Gelatin silver halides

Van Lare, E. J. Symmetrical carbocyanine dyes for optically sensitizing gelatin silver halide emulsions. GB 754546, 1956 Chem. Abstr. 1957, 51, 20893. 

The light-sensitive layer of the present-day photographic material consists essentially of a large number (e.g., 108 per square centimeter) of tiny crystals of silver halide embedded in a layer of gelatin. The tiny crystals, or grains as they are commonly called, of the most sensitive photographic materials are composed of silver bromide, a small percentage of iodide, and a very small but very important amount of silver sulfide (Sheppard, 1) or possibly silver (Carroll and Hubbard, la) or both. The halide in the less sensitive materials may be simply bromide, chloride, or mixtures of the two. 

Direct development by sulfite ion does not take place, probably because the rate of reduction of silver ions by the sulfite is much smaller than the rate of solution of the silver halide (see Section YI of this chapter). The writer has obtained physical development, however, with a solution of silver nitrate and sodium sulfite. A specially hardened gelatin film which was able to withstand the action of the solution at 70° for 40 minutes was used. The developing solution contained 1.7 g. silver nitrate and 13 g. sodium sulfite per liter. Fog formation was relatively high, as might be expected. 

The effect of the cyanine dye and of gelatin on the reaction rate shows that reduction of silver ions from solution is not the rate-controlling process. These influences of adsorbed components on the reaction rate speak against the concept that solution of the silver halide is the rate controlling process. Hence, the silver catalyzed reduction of silver chloride by hydroxylamine takes place substantially at the solid silver/ silver halide interface. 

Materials for use in black and white processes are obtained in the following way. A photographic emulsion is prepared by precipitating either a silver halide or a mixture of silver halides in a polymeric medium which is usually gelatin. This emulsion is then coated as a very thin layer on to a support such as film or paper. The coating is dried, and the support is cut into sizes appropriate to specific applications and stored in light-proof containers. 

The major uses of heterocyclic compounds in black and white processes are in controlling the hardness of gelatin, in optimizing the performance of the silver halide emulsions, in absorbing unwanted radiation and as components in the processing solutions. 

The silver halide crystals can be formed as microscopic grains suspended in a protective colloid, usually gelatin. This dispersion, the photographic emulsion, can be coated on a suitable support to obtain photographic films, plates, and papers. 

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