DEVELOPERS AND THEORY OF DEVELOPMENT 341 



The dates given are those of the earhest literature reference found for this com- 

 pound in photographic developers. 



There are some other organic compounds representing addition products between 

 other aldehydes and bisulphites which have been mentioned to produce the alkalinity 

 needed in developers. 



Before leaving the subject of alkalinity, one other specific effect should be men- 

 tioned, as it appears to have lead to some confusion. This is the effect on pH of the 

 organic reducing agents themselves. Some of these compounds as handled com- 

 mercially are in the form of salts which hydrolize in solution with resultant acid 

 reaction on the solution as a whole. Metol is one particularly common example. If 

 metol is added in increasing quantity to a typical borax developer formula, a point of 



Table IV. — Alkaline Agents for Development 



Approximate pH 

 for Typical 



Working 

 Conditions 



Sodium hydroxide 13.0 



Potassium hydroxide 



Trisodium phosphate (1898) 12.0 



Sodium carbonate 11.6 



Ammonium hydroxide 10 . 7 



Borax (1902) 9.2 



Triethanolamine (1932) 10.1 



Calgon (sodium hexametaphosphate and sodiuni phosphate (1936) 8.5 



Acetone (1902) 



Acetone-bisulphite or sulphite (1902) 



Acetone-formaldehyde (1902) 



Lithium hydroxide 10.5 



Sodium aminoacetate (Pinakol Salt) (1903) 



Sodium silicate (1902) 



Formaldehyde (1890) 



p-Formaldehyde (1890) or trioxymethylene 



Hexamethylenetetramine (1890) 



Formaldehyde-bisulphite compound (1890) 



Trimethylamine (1898) 10 . 



Lithium carbamide (1904) 9.0 



Sodium metasilicate (Metso) (1935) 8.5 



maximum activity is quickly reached such that higher concentrations show less activity 

 than the lower ones. When studied from the view of concentration alone, this 

 behavior is anomalous, but when pH's are determined, it is found that the pH has 

 dropped at the higher concentration, more than offsetting the increase of development 

 rate to be expected from concentration. If now the same experiment is performed 

 with the additional provision that the alkali be changed sufficiently to maintain con- 

 stant pH, the result shows the typical increase of activity with concentration so fre- 

 quentty associated with chemical reactions. 



Restrainer. — In an earlier paragraph it was emphasized that the ability of a 

 reducer to reduce exposed silver halide as distinct from unexposed halide is of primary 

 importance. This differential action is not alone a function of the reducer but is also 

 affected by the other ingredients of the solution and by the conditions surrounding 

 development as well as by the nature of the emulsion being developed. Of these 

 various factors, the last-named, i.e., the nature of the emulsion, is not within the scope 

 of this chapter, but the other two (developer constitution and procedure) are here 

 considered. 



Experience shows that in the majority of formulas greater development differential 

 is produced between exposed and unexposed silver halide when a restrainer is used. 

 The most common chemical for this purpose is potassium bromide and, less commonly, 



