After development, the undeveloped silver bromide is removed by immersion of the print or negative in the "fixing" bath. There are only a few substances which will dissolve silver bromide, and the one which is used in modern photography is sodium thiosulphate, Na2S2O3, 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.

Thiosulphate of Soda or Hypo can be made by boiling together sodium sulphite and sulphur, the sulphur combining with the sodium sulphite according to the equation:

Na2SO3 + S = Na2S2O3

Sodium Sulphite Sulphur Hypo

In practice it is generally made from calcium sulphite residues, the calcium thiosulphite being then converted into the sodium salt by treatment with sodium sulphate. The hypo comes on the market in clear crystals and is usually fairly pure, any foreign substance present being more often due to accidental contamination than of a chemical nature and consisting of dirt, straw or wood dust due to careless handling. Sometimes, however, the hypo contains calcium thiosulphate, which decomposes much more readily than the sodium salt. On the whole, it is not difficult to obtain good hypo; the Eastman Tested Hypo is prepared in the form of granular crystals, easy to dissolve, and free from accidental contamination.

In the process of fixation the silver bromide is dissolved in the hypo by combining with it to form a sodium silver thiosulphate. Two of these compound thiosul-phates 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.

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. This is because the acid converts the sodium thiosulphate into the free thiosulphuric acid, and this substance is quite unstable, decomposing into sulphurous acid and sulphur according to the equation:

Eastman Portrait Film, Artura Print From a Demonstrator's Negative.

Eastman Portrait Film, Artura Print From a Demonstrator's Negative.

H2S2O3 = H2SO3 + S

Thiosulphuric Sulphurous Sulphur Acid Acid

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. The developer which is carried over into the fixing bath is, however, alkaline and consequently a considerable amount of acid is required in a fixing bath which is used for any length of time. If only a small amount is present, it will soon be neutralized by the developer carried over. We are therefore in a difficult position because we require a large amount of acid present, and yet the fixing bath must not be strongly acid. The solution of the difficulty is found by taking advantage of the fact that there are some acids which are very weak in their acidity and yet can neutralize alkali in the same way as a strong acid, so that a large amount of these acids can be added without making the bath so acid that sulphur is precipitated.

The strength of an acid depends upon the fact that when it is dissolved in water some of the hydrogen contained in it dissociates from the acid and remains in the solution in an active form, and the acidity of the solution depends upon the proportion of the hydrogen which is dissociated into this active form. The amount of alkali which the acid can neutralize, however, depends upon the total amount of the hydrogen present, and not on the dissociated portion only. The strongest acids are the mineral acids, such as sulphuric and hydrochloric, while the weakest acids are the organic acids, which are made from vegetable products, such as citric and acetic acids, which are very weak acids indeed.

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 acid can also be used but not so satisfactorily.

Acetic Acid in its dilute form is prepared as vinegar by the fermentation of alcohol, the stronger acid being made from acetate of lime, which is prepared either by neutralizing vinegar with chalk or more usually by neutralizing with lime the crude acetic acid prepared by the destructive distillation of wood. There are three commercial strengths of acetic acid: that known as glacial acetic acid, which contains about 99 % of the acid, and which is called glacial because at moderately low temperatures it freezes to a solid, a solution containing 80 % of the acid, and the 28 % commercial acetic acid. It is not usual for acetic acid to contain any impurities which are likely to be harmful.