There are many different formulas used as developing solutions, but most developers contain the following four essential ingredients: developing agent, preservative, accelerator or activator, and restrainer.

DEVELOPING AGENT

The developing agent, commonly referred to as the reducing agent, is the most important chemical in a developing solution. It is the developing agent that actually converts the silver halide grains in the emulsion to metallic silver. Nevertheless, the other ingredients are necessary to make the solution function properly.

One of the properties of a developing agent is its reducing potential. This refers to its relative ability to develop or reduce the silver halides. An active developing agent attacks silver halides vigorously, whereas one of low potential is slower in its action. For certain purposes, one agent may be preferred over another. The temperature of the solution affects the activity of some agents much more than others. Hydroquinone, Metol, paraphenylenediamine, and phenidone are some of the more commonly used developing agents.

PRESERVATIVE

All developing agents in an alkaline state are affected by oxygen. When the developing agent combines with the oxygen in the air, the efficiency of the developing agent is reduced. When elements combine with free oxygen from the air or water, oxidation occurs. Therefore, a preservative is added to developing solutions to prevent excessive oxidation. The preservative prolongs the useful life of the developing solution and prevents stains caused by the formation of colored oxidation products.

The preservative is a chemical with a great attraction for free oxygen and combines with it when mixed into a solution. A large amount of free oxygen is in the water used for solutions. When the free oxygen is left in the water, it oxidizes most of the developing agent and produces stains before the metallic silver image is developed completely. By adding enough preservative, you may remove practically all of the free oxygen from the solution, the developing agent works as intended, and no stains are produced. Sodium sulfite is the preservative most commonly used in developing solutions.

ACCELERATOR

All developing agents (not developing solutions) are either neutral or slightly acid. Most developing agents must be in an alkaline state to be effective reducing agents, thus solutions require an alkali to activate the developing agent. A developing solution contains an accelerator so the solution becomes alkaline.

The accelerator serves two functions. First, it speeds up the swelling of the gelatin in the emulsion. This swelling permits the solution to penetrate the entire emulsion more quickly. The effect of this action is physical. The second action is completely chemical. As the silver halide salts in the latent image are reduced by the developing agent, the halide elements freed from the silver are absorbed by the accelerator and combined into neutral salts. This prevents harmful effects in the solution.

Because the accelerator is a determining factor in the activity of a developing solution, it affects the degree of graininess produced in the negative. This graininess is dependent upon the clumping action of the silver grains during the development process. The more active the developer, the higher the pH, and the greater the clumping action. Therefore, the milder or less alkaline developers yield finer grain. Common accelerators used in developing solutions are sodium carbonate and potassium carbonate.

Without an accelerator, there is little or no action. With some accelerators there is too much action. By the addition of a bromide restrainer, you may slow down the action of the developing solution to a controllable degree.

RESTRAINER

Without a restrainer most developing solutions act too rapidly and develop unexposed silver halides near the surface of the emulsion. This causes chemical fog, developing streaks, and an image with low contrast. During development, some restrainer is released from the silver and has a restraining action on the reducing agent during development. However, its action is not enough to prevent fog. When a restrainer is added, development time is prolonged and fog is minimized. Contrast is increased because the activity of the developing agent is cut down in unexposed areas. However, too much restrainer greatly retards the amount of development. The chemical most commonly used as a restrainer is potassium bromide.