Photographic Developer

a substance used to convert a latent photographic image, obtained during the exposure of photosensitive motion-picture and still photographic materials to light or other radiation, into a visible image. Photographic developers used for the ordinary, or chemical, development of silver halide photosensitive materials are aqueous solutions; water-alcohol solutions and pastes are also sometimes used. The solutions contain (1) a developing agent; (2) an accelerator (which produces an alkaline reaction of the solution), such as soda, borax, or, more rarely, a caustic alkali; (3) a preservative, usually only sodium sulfite; and (4) an antifogging agent, such as potassium bromide or benzotriazole.

Developing agents reduce silver halide to metallic silver on exposed sections of a photosensitive layer. The properties of photographic developers are determined primarily by the nature of the developing agent. Since the agents are usually effective in an alkaline medium, developers contain alkalies that produce and maintain the necessary pH value and, at the same time, accelerate the development process. Developers containing a caustic alkali usually exhibit pH values of 12-13. Those containing soda and potash have pH 10-11, and those with borax, pH 8-9. Developers containing a caustic alkali are especially active, but they rapidly lose this property during the development process. The preservative protects the developer from rapid oxidation by atmospheric oxygen and regenerates the activity of the developer. The antifogging agent prevents the rapid formation of photographic fog during development.

General-purpose developers contain the above mentioned substances in the following quantities (in moles/liter of solution): developing agent, 0.05; sodium carbonate or potassium carbonate, 0.2-0.3; sodium sulfite, 0.2; and potassium bromide, 0.004-0.04. Developers may also contain other substances. These include Na₂SO₄, which prevents softening of the emulsion layer during high-temperature development; polyethylene glycol, which increases the rate of development; solvents of silver halide, such as potassium thiocyanate, which reduce graininess; and various wetting agents, which ensure uniform developer activity.

In addition to general-purpose developers, there are special developers. These include (1) fine-grain and equalizing developers, with low alkalinity and increased sodium sulfite content; (2) high-speed developers, with high alkalinity and increased quantities of all components; and (3) developers that operate at high and low temperatures.

Developers used in the development of multilayer color motion-picture and still photographic materials have the same basic composition, but they contain para-phenylendiamine derivatives that act as developing agents.

Photographic developers for physical development contain a developing agent, sodium sulfite, and a soluble silver salt, usually AgNO₃. During development, AgNO₃ is reduced to a finely dispersed powder of metallic silver, which is selectively precipitated on the exposed sections of the photosensitive layer. The resultant image has a fine grain structure and low optical densities.