When development is complete, the process must be stopped to prevent
over development. A stop bath is used to discontinue development
action. A weak acid, such as acetic acid, is used in this process.
Developers function at a fairly high pH and so a few seconds in a weak
acid solution (low pH) will halt the developer action
Step Three: Fixing
The goal of the fixing process is to remove the remaining silver
halide crystals that have not been developed. If not removed, they
will react when exposed to appropriate light and the picture will
darken over time. The silver halide crystals are not very soluble in
water and so they must be made soluble and then removed. Sodium
thiosulfate (known as "hypo" in the darkroom) has been the fixer
chosen for this process since 1839. The thiosulfate ion reacts with
the silver ions in the crystal to form silver thiosulfate. The silver
is desorbed from the crystal complex and becomes soluble in water so
that it can be removed from the film.
The time required for fixing depends on the thiosulfate concentration,
the temperature and solution agitation. Different types of emulsions
take different times to fix; for example, fine grain emulsions fix
more quickly than larger grained � a thicker emulsion will take longer
than thinner emulsion of equivalent content.
The film is now developed and fixed. The only thing left to do is
rinse off all the chemicals and byproducts left on the film. This can
be done with large amounts of water or with a chemical rinse product.
The film is then hung to dry in an area free of dust. Dust will cause stray marks on your photograph.