IODIDE OF SILVER. Ag. I = 234. Iodine and silver are sup posed to combine directly and produce iodide of silver, for if a silver plate be placed in a bottle containing iodine and left for a short time, it is tarnished with a yellow film supposed to be iodide of silver. If this experiment be made in the dark, and the tarnished plate exposed to light, either under a negative or in a camera obscura, and then submitted to the fumes of mercury, an image is developed, which proves that silver tarnished with iodine is sensitive to light. The yellow substance which forms the tarnish on the metal may not, however, be pure iodide of silver, and there are other reasons for supposing that pure iodide of silver is not sensitive to light.
Iodide of silver may be obtained by double decomposition, by adding a solution of nitrate of silver to a solution of an alkaline or metallic iodide, such as iodide of potassium or cadmium. The oxygen of the oxide of silver in the nitrate goes to the metal and forms potass or oxide of cadmium ; this is converted into nitrate of potass or cadmium by the nitric acid, which leaves the silver, and the iodine combines with the silver to form iodide of silver, which is a yellow insoluble amorphous substance. When the nitrate of silver is in excess the precipitate is of a strong yellow colour, and is dark ened very quickly to a pale brown tint by exposure to light ; but when the iodide of potassium is in excess the colour of the precipi tate is rather paler, and it is then absolutely insensitive to light. These results are the same however thoroughly the precipitate may be wa,shed in water, frequently changed, before exposing it to light. For the same reason it happens that an excited collodion film or waxed paper never entirely loses its sensitiveness by washing, be cause the iodide of silver was formed from a solution containing nitrate of silver in excess ; but an iodized calotype paper, prepared with double iodide, in which iodide of potassium is in excess, appears to be absolutely insensitive to light, at le,ast so far as the power of developing a picture upon it by the ordinary developers is concerned.
Iodide of silver, with nitrate of silver in excess, is the most sensitive to light of all compounds at present known, and is therefore used in the negative processes of photography. It is darkened more rapidly in vacuo than in air, and is aLso darkened to a coffee colour by being boiled for some minutes with distilled water. When iodide of silver is obtained by double decomposition in the manner described, with nitrate of silver in excess, the nitrate of the alkali or metal also formed in the solution, and from which the precipitated iodide of silver is not thoroughly cleared, affects to some extent the properties of the iodide ; for instance, when nitrate of potass remains, which is a neutral salt, the iodide is more quickly darkened than when nitrate of cadmium, which is an acid salt, remains.
Iodide of silver is insoluble in water and alcohol, and nearly insol uble in ammonia. It is soluble in concentrated solutions of the alkaline chlorides, bromides, and iodides, and aLso in hyposulphite of soda, and cyanide of potassium, forming in every case a soluble double salt of silver and the alkali. Concentrated nitric and sul phuric acids also decompose it. It may be reduced by metallic zinc, forming soluble iodide of zinc, and metallic silver as a black powder. When iodide of silver is fused at a red heat it acquires a red colour. It volatilizes before the blow-pipe, leaving a little silver only behind.
When iodide of silver is dissolved in concentrated solutions of an alkaline iodide, bromide, or chloride, the double salt formed is only soluble in the quantity of water used ; if more be added the solution immediately becomes milky. This effect does not happen when either hypo-sulphite of soda or cyanide of potassium is used as the solvent.