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Silver Chloride

light, solution, reduced and exposed

SILVER CHLORIDE (Formula, AgCI; molecular weight, 143.5; synonyms, chloride of sil ver, argentic chloride).A white insoluble substance obtainable by direct union between the two elements, chlorine and silver, or by double decomposition of silver nitrate with a soluble chloride. If a solution of sodium chloride or hydrochloric acid be added to a solution of silver nitrate, silver chloride is immediately formed as a white precipitate.

Silver chloride also occurs as a native ore, termed, from its peculiar appearance, horn silver.

Silver chloride when exposed to actinic light changes in color from white to purplish tints, and finally to black, but this only takes place if water, organic matter, or chlorine absorbents of some kind be present. If perfectly dry and pure no change takes place. Chloride of silver can be exposed to the light for years in a perfect vacuum without the slightest change taking place./ A certain quantity of humidity is necessary to form a reaction, hence in very cold weather the darkening of silver chloride paper is always slower than in warm and damp weather. By short exposure it becomes converted into what is generally considered to be sub-chloride of silver, the action being similar to that with silver bromide (q.v.), and can then be acted upon by reducing agents or developers, as they are termed.

Silver chloride is but slightly soluble in water, and in strong hydrochloric acid; potassium cyanide and sodium thiosulphate (hypo) dissolve it readily.

If fused silver chloride be covered with hydrochloric acid, and a piece of zinc placed upon it, it will be found entirely reduced after a few hours to a cake of metallic silver; the first por tion of the silver having been reduced in contact with the zinc, and the remainder by the gal vanic action set up by the contact of the two metals beneath the liquid. Ammonia will readily dissolve silver chloride, the solution depositing colorless crystals of the chloride when evaporated; if the ammonia solution is very strong, ammonia-chloride of silver will be formed.

Silver chloride melts at 26o deg. Fahr., is not decomposed if heated with carbon, but can be reduced by heating in a current of nascent hydrogen.

It is largely employed in photography for printing out processes owing to its darkening properties when affected by light, and in the manufacture of chloride, and chloro-bromide emul sions by reason of properties it further possesses of being acted upon by certain developing agents, when exposed to light.