DEVELOPERS AND THEORY OF DEVELOPMENT 339 



metol particularly in nietol-hydroquinone-borax formulas. Hydroquinone represents 

 one of the most popular current cases of the preservative effect of the second reducer. 

 Experiments have shown that the primary function of the hydroquinone in the 

 formulas commonly compounded is that of preservative. Freshly mixed developer, 

 lacking the hydroquinone, shows relatively little difference in its activity from the 

 complete formula, but it is almost impossible to store it any length of time without a 

 decrease in its activity, whereas the complete formulas keep reasonably well. 



A few other preservatives have been suggested and tested but are far less important 

 than sulphite. They include: acetone bisulphite; formaldehyde sodium sulphoxylate 

 (known as Rongalite C); Mannitol; Sorbitol; lactic acid; stannous chloride; and 

 glycollic acid. In a number of these cases, sulphite has been present also. 



Other Effects of Sulphite Preservative. — While considered usually as a preservative, 

 sulphite contributes greatly to the action of a developer in other ways. The usual 

 photographic developing agents do not act so rapidly and energetically in the absence 

 of sulphite as in its presence. Thus in spite of the fact that, as a reducing agent alone, 

 sulphite cannot develop a photographic emulsion, it plays an important part in that 

 development. In the brief description of the process of development given under 

 Organic Reducing Agents, mention was made of the fact that one over-all effect 

 of the development process is the increase of the halide content of the developing 

 solution. That does not take place simply as an increase in the ion content of the 

 solution and the sulphite is usually considered as the "halogen acceptor" which acts 

 in the cooperative way required to balance the system. It has also been suggested 

 that the sulphite itself can regenerate the original reducing agents from their oxidation 

 products, while it is itself oxidized to the sulphate. With some reducers, the sulphite 

 may form compounds which are more active as developers than the parent substances. 



A second action of sulphite in a developing solution is its behavior as a silver halide 

 solvent. It is relatively simple to demonstrate the existence of this effect by analyti- 

 cal means, though the action is not rapid and large like the solvent action of hypo. 

 This solvent action has been suggested as a contributing cause of such fine-grain 

 characteristics as the usual metol-hydroquinone-borax negative developers possess. 

 The mechanism suggested for this action is this : The surface of the silver halide grains 

 is dissolved by the relatively high sulphite content of these developers thus increasing 

 slightly their separation and decreasing the chance of clumping during development. 

 With the emulsions used on manj^ printing papers, the sulphite content of the devel- 

 oper is of importance as it influences the tonal quality of the final print. The size 

 of the particles making up the image is influenced by the solvent action of the sulphite, 

 and the resultant deposit may be "cold" and blue-black in tone or "warm" and 

 brown-black in tone. 



Accelerator. — Experience has shown that developing agents increase in activity 

 as the alkalinit}^ increases. Thus the alkali in the developer has been termed the 

 "accelerator." A considerable range of alkalies has been used in different photo- 

 graphic developing formulas. Their primary difference lies in the alkalinity produced, 

 and there is little or no evidence of other or specific effect on the developer itself. 

 Such specific action has been sought in the attempt to determine whether or not one 

 alkali is better than another in'results produced. Considering the complex nature 

 of the reactions occuring, the possibility of such specific effects cannot be ruled out on 

 theoretical grounds but must be the subject of careful experiments. As the result of 

 such tests, the consensus is that the pH or alkalinity produced in the solution is the 

 prime control factor affected by the alkali. 



This statement is not to be construed to mean that the choice of alkali is of no 

 significance, for there are various secondary effects of considerable importance. One 

 of these is the stabihty of the solution. If a low alkahnity is desired, (pH but slightly 



