Silver peroxide, Ag0 or is formed by the electrolysis of silver solutions under certain conditions, or by the action of potassium permanganate upon argentic oxide. That formed by electrolysis is generally found combined with some of the silver salt forming the electrolyte, and compounds such as and are said to have been produced. Higher oxides of silver have been reported but not confirmed.
Halogen Compounds.—Silver combines with all the halogens to form definite and very stable compounds. Silver chloride, AgC1, is found native in the mineral horn-silver or cerargyrite ; mixed with clay, it is called butter-milk ore by the German miners. It is obtained as a precipitate by the action of hydrochloric acid or any soluble chloride upon a solution of silver. It forms a white curdy precipitate which rapidly darkens on exposure to light, and this reaction is the basis of nearly all the processes used in pho tography (q.v.). Silver chloride is nearly insoluble in pure water and in dilute nitric acid (I part in 50,000). It is, however, much more soluble in concentrated solutions of hydrochloric acid and metallic chlorides, although, on dilution, most of the silver chloride is often precipitated. Silver chloride dissolves readily in am monia and evaporation of the solution yields rhombic crystals of the composition It is also easily soluble in solutions of sodium thiosulphate and potassium cyanide. Silver chloride melts at about C to a yellow transparent liquid, which, on cooling, solidifies to a colourless horny mass. It is easily reduced to metallic silver (I) by contact under acidulated water with certain metals (zinc, iron, etc.), an equivalent of which dissolves as chloride; (2) by fusion with alkaline hydroxides, carbonates or cyanides; (3) by heating in a stream of hydrogen or coal gas; or (4) by digesting alkaline solutions with grape sugar or other reducing agents.
Silver bromide, AgBr, found native in the mineral bromargyrite in Chile and Mexico, is obtained as a yellowish-white precipitate by the addition of a soluble bromide to a silver solution. It resembles the chloride in most of its properties but is less soluble in ammonia. It melts at about 425° C, and is even more sensitive to light than the chloride, although its change in colour is less noticeable. Certain minerals, chiefly found in Chile, consist of
variable mixtures of AgC1 and AgBr; they are embolite, mega bromite and microbromite. Silver iodide, AgI, found native in the mineral iodargyrite or iodyrite, is produced by precipitating a silver solution with any soluble iodide or by acting upon the metal with hydriodic acid or iodine. It forms a yellow powder, practi cally insoluble in water, acids, or ammonia, but soluble in strong hydriodic acid, sodium thiosulphate, or strong solutions of potas sium iodide, forming in the last case a double iodide, KAgI2. Silver iodide is dimorphous ; at ordinary temperatures it is hexagonal and deep yellow, but at 146° C it changes to a cubic, light yellow form. Like the chloride and bromide, it is partly decomposed by light, but without apparent change of colour as it requires a developer to demonstrate the change. Silver iodide melts at 352° C, and after resolidifying it shows the anomalous behaviour of expansion on cooling.
Silver fluoride, AgF, produced by dissolving the oxide in hydro fluoric acid, is soluble in water, the solution possessing an alkaline reaction ; it crystallizes with one molecule of water and is unacted on by light.
Other Compounds.—Silver nitrate, by far the most important compound of silver, is almost always produced by dis solving the metal in somewhat diluted nitric acid and crystallizes on evaporation of the solution as anhydrous, colourless, rhombic plates. Silver nitrate is exceedingly soluble in water, dissolving in less than half its own weight at ordinary temperatures; at Doe C one part of water dissolves more than eleven parts of the nitrate ; it is also soluble in alcohol and ether. Silver nitrate melts at 209° C and decomposes quantitatively above 320° C into silver nitrite, AgNO2, and oxygen. When pure, it is not decom posed by light, but in the presence of organic matter blackening takes place. Either in the solid state or in solution it stains the skin black, and a dilute solution in ether is sometimes used as a hair-dye. The nitrate cannot be used as the basis of a marking ink for linen, since the strong acid liberated when the silver is de posited would corrode the fabric, and hence silver salts of various weak organic acids are used for this purpose.