AgBr

Silver bromide (AgBr) is an inorganic compound that forms pale yellow to white crystals. It is one of the classic silver halides and adopts a rock-salt type structure, with Ag+ and Br− ions in a face-centered cubic lattice. It is very sparingly soluble in water, with a solubility product around 5×10−13 at room temperature, and it dissolves in concentrated ammonia or cyanide solutions to form soluble complexes.

A key property of AgBr is its photochemical sensitivity. Exposure to light induces a photochemical reduction of Ag+ to metallic silver, forming a latent image that can be amplified by chemical development. This behavior underpins traditional photographic processes that use silver halide emulsions in gelatin. In the dark, AgBr remains relatively inert, while light exposure initiates the formation of metallic silver grains.

Occurrence and preparation: In nature, the mineral bromargyrite consists of AgBr. Commercially, AgBr is produced by precipitating silver ions with bromide sources, such as mixing silver nitrate solutions with potassium bromide or hydrobromic acid to yield the AgBr precipitate.

Reactions and applications: Beyond photography, AgBr participates in halide exchange and forms soluble complexes with ammonia, cyanide, or thiosulfate under suitable conditions. In photographic practice, fixing agents like sodium thiosulfate dissolve unexposed AgBr, leaving the developed image of metallic silver.

Safety: As a silver salt, AgBr is relatively insoluble and handled with standard laboratory precautions. Prolonged exposure to silver compounds can have health and environmental considerations, and appropriate disposal is advised.