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. TYPES  OF  BLACK-AND-WHITE DEVELOPERS As stated previously, when a photographic emulsion is exposed to light, there is an invisible change produced in the minute crystals of silver halides that results in a latent image. To make the image visible, you must treat the  exposed  emulsion  in  a  solution  known  as  a developer. This solution converts the halides affected by light to black metallic silver. These black metallic silver particles  form  the  visible  image  on  the  negative. At  the  beginning  of  development,  there  is  little difference in density between the highlight and shadow areas of the film. However, during normal development, this  difference  increases  because  the  highlight  densities continue  to  increase  after  the  shadow  areas  are completely  developed.  Development  should  stop  when the contrast between the shadows and highlights reaches a desired difference. The activity of the developer, and to some extent the type of film, primarily determines this developing  action. 10-2