Silver bromide, AgBr

Precipitation of a solution of a silver salt with a bromide solution yields a flocculent to pulverulent, amorphous precipitate of silver bromide, AgBr, its colour being white, yellowish-white, or lemon-yellow according to the conditions of precipitation. Its melting-point is given as 422° C. and 427° C., and the density of the precipitated salt as 6.39 to 6.52, and after fusion as 6.32 to 6.49.

Addition of water to a 0.1 per cent, solution of the amorphous form in concentrated ammonium hydroxide yields hexagonal plates; slow evaporation of a similar solution produces green, lustrous octahedra.

At 18° C. the solubility of the bromide is 0.109 mg. per litre of water. Besides ammonium hydroxide, it dissolves in sodium-thiosulphate solution, forming the double salt 2Na₂S₂O₃,Ag₂S₂O₃,2H₂O, and in solutions of bromides and potassium cyanide. Its heat of formation from its elements is given as 22.7 Cal., and 20.7 to 23.7 Cal. according to the condition of the bromide produced.

In ammoniacal solution at 200° C., silver bromide is converted by iodic acid into the iodide. Double compounds with ammonia of the formulae AgBr,3NH₃ (8.64); AgBr,1½NH₃ (9.95); and AgBr,NH₃ (10.65) have been prepared, the figures in parentheses indicating the calculated heats of formation in large calories.

The action of light is similar to that on silver chloride, silver photo-bromide being produced with liberation of bromine. The change in weight under the influence of light does not exceed 2.4 per cent. Silver bromide is more sensitive to light than any other substance, and is extensively employed in the manufacture of dry photographic plates. The glass is coated with an emulsion produced by addition of ammoniacal silver nitrate to a solution of potassium bromide containing gelatin, the mixture being digested at 40° to 45° C. for about an hour to increase the size of the bromide granules. The emulsion is solidified by cooling with ice, washed with water, liquefied, and poured over the glass. It is usual to add a small proportion of silver iodide as a decelerator, and a slight excess of potassium bromide to eliminate Silver nitrate.

A short exposure in the camera to light produces the "latent image," the process being attended by slight reduction, and the formation of photobromide, probably a solid solution of silver and silver subbromide in silver bromide. The latent image is developed by immersing the plate in an alkaline reducer, such as pyrogallol or quinol, in presence of alkali. The reduction takes place first at those points where it has been initiated by the action of light. Development must not be continued so long as to cause general blackening of the plate or "chemical fog." When it is complete, the image is "fixed" by dissolving the unaltered silver salt in a solution of sodium thiosulphate. The velocity of reduction is lowered by the presence of bromine ions, so that the operator can control the rate of reduction by addition of a solution of potassium bromide to the developer.

Collodion can be substituted for gelatin in the preparation of the emulsion, but the plates are less sensitive than the gelatin plates. Gelatin exerts a reducing action on silver bromide, but collodion does not; the collodion plates are consequently free from the trace of fog characteristic of gelatin plates, and therefore give a very sharp, well-defined image suitable for technical reproduction. Collodion plates are rendered more sensitive by the presence of Silver nitrate in the emulsion, but such plates have to be exposed in the wet condition, and are not well adapted for field work. The increase in sensitiveness depends on the reaction

6 AgNO₃ + 3H₂O + 3Br₂ = 5 AgBr + AgBrO₃ + 6HNO₃.

Silver bromide is most sensitive to blue light, but can be rendered sensitive to the green, red, and ultra-red portions of the spectrum by dyeing the emulsion with members of the eosin group or the cyanine group, the resulting plates being orthochromatic or panchromatic. The sensitiveness of gelatino-bromide plates is much diminished by immersion in water or developer, especially for yellow and green rays.