Basic Photography Course

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
requirements:

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.

Preservative

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.

Neutralizer

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.

Hardening Agent

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
washing process.

Antisludge Agent

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

factors:

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
fixation.

10-6

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Basic Photography Course

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DOFMaster for Windows On-line Depth of Field Calculator DOFMaster for Mobile Devices On-line Depth of Field Table Hyperfocal Distance Chart Articles FAQ Recommended Books Support Contact Links Home As an Amazon Associate I earn from qualifying purchases.	contents <- previous page next page -> 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 requirements: 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. Preservative 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. Neutralizer 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. Hardening Agent 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 washing process. Antisludge Agent 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 factors: 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 fixation. 10-6 contents Basic Photography Course <- previous page next page ->	As an Amazon Associate I earn from qualifying purchases.
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