b. Reducers.
(1) Reduction process. As described previously, the latent image site is a
speck of silver capable of initiating development. The latent image site provides a place
where the reducers accomplish the process of breaking down the silver bromide crystal
to black metallic silver. The reducing agents act as electron donors to the latent image
site, giving it a negative charge. Thus, the positively charged silver ions may move into
the areas of the sensitivity speck and become attracted to it. As this process continues,
more silver ions are attracted and deposited as atoms of silver. The final result is the
breakdown of the entire crystal to black metallic silver.
(2) Reducing agents. The agents commonly used are ElonTM (another trade
name is MetolTM) and hydroquinone. The activity of these chemicals requires their
presence in an alkaline solution. Chemical functions differently in attacking the
emulsion. The ElonTM starts development by attacking the exposed silver bromide
crystals swiftly with resultant production of gray tones in the image. ElonTM is
unpredictable above 75F and hydroquinone ceases its activity below 60F. The activity
of the hydroquinone is slower, but it serves to build up the black tones required in the
image. The reducers, acting as a team, produce a good image with satisfactory
contrast in a minimum of time, as long as the temperature of the solution remains in the
optimum range. Reducers are not too stable in the presence of oxygen, which they can
readily absorb from the air or from the water.
c. Restrainer. The restrainer (potassium bromide) limits the action of the
reducing agents to the breaking up of the exposed silver bromide crystals only without
attacking the unexposed crystals in the emulsion during the normal development. If the
restrainer is omitted, the reducers are hyperactive and break down the unexposed
crystals, fogging the film. If the film is left in the developer too long, the reducers will
override the restrainer and chemical fog will result. Also, the bromide released from the
crystals into the solution will gradually restrain the action of the reducing agents to a
point where they no longer function efficiently. Therefore, when a replenisher solution is
mixed, it should not include the restrainer.
d. Preservative. The preservative (sodium sulfate) retards the activity of the
reducing agents to within controlled limits so that the "life" of the developing solution is
maintained over a reasonable period of time. Since the reducing agents react quickly
with oxygen, this reaction must be controlled or the developing solution will not last very
long. Sodium sulfate works very well as a preservative because it retards oxidation of
the reducing agents and prevents the formation of stains on the film.
1-9.
AUTOMATIC DEVELOPER SOLUTION
Because of the increased speed and temperatures used for automatic
processing, the chemicals used in the manual developer are not adequate. Therefore,
although the functions of the chemicals are basically the same, some of the chemicals
MD0954
1-10