334 HANDBOOK OF PHOTOGRAPHY 



below 5o°F. The keeping quality is 3 or about one-third that of metol, according to 

 Strauss. 



In developing action hydroquinone is slow but tends to build strong contrasts. 

 However, a minute trace of metol will accelerate the action of hydroquinone. The 

 dye safranine has also been observed to act as a catalytic agent in starting hydro- 

 quinone to develop. On the other hand, hydroquinone is exceedingly sensitive to 

 soluble bromides, producing high transparency in the slightly exposed areas, and for 

 this reason is an excellent developer for line copy work where pure black and white are 

 required. 



Generally hydroquinone is used in combination with more active developing 

 agents, usually metol. When hydroquinone develops gelatino-silver halide emulsions, 

 particularly if the sulphite concentration is a minimum, the oxidation products of the 

 hydroquinone locally harden the gelatin wherever silver is produced. Use has been 

 made of this action to produce relief images which, being subsequently dyed, are used 

 to transfer dye images in color photography. 



p-Aminophenol. — In 1888 Andresen recommended ?>-aminophenol as a developing 

 agent and thus opened the way to the largest class of developers, the aminophenols. 

 A legion of derivatives of this basic agent have been suggested inasmuch as one of the 

 amino hydrogens can be readily substituted by alkyl, aryl, and heterocyclic groups. 



The free base of p-aminophenol is only sparingly soluble in water, soluble in alco- 

 hol, arid insoluble in ether, while its sulphate, hydrochloride, oxalate, and tartrate 

 salts are readily soluble in water. The hydrochloride salt is soluble to the extent of 

 3 per cent in cold water (60°F.), but this solubility is reduced to 0.3 per cent in a 5 per 

 cent sodium sulphite solution at the same temperature. The stability of a p-amino- 

 phenol developer is 9, or almost as great as that of metol, according to the Strauss 

 scale. The various salts of p-aminophenol have appeared under the trade names: 

 Citol, Diutall, Kodelen, Energol, Freedol, Indianol, Kathol, Para, and Unal. The 

 free base of p-aminophenol forms the active ingredient in the Rodinal type of devel- 

 opers. Relatively large amounts of the salt dissolve in strong alkali, e.g., sodium 

 hydroxide, to form p-amino-sodium phenolate, which is the active developing ingre- 

 dient of the p-aminophenol developers. However, this compound is not very stable, 

 so a small amount of the base itself is allowed to remain in the developer, thus acting 

 as a preservative. The following is typical of various methods suggested for the 

 preparation of the Rodinal type of developer: 



Dissolve 1 g. of potassium metabisulphite in 250 cc. of boiling hot water. When 

 dissolved, add 20 g. of p-aminophenol hydrochloride with constant stirring, and finally 

 add 60 g. of potassium metabisulphite. The mixture is stirred until all the meta- 

 bisulphite has dissolved. Then add, with constant stirring, 40 per cent sodium 

 hydroxide solution until the mixture gradually becomes clear. The addition of sodium 

 hydroxide must be stopped just before the last trace of the cloudj'^ precipitate disap- 

 pears. The solution is then made up to 400 cc. and stored in rubber-stoppered 

 bottles. For use the stock solution is diluted with 20 to 30 parts of water. The high 

 concentration of the stock solution is an advantage under some working conditions 

 and, in fact, is the major advantage of this type of developer. It has appeared also 

 under the trade name of Azol, Aetivol, Artinal, Kalogen, and Paranol. 



p-Aminophenol develops rapidly but does not build up great contrast and tends to 

 produce less fog than most other rapid developers when development is carried out 

 with warm solutions; for this reason, p-aminophenol is largely recommended for use in 

 tropical countries or for warm weather use. 



Metol. — In 1891, Bogisch observed the developing action of methylated-p-amino- 

 phenol and in that same year Hauff introduced metol as a developing agent. The 

 exact structural formula of the earlv metol is not certain and there is evidence to 



