Developer magnifies the chemical change described above (on the last page) so that it becomes visible to the naked eye. A reducing (or developing) agent is
the primary chemical involved in development. Hydroquinone, metol, and
phenidone are the most common developing agents. Developer reduces the
silver ions to free silver (the developer gives electrons to the
positive silver ions). The greater the number of silver nuclei on a
grain, the faster the developer will reduce the remaining silver to
metallic silver. The grains that had more light hit them will be
reduced more quickly and will have a more intense color (black) from
the silver formed.The developer is selective and does not reduce the
grains which were not exposed.
There are several other parts of an effective developer solution. A
preservative (often sodium sulfite) can be added to prevent oxidation
of the developer in the air. An accelerator activates the developing
agent, speeding up the developing process. Sodium hydroxide and sodium
carbonate are examples of accelerators. A restrainer is also added to
the developer, it helps reduce chemical fog. Restrainers slow down
development in areas that received less exposure; potassium bromide is
the most common restrainer.