Composition of a chemical developer

There are many different formulas for developer solutions, but almost all practical formulas contain four essential ingredients:

Developer

The developer reduces the silver from its compound to create the image. The main active ingredients, used singly or in combination, are metol developer and hydroquinone.

Accelerator

An alkali, such as sodium carbonate, borax, or sodium or potassium hydroxide, is added as an accelerator to make the developer sufficiently active.

Preservative

The preservative (sodium sulfite) is necessary to minimize the effect of air oxidation of the developer and tends to keep the solution colorless during mixing and storage.

Retaining Agent

Many formulas also require a small amount of potassium bromide to prevent the developer from acting on the unexposed silver halide crystals, thereby creating a gray haze. In addition to these main ingredients, developer solutions often contain other substances such as sodium bisulfite, sodium sulfate, alcohol, etc., which are added for specific purposes