DEVELOPERS AND THEORY OF DEVELOPMENT 335 



indicate that the first inetol was p-methyl-amino-m-cresol. For a number of years 

 now, however, metol has been considered to indicate the sulphate salt of p-methyl- 

 aminophenol. This particular aminophenol salt has probably appeared under the 

 guise of more different trade names than any other developing agent; a few of these 

 trade names being Rhodol, Enol, Elon, Viterol, Scalol, Genol, and Satrapol. 



Metol is soluble in water at 60°F. up to a concentration of 4.8 per cent and soluble 

 to almost the same degree in 5 per cent sodium sulphite solution. It is insoluble in 

 ether and alcohol, thus establishing one test of differentiation between it and p-amino- 

 phenol. The stability of metol, as a developer, is 10, the highest rating according to 

 Strauss. Like p-aminophenol, metol is a soft-working developer but the combination 

 with hydroquinone yields a developer which acts strongly on the weakly exposed 

 portions and also builds strong contrasts. Thus, by varying the concentrations of 

 these two agents, the metol working energetically even on the underexposed areas, with 

 hydroquinone which favors the more exposed areas, developers can be compounded to 

 take care of a wide range of desired results. 



Metol will tolerate relatively large amounts of soluble bromide, in fact, metol 

 developers tend to produce fog unless some bromide is included in the solution. 



Glycine. — Instead of methylating one of the amino hydrogens of p-aminophenol, 

 Bogisch, in 1891, substituted one of the amino hydrogens, by means of methyl carboxyl 

 ( — CH2COOH), to form p-hydroxyphenyl aminoacetic acid or glycine. 



Glycine is practically insoluble in water, alcohol, and ether; thus it is distinguished 

 from the other commonly used developing agents. However, in a 5 per cent sodium 

 sulphite solution at 60°F., glycine will dissolve to a concentration of almost 13 per cent 

 by weight. As a developer it is slow acting but rather powerful. Its action resembles 

 in many respects the characteristics of some metol-hydroquinone (MQ) developers. 

 Even in the absence of soluble bromides glycine produces practically no fog. It yields 

 fine-grained silver images and has been recommended for this type of development, 

 either alone or in combinations, particularly with p-phenylenediamine. Glycine, 

 above all other developers in use today, offers more resistance to oxidation by the air 

 and thus is highly recommended for deep tank work. p-Hydroxyphenylglycine has 

 also been marketed as Iconyl. 



Amidol. — In the year 1891, Andresen introduced 2 : 4-diaminophenol as a 

 developer, under the name Amidol. Amidol belongs to a rather small class of develop- 

 ing agents which can develop in the absence of alkalies; in fact, Amidol will develop in 

 weak acidic solutions, but such development requires prolonged time. Amidol is 

 soluble to concentrations of almost 16 per cent in water at 60°F., but only to 10 per 

 cent in 5 per cent sodium sulphite solution at the same temperature. It is verj- 

 slightly soluble in alcohol and insoluble in ether. 



Amidol is the most energetic working developer in common use and is next to 

 pyrogallol in its instabihty. It builds contrast slowly and tends to produce heavy 

 fog when used in alkaline solutions. It will develop feebly in plain aqueous solution. 

 Generally the alkalinity supplied by aqueous sodium sulphite is sufficient to cause 

 Amidol to develop energetically, and the usual developers of this agent contain only 

 Amidol and sodium sulphite and usually a small amount of potassium metabisulphite. 

 It has sometimes been called the "teaspoon" developer since a teaspoon and a mixing 

 vessel are the only measuring means required to mix the developing solution; the 

 relative proportions of the developer being 1 part potassium metabisulphite, 4 parts 

 sodium sulphite, 1 part Amidol, and 200 parts of water. Because of its instability, 

 various agents have been recommended as preservatives, these include boric acid, 

 glycollic acid, lactic acid, and tin chloride. 



In 1888 Andresen described the developing action of p-diaminobenzene or p-phenyl- 

 enediamine. Andresen found it necessary to use caustic alkali to obtain sufficient 



