a chemical reducing agent used in reducing silver halides in the photosensitive layers of motion-picture and still photographic materials to metallic silver during photographic development. Developing agents are the basic constituent of photographic developers. Only those compounds that are able to reduce the silver compounds selectively in the exposed sections of a photosensitive layer may serve as developing agents; that is, these compounds must ensure a sufficiently high reduction rate for the exposed sections during the development process in comparison with unexposed sections. If the reverse is true, the resultant image will be densely covered with photographic fog.

Developing agents may be organic, such as hydroquinone, or inorganic substances, such as vanadium compounds and complex compounds of bivalent iron. The most valuable are organic developing agents whose molecules contain the grouping

where α₁, and α₂ represent —OH, —NH₂, —NHR, —NR₂groups (R is an alkyl or another substituent), n is zero or a small integer, C is a carbon atom, and A is a carbon or nitrogen atom. The exception to this formula is Phenidone, which has low developing power. In certain cases, the rate of development achieved with a mixture of developing agents is higher than the sum of the rates achieved with the same developing agents individually. This property serves as a basis for the preparation of mixed developers, such as metol-hydroquinone and Phenidone-hydroquinone, which are widely used.

The following developing agents are also used alone: hydroquinone, a high-contrast developing agent in an alkaline medium with pH ≥ 10.5; metol, a soft-working developing agent in neutral and alkaline mediums; amidol, a soft-working developing agent in all mediums, producing very fine grain; pyrocatechin and pyrogallol, which display tanning properties; para-phenylendiamine, a fine-grain developing agent; diethyl-para-phenylendiamine, used in color photography; and para-aminophenol, which works very slowly but develops high-quality images without fog.