After development, the undeveloped silver bromide is removed by immersion of the negative or print in what is called the "fixing bath". There are only a few substances which will dissolve silver bromide, and the one which is universally used in modern photography is sodium thio-sulphate, which is known to photographers as hyposulphite of soda, or more usually as hypo, though the name hyposulphite of soda is used by chemists for another substance.

In the process of fixation the silver bromide is dissolved in the hypo by combining with it to form a compound sodium silver thiosulphate. Two of these compound thio-sulphates exist, one of them being almost insoluble in water, while the other is very soluble. As long as the fixing bath has any appreciable fixing power, the soluble compound only is formed.

Fixing is accomplished by means of hypo only, but materials are usually transferred from the developer to the fixing bath with very little rinsing so that a good deal of developer is carried over into the fixing bath, and this soon oxidizes in the bath, turning it brown, and staining negatives or prints. In order to avoid this the bath has sulphite of soda added to it as a preservative against oxidation, and the preservative action is, of course, greater if the bath is kept in a slightly acid state. In order to prevent the gelatine from swelling and softening it is also usual to add some hardening agent to the fixing bath so that a fixing bath instead of containing only hypo will contain in addition sulphite, acid and hardener.

Now, if a few drops of acid, such as sulphuric or hydrochloric acid, are added to a weak solution of hypo, the hypo will be decomposed and the solution will become milky, owing to the precipitation of sulphur. The change of thiosulphate into sulphite and sulphur is reversible, since, if we boil together sulphite and sulphur we shall get thiosulphate formed, so that while acids free sulphur from the hypo, sulphite combines with the sulphur to form hypo again. Consequently, we can prevent acid decomposing the hypo if we have enough sulphite present, since the sulphite works in the opposite direction to the acid. An acid fixing bath, therefore, is preserved from decomposition by the sulphite, which also serves to prevent the oxidation of developer carried over into it.

Since in fixing baths what we require is a large amount of a weak acid, the best acid for the purpose is acetic acid. Citric or tartaric acids can also be used.

In order to make sure that the films are properly fixed they should be left in the fixing bath twice as long as is necessary to clear them from the visible, white silver bromide. If considerable work is being done, the best course is to use two fixing baths, transferring the films or prints to the second clean bath after they have been fixed in the first. Then, when the first bath begins to work slowly, it can be discarded and replaced by the second bath, a fresh solution being used for the second bath. These precautions are necessary because, as has already been said, silver forms two compound thiosulphates, the first of which is almost insoluble in water but is transformed into the second, which is soluble, by longer treatment with hypo. Consequently when a film first clears, it still contains the first insoluble thiosulphate of silver, and if it is taken out of the fixing bath and washed some of the silver will be left behind and not washed out. Then, on keeping, this silver thiosulphate left in the negative will decompose and produce stains. If a negative or print is properly fixed and washed it will be permanent.

The actual rate of washing may be understood by remembering that the amount of hypo remaining in the gelatine is continually halved in the same period of time as the washing proceeds. An average negative, for instance, will give up half its hypo in two minutes, so that at the end of two minutes half the hypo will be remaining in it, after four minutes one-quarter, after six minutes one-eighth, after eight minutes one-sixteenth, ten minutes one-thirty-second, and so on. It will be seen that in a short time the amount of hypo remaining will be infinitesimal. This, however, assumes that the negative is continually exposed to fresh water, which is the most important matter in arranging the washing of either negatives or prints.

If a lot of prints are put in a tray and water allowed to splash on the top of the tray, it is very easy for the water on the top to run off again, and for the prints at the bottom to lie soaking in a pool of fairly strong hypo solution, which is much heavier than water and which will fall to the bottom of the tray. If the object is to get the quickest washing, washing tanks should be arranged so that the water is continuously and completely changed and the prints or negatives are subjected to a continuous current of fresh water. If water is of value, and it is desired to economize in its use, then by far the most effective way of washing is to use successive changes of small quantities of water, putting the prints first in one tray, leaving them there for from two minutes to five minutes, and then transferring them to an entirely fresh lot of water, repeating this until they are washed.

The progress of the washing can be followed by adding a little permanganate solution to the wash water after the prints are taken out of it in order to see how much hypo is left in it, the presence of hypo being seen by decoloration of the permanganate. An even simpler test is to taste the prints. Six changes of five minutes each should be sufficient to eliminate the hypo effectively from any ordinary material.