Changes in fixers with use

The composition of a fixer, like that of a developer, changes with use. We can best understand the reasons for these changes by considering the sequence of events following development, assuming that an acid-hardening fixing solution is used, preceded by a plain rinse.

• The rinse removes much of the gross contamination of the film by the developing solution and slows development.

• The film is transferred to the fixer where the acid neutralizes the alkali of the developing solution in the emulsion layer and stops development.

• The thiosulphate converts the silver halide in the emulsion to soluble complex sodium argentothio-sulphates and halide ions. Thus the concentration of silver complexes and halide ions increases.

• The hardening agent diffuses into the gelatin and begins its hardening action.

As the solution continues to be used, more and more of the alkaline developer is carried over and the acid becomes exhausted. A good guide to the degree of acid exhaustion is provided by its pH value. The pH of the solution should preferably not be allowed to rise above about 6.0. If the solution becomes alkaline, it will not stop development and there will be risk of staining. If the solution is a hardening-fixing solution there will also be a serious reduction of hardening power. If a hardening fixer becomes very alkaline, a precipitate may be formed from the interaction of the hardening agent and the alkali. This will tend to deposit on negatives in the form of a scum, which may be difficult to remove.

Not only does the acidity of the solution decrease as it is used, but the fixing action becomes slower. This is partly due to exhaustion of the thiosulphate, but with negative materials it is also due to a concentration of iodide derived from the emulsion, which builds up in the solution. Silver iodide, present in many negative emulsions, is extremely difficult to dissolve in a solution of sodium thiosulphate, and has the effect of depressing the solubility of silver bromide and so retarding the clearing process as a whole. We thus have a symptom of apparent exhaustion which is brought about by the accumulation of an antagonistic end-product. Among other important products which build up in the solution are the complex sodium argentothiosulphates. These also retard clearing but their most important effect is upon the permanence of the negatives and prints. Silver indicator test papers are available; these change colour on immersion in a fixer containing silver. These papers are calibrated so that the intensity of the coloration may be read from a chart which relates the colour to the silver content.

A further cause of exhaustion of a hardening fixer can be exhaustion of sulphite, but the quantity employed is usually sufficiently large to ensure that trouble from this source is rare. Lastly, since a small volume of water from the rinse is carried into the fixer on each film and a small volume of the fixing solution is carried out, the fixer becomes progressively diluted with use. This also leads to an increase in clearing time.

For large-scale use fixers, like developers and all other processing solutions, are replenished (see Figure 17.7). The first property of a fixer to decrease with use is its acidity, then its hardening properties. The life of a fixer may, therefore, be usefully increased by the addition of a suitable acid mixture. If this is made to contain a hardener, both acidity and hardening power can be restored to a certain extent. This process cannot, however, be continued indefinitely because of the accumulation of iodide and silver in the fixer. It is therefore, not worthwhile to

Figure 17.17 A small-scale electrolytic silver recovery unit: A, tank; B, rotation and control unit; C, rotating cathode

attempt to replenish a fixer by the addition of thiosulphate unless some means of removing the silver is available, as, for example, by electrolytic silver recovery (see below).

Continue reading here: Fractional gradient

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