Development: Transforming the latent image to a visible image by chemically reducing exposed silver halides to black metallic silver.

A developer is made up of the following components:

1. Developing Agent: i.e. Metol, Hydroquinone, Phenidone.
2. Alkali (accelerator): i.e. Borax, Sodium Carbonate, Sodium hydroxide.
3. Preservative: i.e. Sodium Sulphite, Potassium Metabisulphite.
4. Restrainer: i.e. Potassium Bromide.
5. Additives. Wetting Agent, Anti-sludging agent etc.

Purpose of individual components.

Developing agent. Converts the latent image to a visible silver image by a process of chemical reduction.
Alkali. Shortens developing time for practical purposes. Development will still take place in the absence of an alkali but it will take considerably longer. Developing agents work more effectively in an alkaline environment hence the addition of an alkali in the developer formula substantially reduces the development time and the subsequent use of an acid stop bath halts development abruptly by neutralising the alkali and producing an acid environment.
Preservatives. Slows down oxidation of the developing agent. Developing agents oxidise when exposed to air. Leave a tray of developer out overnight and note the change in colour from pale yellow to brown or dark brown if you leave it long enough .
Restrainer. Keeps fog level down.
Other additives. Perform various functions such as preventing deposits forming in the solution or helping promote even ingress of chemicals into the emulsion.

Developer formula (ID 11)

To make 1 litre:

Weigh/measure out the following:

Metol 2 grammes
Sodium Sulphite (anhydrous) 100 grammes
Hydroquinone 5 grammes
Borax 2 grammes
Water 1000cc

Starting with approx. 750cc of water at about 50 C add , in the following order, a little of the Sodium Sulphite, to stop the Metol oxidising when you mix it, then gradually add the Metol , stirring until it is all dissolved. Add the Sodium Sulphite, again stirring until dissolved then do the same with the Hydroquinone and Borax. Finally add water to make up to 1000cc.

To make 1 litre:

Metol 3 grammes
Sodium Sulphite (anhydrous) 100 grammes
Hydroquinone 7.5 grammes
Borax 20 grammes
Water 1000cc
Prepare as above. Add to developer to maintain volume as required. You can add an equal volume of replenisher before discarding the solution although it is always best to discard solutions well before exhaustion.