), but in that year Deville showed that Anhydrous Nitric Acid, or Nitric Anhydride, (see NITROGEN), might be obtained in transparent colourless 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.
It is a very unstable compound, and sometimes explodes spontaneously. It dissolves in water with evolution of much heat, and forms hydrated nitric acid. Nitric Acid (sym. ; equiv. 63; sp. gr. 1.530), when perfectly pure, is a colourless, limpid, fuming, powerfully caustic fluid, possessing an intensely acid reaction, as shown by its action on litmus. It boils at ( C.). It parts very readily with a portion of its oxygen to most metals, and hence is used in the laboratory as an oxidising agent. Whether in the concentrated or in a more dilute form, it acts energetically on organic matters. As examples of such actions we may refer to its power of decolorising indigo; of staining the skin and all albuminous tissues of a bright-yellow colour; of coagulating fluid albumen; and of converting cotton fibre into an explosive substance (see GUN-COTTON). The applications of this acid in the arts, in manufactures, and in chemical processes are very extensive.
The concentrated acid, , is by no means a stable compound. If it be exposed to the action of light it is decomposed with the formation of lower oxides of nitrogen; 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, , which distils at ( C.) 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 called in question by Roscoe. The so-called Fuming Nitric Acid is merely a mixture of the pure acid with one of the lower oxides.
Nitric acid does not occur naturally in a free state; but it is found tolerably abundantly in combination with potash, soda, lime, and magnesia; and after thunderstorms 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 nitre (nitrate of potash) and oil of vitriol (hydrated sulphuric acid) placed in a retort. A combination 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 :
| Nitre. | Sulphuric Acid. | Nitric Acid. | Bisulphate of Potash. |
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. In this operation two equivalents of oil of vitriol are taken for one of nitre, 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 (see NITRE) is substituted for nitre, 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, &c., are anhydrous, while others combine with a certain number (often six) of equivalents of water of crystallisation. Most of them are soluble in water, crystallisable, 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 smouldering mode characteristic of touch-paper. This property is, however, shared by 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 solution of sulphate of iron, and then carefully pouring sulphuric acid through a tube to the bottom of the glass so as to form a lower layer. If much nitric acid is present a black colour is produced; if only a small quantity is present the liquid becomes reddish brown or purple; the dark colour being due to the formation of nitric oxide by the deoxidising action of a portion of the iron salt on the nitric acid.
Medicinal Uses.—In the British pharmacopœia there is both a strong and a dilute acid. The strong acid has a specific gravity of 1.42, contains 70 per cent. of real acid, , while the diluted acid is prepared by mixing six ounces of the former with about twenty-five of distilled water, and has a specific gravity of 1.101. The dilute acid is used internally as a tonic in conjunction with bitter infusions. In many cases of biliousness, of chronic inflammation of the liver, and in syphilitic cases in which mercurials are inadmissible, it may be prescribed with great benefit, either alone or in conjunction with hydrochloric acid, externally as a bath or lotion, or internally in doses of about 20 minims properly diluted. The strong acid is useful as an escharotic; e.g. to destroy warts, some kinds of polypi, the unhealthy tissue in sloughing ulcers, &c., and as an application to parts bitten by rabid or venomous animals. Largely diluted—as 50 or 60 drops of the strong acid to a pint or more of water—it forms a stimulative application for torpid ulcers.