The standard coin of the realm is an alloy of silver and copper, containing about one-eleventh of the latter metal. It may be converted into nitrate of silver, sufficiently pure for photographic purposes, by dissolving it in nitric acid and evaporating the solution to the crystallizing point: or, if the quantity be small, the solution may be boiled down to complete dryness, and the residue fused strongly; which decomposes the nitrate of copper, but leaves the greater portion of the silver salt unaffected. (N. B. Nitrate of silver which has undergone fusion contains nitrite of silver, and will require the addition of acetic acid if used for preparing the collodion sensitive film.)
Crystallized nitrate of silver absorbs ammoniacal gas rapidly, with production of heat sufficient to fuse the resulting compound, which is white, and consists of 100 parts of the nitrate + 29·5 of ammonia. The compound however which photographers employ under the name of ammonio-nitrate of silver, may be viewed more simply as a solution of the oxide of silver in ammonia, without reference to the nitrate of ammonia necessarily produced in the reaction.
Very strong ammonia, in acting upon oxide of silver, converts it into a black powder, termed fulminating silver, which possesses the most dangerous explosive properties. Its composition is uncertain. In preparing ammonio-nitrate of silver by the common process, the oxide first precipitated occasionally leaves a little black powder behind, on re-solution; this does not appear, however, according to the observations of the author, to be fulminating silver.
In sensitizing salted paper by the ammonio-nitrate of silver, free ammonia is necessarily formed. Thus:—
Chloride of ammonium + oxide of silver in ammonia = chloride of silver + ammonia + water.
Silver, Oxide of.
Symbol, AgO. Atomic weight, 116.
If a little potash or ammonia be added to solution of nitrate of silver, a brown substance is formed, which, on standing, collects at the bottom of the vessel. This is oxide of silver, displaced from its previous state of combination with nitric acid by the stronger oxide, potash. Oxide of silver is soluble to a very minute extent in pure water, the solution possessing an alkaline reaction to litmus; it is easily dissolved by nitric or acetic acid, forming a neutral nitrate or acetate; also soluble in ammonia (ammonio-nitrate of silver), and in nitrate of ammonia hyposulphite of soda, and cyanide of potassium. Long exposure to light converts it into a black substance, which is probably a suboxide.