Weight for weight, ammonium thiosulfate has approximately three times the fixing power of sodium thiosulfate. It is
the clearing agent in liquid high-speed fixing concentrates, while hypo is used in regular-speed formulas.
It is essential that the fixing solution neutralize the alkaline developer adhering to the film. In other words,
development must stop before fixing can begin. The neutralizer is an acid; the most suitable of which are acetic and
sulfuric acid in weak concentration. If a fixing bath is to be used for a long period of time, a large quantity of acid is
necessary to neutralize the alkalinity of the developer. Fixing is accomplished by means of the thiosulfate only.
However, in transferring the film from one stage to another, materials from the developing solution may be transferred
to the fixer bath. These may oxidize in the fixer causing stains on the film. To counteract this condition, sodium
sulfate is employed as a preservative.
Because X-ray films are handled frequently and are subject to more abuse than photographic negatives, it is customary
to use a hardening agent. This hardening agent or hardener tans and toughens the emulsion. Some of the common
hardeners are alum and aluminum and chloride for high-speed fixers. One of the distinct advantages of the hardener
used in a high-speed fixer is the production of a hardened film which will not melt in water as hot as 85°C (175°F)
after the film is dried.
When a film is removed from the developing solution the undeveloped areas are swollen and yellow in appearance.
Sometime after immersion in the fixer this yellow becomes transparent; this change may be observed and recorded.
The time required for this change is known as the "clearing time." To adequately fix a film it SHALL be immersed in
the fixer at least two times as long as it took to clear. This period SHALL not exceed fifteen minutes. For example, if
the clearing time is two minutes then the fixing time is four minutes. The fixing solution will become deficient with
use. This deficiency is insidious and may be overcome by adjusting the fixing time up to the maximum fifteen-minute
time period. The cause of the fixer degradation may be one or more of the following:
a. Accumulation of soluble silver salts. This condition will gradually prevent the fixer from dissolving
unexposed silver halide from the film emulsion. Therefore, making the fixer incapable of properly
clearing the radiographic film.
b. Loss of chemical activity is evident when long periods of time are required to clear a radiograph. This
situation will cause colored stains on the radiograph, swelling of the emulsion that inhibits hardening
and results in long drying times, and reticulation or sloughing during drying.
c. Reduction of activity caused by dilution of the fixer solution when stop bath, rinse water, and developer
solution are carried in by the film being processed. The effects of this dilution/contamination are
reduced by allowing the radiograph to drain into the stop bath prior to being put in the fixer. Care
should be used not to contaminate the developer.
The purpose of washing films is to remove the chemicals present after fixing. Because hardeners are used in X-ray
fixing solutions, it is difficult to remove small quantities of the fixer retained by the gelatin. The speed of washing is
determined by the speed with which the clearing agent diffuses out of the film into the water. The quantity of clearing
agent remaining in the gelatin is continually halved in the same period of time as washing continues. For example, if a
film gives up one-half its clearing agent in 1 minute, then after 2 minutes one-quarter remains, after 3 minutes one-
eighth, in 4 minutes one-sixteenth, and so on, providing the film is continually exposed to fresh water. It is obvious
that washing will never remove the last traces of fixer. The object in washing is to remove enough fixer so the film
may be kept without fading for a given period of time. For most practical purposes, X-ray films will be washed
sufficiently in 30 minutes if the water changes at the rate of four to eight times per hour (see Table 6-16). The wash
Water temperature should be between 65°F and 80°F. Regardless of the type of fixer used, if the film is allowed to fix
twice the required time, three times a normal washing time is required.