342 HANDBOOK OF PHOTOGRAPHY 



fhloride or iodide. A simple picture may be painted of the action of bromine ions 

 as restrainers of development of a silver bromide emulsion. In this simple case it 

 appears probable that the bromine ions in solution from the potassium bromide 

 depresses the ionization of the minute quantities of silver bromide dissolving from 

 the emulsion grains, thus reducing the rate of reaction, i.e., the rate of production of 

 photographic density. This simple picture gives no key to the magnitude of the 

 effect nor does it give a basis upon which to predict whether or not development of 

 image would be more or less affected than development of fog. Experience shows that 

 in the majority of cases fog is restrained more effectually than image, thus increasing 

 the differential nature of development. 



A more complex situation exists in many practical cases as numerous emulsions 

 consist of mixtures of at least two silver halides. Thus mixtures of silver bromide and 

 iodide and of silver chloride and bromide are frequentlj^ encountered. In all such 

 cases the chloride or the bromide predominates, as the iodide is never used commer- 

 cially to a large extent. The simple picture presented in the previous paragraph is not 

 adequate to describe completely the phenomenon, but the same sort of effects is 

 produced, i.e., fog is restrained more than image development. 



When potassium iodide is used in the developer formula as a restrainer, the 

 restraining effect is somewhat increased, as silver iodide is much less soluble than either 

 silver bromide or chloride. Accordingly, when potassium iodide is used in a developer, 

 smaller concentrations are used than of bromide. 



Sometimes a combination of bromide and iodide is used in a developer, and some 

 results of special interest have been achieved by properlj^ proportioning the two 

 compounds. When the developer is fresh, its action is restrained predominantly b^- 

 the iodide and secondarily by the bromide. As the developer is used repeatedly, the 

 iodide tends to replace the bromide in the emulsion passing through, owing to the 

 extremely low solubility of the former. Thus the iodide content of the developer is 

 reduced, while the bromide content is concurrently built up, and the reducers become 

 less active through use. When correctly proportioned, the tendency to increase in 

 activity owing to reduction of iodide content may be balanced against the decrease 

 of activity due to accumulation of bromide and exhaustion of the reducers, with a 

 resultant evening of developer activity throughout its life. In one such formula, 

 designed for positive motion-picture film, the iodide was present only to the extent 

 of Mo g- per 1., but this small quantity modified markedlj' the exhaustion character- 

 istics of the bath in the manner indicated. 



Little has been said of sodium or potassium chloride as a restrainer, for, while they 

 have restraining properties, neither has received the sanction of widespread continued 

 use accorded potassium bromide. 



Restrainers of the type discussed are primarily intended to decrease inherent 

 chemical or emulsion fog, i.e., the fog developed spontaneously. When development 

 is carried on under conditions such that the emulsion is permitted to come in contact 

 with the air while wet with developer, a fog is sometimes caused which is known as 

 "aerial " fog. This appears to be connected with the aerial oxidation of the developer 

 chemicals. Many desensitizers tend to prevent its formation. Because of this, the 

 suggestion has been made that this fog is due to chemiluminescence accompanying 

 the oxidation of the reducing agent by the oxygen of the air. In most photographic 

 processing an attempt is made to reduce to a minimum the exposure to the air of the 

 emulsion while wet with developer. This is usually the simplest and cheapest way 

 of avoiding such fog troubles. When this procedure is not sufficient, desensitizers may 

 be used either in the developer or as a preliminarj' bath. A number of such compounds 

 are known of which the most important are pinakrj'-ptol green, pinakryptol yellow, and 

 p henosa f ranine . 



