By putting more and more negatives or prints into a fixing bath in an attempt to saturate the solution with silver salts, the disappearance of the silver salts would be rendered very slow, and the last samples to be fixed in a reasonable time would certainly become yellowish after some days, due to silver sulphide, formed from the insoluble complex hyposulphite and occurring fairly uniformly throughout the gelatine layer.
The first determinations in this field having an immediate practical bearing on photography were those of Lumiere and Seyewctz (1907). These authors did not confine themselves to the determination of the maximum solubilities of silver bromide in pure hyposulphite, hyposul phite with bisulphite, with and without chrome alum ; they also determined the silver content at which any of these baths must be considered as unsuitable for further use, at least when fixation is carried out in a single bath.
The above tables summarize the results— These results show that with single-bath. fixation the efficiency of action of the hyposul phite decreases as its concentration increases, and is still further depressed by acidification of the bath, especially in the absence of alum.
It is somewhat curious that, in spite of the fact that silver chloride is more soluble than the bromide in hyposulphite of soda, the prac tical limit of fixation is more rapidly reached. in the case of the chloride ; solutions of hypo sulphite saturated with silver chloride more easily deposit the insoluble complex hyposulphite.
405. Speed of Fixation—Various Factors. The influence of the concentration of the bath and its temperature on the rate of fixation has been very carefully investigated by Welborne Piper' (1912-1914) ; the two graphs (Figs. 173 and 174) show, for a given emulsion, the nature of the variations which occur when the first phase of fixation (disappearance of the milky film of halide) in pure sodium hyposulphite is considered.
It is seen that whatever the concentration of the fixer, the process is most rapid at the higher temperatures, and the greatest speed is always obtained at a concentration of 40 per cent. At higher concentrations fixation becomes slower as the concentration rises, on account of the increased difficulty with which these solutions diffuse into the gelatine.
The rate of fixation is reduced as the bath becomes charged with more silver ; the following table (P. Strauss, 1925) shows the influence of silver salts on the time of disappearance of silver bromide in a 25 per cent solution of hyposulphite Silver chloride retards fixation less than an equal weight of the bromide, despite the fact that the chloride contains 75 per cent of silver and the bromide only 57 per cent. From this it is evident that the exhaustion of the bath, caused by the formation of silver hyposulphite, is not the only factor to be considered, but that the rate of fixation must be influenced by other salts arising from the reactions occurring. As a matter of fact, the addition of sodium bromide to the bath retards fixation, whilst sodium chloride accelerates it.
By reason of the very small solubility of silver iodide, the presence of a very little iodide in a solution of hyposulphite is sufficient to retard fixation considerably ; the following table shows the retarding influence of this salt on a 25 per cent solution of hyposulphite Potassium iodide in 'co c.c. of the bath . . . woo 0-02 cot 0-32 gem.
Time of fixation . . . 73 91 558 sec.
Sulphate of soda slightly retards fixation, and all the salts of the heavy metals (copper, lead, etc.) behave in the same way. The nitrates of soda or potash accelerate fixation when present in small quantities, but at concentrations of 4 per cent or above they retard it. The salts of ammonia, particularly the chloride (ammonium chloride, or sal ammoniac), have a very strong accelerating action, which is, however, not so great with emulsions containing silver iodide (Agfa, 1906; Lumiere and Seyewetz, 1908 and 1924), in spite of the fact that the addition of ammonium chloride to a solution of hyposulphite allows it to dissolve more silver iodide.
The accompanying graph (Fig. 175) sum marizes the observations of TIT 11 t .1 _ vv einorne-riper ti914) on this subject. It is seen that for each concentration of hyposulphite there is an optimum concentration of ammonium chloride. This optimum becomes smaller as the concentration of hyposulphite is increased.