Nitrate of Silver is prepared by dissolving metallic Silver in Nitric Acid. Nitric Acid is a powerfully acid and corrosive substance, containing two elementary bodies united in definite proportions. These are Nitrogen and Oxygen; the latter being present in greatest quantity.
Nitric Acid is a powerful solvent for the metallic bodies generally. To illustrate its action in that particular, as contrasted with other acids, place pieces of silver-foil in two test-tubes, the one containing dilute Sulphuric, the other dilute Nitric Acid; on the application of heat a violent action soon commences in the latter, but the former is unaffected. In order to understand this, it must be borne in mind that when a metallic substance dissolves in an acid, the nature of the solution is different from that of an aqueous solution of salt or sugar. If salt water be boiled down until the whole of the water has evaporated, the salt is recovered with properties the same as at first; but if a similar experiment be made with a solution of Silver in Nitric Acid, the result is different: in that case metallic Silver is not obtained on evaporation, but Silver combined with Oxygen and Nitric Acid, both of which are strongly retained, being in fact in a state of chemical combination with the metal.
If we closely examine the effects produced by treating Silver with Nitric Acid, we find them to be of the following nature:—first, a certain amount of Oxygen is imparted to the metal, so as to form an Oxide, which Oxide dissolves in another portion of the Nitric Acid, producing Nitrate of the Oxide, or, as it is shortly termed, Nitrate of Silver.[1]
[1] The preparation of Nitrate of Silver from the standard coin of the realm is described in Part III., Art. "Silver."
It is the instability of Nitric Acid therefore—its proneness to part with Oxygen—which renders it superior to the Sulphuric and to most acids in dissolving Silver and various other substances, both organic and inorganic.
Properties of Nitrate of Silver.—In preparing Nitrate of Silver, when the metal has dissolved, the solution is boiled down and set aside to crystallize. The salt however as so obtained is still acid to test-paper, and requires either re-crystallization, or careful heating to about 300° Fahrenheit. It is this retention of small quantities of Nitric Acid, and sometimes probably of Nitrous Acid, which renders much of the commercial Nitrate of Silver useless for Photography, until rendered neutral by fusion and a second crystallization.
Pure Nitrate of Silver occurs in the form of white crystalline plates, which are very heavy and dissolve readily in an equal weight of cold water. The solubility is much lessened by the presence of free Nitric Acid, and in the concentrated Nitric Acid the crystals are almost insoluble. Boiling Alcohol takes up about one-fourth part of its weight of the crystallized Nitrate, but deposits nearly the whole on cooling. Nitrate of Silver has an intensely bitter and nauseous taste; acting as a caustic, and corroding the skin by a prolonged application. Its aqueous solution does not redden blue litmus-paper.
Heated in a crucible the salt melts, and when poured into a mould and solidified, forms the white lunar caustic of commerce. At a still higher temperature it is decomposed, and bubbles of Oxygen Gas are evolved: the melted mass cooled and dissolved in water leaving behind a black powder, and yielding a solution, which is faintly alkaline to test-paper, from the presence of minute quantities of Nitrite or basic Nitrite of Silver.[2]
[2] Nitrite of Silver differs from the Nitrate in containing less Oxygen, and is formed from it by the abstraction of two atoms of that element; it is described in the vocabulary, Part III.
THE CHEMISTRY OF THE CHLORIDES OF SILVER.