DEVELOPING AGENTS

The most common developing agents are Metol, Hydroquinone and Phenidone.

None of these work very well independently, and they are usually combined as either Metol and Hydroquinone [an MQ developer] or Phenidone and Hydroquinone [a PQ developer]. The term super-additivity is used to mean that when these agents are paired in a solution together there is more activity than if the activity of each were added. All developing agents oxidise easily, they absorb oxygen and become ineffective usually going brown. To prevent oxidation a preservative, usually Sodium sulphite is also in the solution - this salt is converted to sodium sulphate by oxygen in the air before the developing agent and thus has a protective action. All developing agents operate only in alkaline solutions [i.e. of high pH] and Sodium carbonate or Borax [Sodium potassium borate] is used in most black and white developers. These alkaline salts are called accelerators because the higher the concentration the more active the solution. Next, we add an accelerator to give the required amount of contrast and speed of developing.

For positive developers we use Sodium Carbonate and for negative developers Borax powder. During development sodium bromide is released into the solution, which has the effect of reducing the activity, increasing the contrast and reducing the base density. Most developer formulations, but not all, have some potassium bromide added from the start. This helps to provide control and repeatability and ensures that replenishment does not result in major changes of activity [see below].

The final ingredient is a restrainer so that the clear areas remain clear. A developer without a restrainer would give a high fog level which is the name we give to image formed by anything other than deliberate light exposure. For example manufacturing produces fog, as does the handling in the laboratory. The usual restrainer is Potassium Bromide.