Sodium and ammonium thiosulfate changes
undeveloped silver halide to soluble silver sodium
thiosulfate. It removes this compound from the
emulsion and refills the space it occupied with
nonexhausted fixing solution. Therefore, the function of
the fixing agent is to convert the silver salts remaining
in the emulsion after development to soluble compounds
and to remove these soluble compounds by constantly
diluting and replacing them in the emulsion. The number
of substances capable of functioning as fixing agents is
small because a good fixer must meet the following
It must dissolve silver salts without affecting the
metallic silver image.
The compounds it forms must be soluble so they
can be removed from the emulsion.
The fixer should neither swell excessively nor
soften the gelatin.
A preservative prevents oxidation of the developing
agents that are carried over into the fixing bath by the
film. It also prevents decomposition of the fixer.
Oxidized developer in a fixing bath produces stains.
Strong acids may cause a fixing agent to decompose
(sulfurize). You must add preservative (sodium sulfite)
in the fixer to prevent sulfurization. The preservative
prevents the acid from decomposing hypo into free
sulfur, prevents discoloration of the solution because of
oxidation, and aids in preventing stains.
After development, the pores of the swollen
emulsion retain a portion of the developer. If allowed to
remain, the developer continues its activity. Even when
the emulsion is thoroughly rinsed in a water bath before
being placed in the fixer, some developer activity
remains. This causes uneven stains in the gelatin of the
emulsion and makes the negative unusable. To stop
development and prevent stains, you must add an acid
neutralizer to the fixer. The most frequently used
neutralizer is acetic acid.
During development, the gelatin becomes softened
and swells. Frilling, reticulation, scratches, and other
undesirable effects may result when processing is
continued without hardening the emulsion. A hardening
agent is included in the fixer to harden the emulsion in
the fixing bath. The most common hardening agent is
potassium alum. The hardening and toughening of the
gelatin by the alum stops the tendency of the emulsion
to swell but leaves it expanded and rigid enough for the
The pH range of the fixer is limited. It must be low
enough to neutralize the activity of the developer and
also be high enough to prevent sulfurization. The
reduced acidity of the bath is gradually neutralized by
the alkali of the developer carried into the fixer by the
film. When the active acidity is neutralized too far, a
sludge of aluminum sulfite forms that can make the fixer
useless. An antisludge agent, such as boric acid, is added
to the fixer. This agent is capable of absorbing a large
quantity of the developer before sludge occurs, thus
lengthening the useful life of the fixer.
Time Required for Fixing
The time required for film to fix depends on several
The type of emulsion and its thickness. All else
being equal. fine-grain emulsions fix faster than
coarse-grained ones. Thin emulsions require less time
to fix than thick emulsions.
The type of fixing bath and degree of exhaustion.
When sodium thiosulfate is the fixing agent, a
concentration of about 75 percent gives the fastest rate
of fixation. However, because of the tendency of hypo
to bleach out the image, most fixers for negatives have
a concentration between 20 and 40 percent.
The fixing bath temperature. An increase in the
temperature increases the rate of fixation. (Do not
interpret this to mean that you can raise the temperature
of the fixer above the temperature called for by the
particular process being used.) The temperature of the
fixer is not as critical as the temperature of the developer.
However, you should keep all processing solution
temperatures constant to avoid an increase of graininess.
The amount of agitation. The rate of fixation is
affected by diffusion of the chemicals, so agitation
reduces fixation time.
The amount of exposure. The more exposure the
film has to light, the less unused silver halide to be
removed by the fixer, and hence the faster the rate of