The nitric acid of commerce may be freed from the impurities alluded to above by one or other of the following methods:—
1. By the addition of a little nitrate of silver, as long as it produces any cloudiness, and, after repose, decanting the clear acid, and rectifying it at a heat under 212°. To ensure a perfectly colourless product, a small portion of pure black oxide of manganese should be put into the retort. (Murray.)
2. By agitating the acid with a little red oxide of lead, and then rectifying it, as before.
3. By adding 1% of bichromate of potassa to the acid before rectifying it. This answers well for acid not stronger than sp. gr. 1·48.
4. By rectification at a gentle heat, rejecting the first portion that comes over, receiving the middle portion as genuine acid, and leaving a residuum in the retort. (Ure.)
According to Apjohn and others, the strongest liquid nitric acid, sp. gr. 1·520, is a monohydrate; that of the sp. gr. 1·500, a sesquihydrate; that of 1·486, a binhydrate; and that of 1·244, a quadrihydrate; or containing respectively 1, 11⁄2, 2, and 4 atoms of water. (See below.)
Nitric Acid, Anhy′drous. N2O5. Syn. Nitric anhydride. This interesting substance was first obtained in a separate form by M. Deville, in 1849.
Prep. (M. Deville.) Nitrate of silver is dried by exposure to a current of dry carbonic acid at a temperature of 356° Fahr., and the tube containing it is then immersed in a water bath heated to 203° Fahr.; pure dry chlorine gas is next passed through the apparatus, and, as soon as the reaction commences, the temperature
is retraced to 154° or even 136°, but not lower; the production of crystals in the receiver, which must be cooled by a powerful freezing mixture, soon commences; lastly, the liquid portion of the product is removed by a current of dry carbonic-acid gas.
Prop., &c. Colourless prismatic crystals, which melt at 86° Fahr., boil at about 115°, and at 122° begin to suffer decomposition; added to water, much heat is generated; it rapidly attacks organic bodies, even caoutchouc; sometimes it explodes spontaneously. The process for its preparation is tedious and difficult.