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Fixation 397

silver, soda, hyposulphite, bromide, water, fixing and solution

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FIXATION 397. The Purpose of Fixation. Fixing and wash ing are necessary in order, firstly, to convert the salts of silver remaining in the image, after its development, into soluble substances, and, secondly, to remove them.' Contrary to common opinion, the former of these processes is by far the more important ; washing, no matter how prolonged, can only remove soluble substances, and is not effective if the previous process of solu tion has not been carried to completion by fixing.

398. Solvents of the Silver Halides. It must be said at the outset that there are no true solvents of the chloride, bromide, or iodide of silver. When sugar is dissolved in water and the latter is evaporated, spontaneously or by boiling, the sugar is recovered in its original condition ; this is really a case of true solution. If silver bromide be submitted to the action of one of the saline solutions which are generally considered as its solvents, the evaporation of the liquid so obtained will never leave behind silver bromide ; the residue will consist of trans formation products of this salt, due to the chemical interaction of the silver salt and the fixing salt. This is not a purely academic distinction ; we shall see later that it is of great practical importance.

The first practical fixer to be used was hypo sulphite of soda, employed for this purpose by Herschel (son of the astronomer) in 1839. Hyposulphite of soda, though it deals effectively with the chloride and bromide of silver, is only a very mediocre fixing agent for photographic coatings containing mainly iodide of silver. It was abandoned on the introduction of the wet collodion process, in which fixing was done by a. solution of cyanide of potassium.' Hyposul phite returned to favour when the gelatino bromide plate replaced, except for certain special applications, the collodion processes_ It may be considered as being practically the only fixer in common use.

The various other substances capable of converting the halides of silver into soluble substances are considerably more difficult to use. The solutions so obtained are not very stable and are precipitated on dilution, and do not permit of final removal by washing. This

is notably the case with ammonia (ineffective for iodide of silver), with sulphites and bisul phites (only slightly active), with sulphocvanides (thiocyanates), with thio-urea and its deriva tives.

399. Hyposulphite of Soda. Hyposulphite (or thiosuiphate) of soda' occurs in crystals of varying size, of density 1-7, con taining 64 per cent of the active substance (anhydrous hvposulphite) and 36 per cent of water. It is deliquescent in moist air ; it is very soluble in water, but insoluble in alcohol. It melts at about 122° F. in its own water of crystallization, and is completely dehydrated by heating to a temperature above F., but this leads to partial decomposition unless special precautions are taken, and thus the price of anhydrous hvposulphite is excessive.

On dissolving in water, hyposulphite of soda lowers the temperature, thus forming a freezing mixture ; for this reason the preparation of solutions should not be left until they are required for use. For making-up quantities of hyposulphite of soda on the commercial scale, a 13aurne hydrometer should be used, together with the following table, which refers to a temperature of 6o° F.— Concen tration of solution - eo% 15% 20% 25% 3405°,, Degrees, &wine . 7" 10" Solutions of hyposulphite decompose slowly, even when kept from air and light ; sulphur is deposited, whilst a little sulphite is formed in the solution.

With very few exceptions, acids (even the weakest) and acid salts decompose hyposulphite of soda more or less rapidly, according to the concentration and strength of the acid. This decomposition manifests itself by the gradual formation of sulphur, which is set free in such a condition (colloidal sulphur) that it is only seen with difficulty. The particles unite with one another, and at first a bluish opalescence appears, then a white turbidity, and finally a yellow precipitate is formed ; at the same time sulphur dioxide and sometimes sulphuretted hydrogen are set free, whilst in the solution sulphate of soda and thionates (Seyewetz and Chicandard, 1895) are formed. Once started, this decomposition goes on until the hyposul phite is completely destroyed.

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