3871. Nitro-Sulphuric Acid

3871.    Nitro-Sulphuric Acid. Dissolve

1 part nitre in 9 parts sulphuric acid. This is used to separate the silver from the copper and solder of old plated goods. At about 200° Fahr. it readily dissolves silver, but scarcely acts on copper, lead, or tin, unless diluted, or assisted by a much higher temperature.

3872. Nitric Acid

3872.    Nitric Acid. There are five compounds of nitrogen and oxygen. The union of 1 equivalent of nitrogen with 1 of oxygen produces nitrous oxide, or laughing gas; with

2  oxygen, nitric oxide; with 3 oxygen, nitrous acid; with 4 oxygen, hyponitric acid; and with 5 equivalents of oxygen, nitric acid. Pure liquid nitric acid is colorless, highly corrosive, and possesses powerful acid properties. It is employed in assaying, to dye silk and woolens yellow, and to form various salts. In medicine, it is used as a caustic, etc.. The officinal strength of nitric acid of the U. S. and British pharmacopoeias has a specific gravity of 1.42, and boils at 250° Fahr.

Nitric acid of less density than 1.42 parts with water and becomes stronger at lower temperatures; but acid of higher specific gravity is weakened by exposure to heat. It freezes when exposed to extreme cold. It rapidly oxidizes the metals, and unites with them and the other bases, forming salts called Nitrates. Two strengths of this acid occur in the arts, known as double and single aqua-fortis. Double aqua-fortis has usually a specific gravity of 1.36, and single, or ordinary aqua-fortis 1.22. Both are frequently sold at lower strengths. This can easily be ascertained by acidimetrv. (See No. 78.)

3873. To Obtain Nitric Acid

3873.     To Obtain Nitric Acid. The usual method adopted for obtaining this acid is to add to nitrate of potassa in coarse powder, contained in a glass retort, an equal weight of strong sulphuric acid, poured in through a funnel, so as not to wet the neck of the retort. The materials should not exceed two-thirds of the capacity of the retort. A moderate heat is at first applied, increasing as the materials begin to thicken. Red vapors will at first arise and pass over into the receiver ; these will disappear in the course of the distillation, but subsequently renewed, showing that the process is completed. The pale yellow acid thus obtained may be rendered colorless, if desired, by heating it gently in a retort.

3874. To Purify Nitric Acid

3874.     To Purify Nitric Acid. The nitric acid of commerce frequently contains chlorine, muriatic and sulphuric acids, and sometimes iodine, from which it may be purified 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° Fahr. A perfectly colorless product may be obtained, by introducing a small portion of pure black oxide of manganese into the retort. Nitric acid may also be purified by rectification at a gentle heat, rejecting the first liquid that comes over, receiving the middle portion as genuine acid, and leaving a residuum in the retort. Another method is to agitate it with a little red-lead before rectification.