§ 79. General Properties.—Nitric acid—commonly known in England as aqua fortis, chemically as nitric acid, hydric nitrate, or nitric monohydrate—is a mono-hydrate of nitrogen pentoxide (N2O5), two equivalents, or 126 parts, of nitric acid containing 108 of N2O5, and 18 of H2O. Anhydrous nitric acid, or nitrogen pentoxide, can be obtained by passing, with special precautions, dry chlorine over silver nitrate; the products are free oxygen and nitrogen pentoxide, according to the following equation:—
| Silver Nitrate. | Chlorine. | Silver Chloride. | Nitrogen Pentoxide. | Oxygen. | ||||
| Ag2O,N2O5 | + | 2Cl | = | 2AgCl | + | N2O5 | + | O |
By surrounding the receiver with a freezing mixture, the acid is condensed in crystals, which dissolve in water, with emission of much heat, forming nitric acid. Sometimes the crystals, though kept in sealed tubes, decompose, and the tube, from the pressure of the liberated gases, bursts with a dangerous explosion.
Pure nitric acid has a specific gravity of 1·52, and boils at 98°. Dr. Ure examined the boiling point and other properties of nitric acid very fully. An acid of 1·5 specific gravity boils at 98·8°; of specific gravity 1·45, at 115·5°; specific gravity 1·40, at 118·8°; of specific gravity 1·42, at 122·8°. The acid of specific gravity 1·42 is the standard acid of the British Pharmacopœia. It can always be obtained by distilling either strong or moderately weak nitric acid; for, on the one hand, the acid on distillation gets weaker until the gravity of 1·42 is reached, or, on the other, it becomes stronger.
There is little doubt that acid of 1·42 gravity is a definite hydrate, consisting of 1 atom of dry acid and 4 atoms of water; it corresponds to 75 per cent.[102] of the liquid acid HNO3. There are also at least two other hydrates known—one an acid of 1·485 specific gravity, corresponding to 1 atom of dry acid and 2 of water, and an acid of specific gravity 1·334, corresponding to 1 atom of dry acid and 7 atoms of water.
[102] The British Pharmacopœia states that the 1·42 acid equals 70 per cent. of HNO3; but this is not in accordance with Ure’s Tables, nor with the facts.
In Germany the officinal acid is of 1·185 specific gravity, corresponding to about 30 per cent. of HNO3. The dilute nitric acid of the Pharmacopœia is a colourless liquid, of specific gravity 1·101, and should contain about 17·4 per cent. of acid. The acids used in various industries are known respectively as dyers’ and engravers’ acid. Dyers’ acid has a specific gravity of 1·33 to 1·34 (66° to 68° Twad.), that is, strength from 56 to 58 per cent. of HNO3. Engravers’ acid is stronger; being of 1·40 specific gravity (80° Twad.); and contains 70 per cent. of HNO3. Although the pure acid of commerce is (and should be) almost colourless, most commercial specimens are of hues from yellow up to deep red. An acid saturated with red oxides of nitrogen is often known as “fuming nitric acid.”
§ 80. Use in the Arts.—Nitric acid is employed very extensively in the arts and manufactures. The dyer uses it as a solvent for tin in the preparation of valuable mordants for calico and other fabrics; the engraver uses it for etching copper. It is an indispensable agent in the manufacture of gun-cotton, nitro-glycerin, picric acid, and sulphuric acid; it is also used in the manufacture of tallow, in preparing the felt for hats, and in the gilding trades. It is said to be utilised to make yellowish or fawn-coloured spots on cigar leaves, so as to give them the appearance of age and quality. It is also used as a medicine.