DEVELOPERS AND THEORY OF DEVELOPMENT 337 



gradation with p-phenylenediamine developers. In 1904 Lumi^re and Seyewetz 

 recommended this agent as a substitute for physical development to produce fine- 

 grain images particularly suitable for lantern slides because of the fineness of the 

 deposit and the pleasing tone of the image. 



p-Phenylenediamine base is slightly soluble in cold water and fairly soluble in 

 5 per cent sodium sulphite solution. The hydrochloride salt is soluble in water and 

 insoluble in ether and alcohol. The dry developer exerts a rather foxic action on the 

 human skin, particularly so in the case of persons who are allergic to its constituents. 



The fine-grain characteristics of p-phenylenediamine developers, noted as early 

 as .1904, have been widely applied in recent years for the development of miniature 

 films. This particular characteristic is due, in part, to the fact that the developing 

 energy of p-phenylenediamine is the least of those developing agents thus far evaluated 

 by the reduction-potential method. The second probable cause for the fine-grain 

 developing action of this developer is the solvent action exerted by the diamine on 

 silver halides. 



Owing to its weak developing energy, developing agents of more energetic action 

 have been combined with p-phenylenediamine, either in the dry form or in the develop- 

 ing solution. These "booster" agents include: metol, Amidol, glycine, hydroquinone, 

 and pyro. 



Early in 1900, p-phenylenediamine was marketed in this covmtry under the name 

 of Metacarbol. 



Method for Identification of Developing Agents. — The above developing agents 

 and a number of those mentioned previously are included in the scheme for their 

 identification, which was arranged by Plauman and given in Table II. 



Reduction Potential and Velocity Constant. — The many compounds which have 

 been found to be developers differ rather widely in their activity and power. It has 

 proved difficult to compare them completely, since some will develop under conditions 

 of temperature and alkalinity in which others are essentially inert, yet with change 

 of conditions, the relative activity may change considerably. To bring a measure of 

 order out of the complex situation, the concept of "reduction potential" has been 

 introduced as one measure of developing power or activity. Reduction potential is 

 measured by the ability of the reducer to overcome the restraining action of potassium 

 Ijromide. With some developers, those of low reduction potential, there is produced a 

 considerable reduction in density by the addition of a small standard quantity of 

 bromide. With others of high reduction potential there is less density depression by 

 a similar bromide addition. For purposes of numerical comparison, hydroquinone is 

 assigned a value of 1 in the reduction-potential scale. 



Table III gives the reduction potentials found for a number of developing agents. 



Another property which seems closely allied to, but not identical with the reduction 

 potential, is the rate of development, as represented by the "velocity constant" (see sec- 

 tion on Development Quantitatively Considered). In general, reducers having high 

 reduction potential also have high velocity constant, and, similarly, reducers of low 

 reduction potential have low velocity constants. In spite of this, as a general trend, 

 the literature indicates that small differences of reduction potential may not be repre- 

 sented by corresponding differences in velocity constant. 



Preservative. — -Reducers of the type described are readily oxidized by the air 

 and provision must be made in the compounding and use of developing solutions to 

 guard against deterioration through such aerial oxidation. The organic compounds in 

 common use keep reasonably well in the dry state, but when damp or actually in 

 solution, deterioration may become very rapid. Pyro shows the greatest rate of 

 deterioration of the common reducers and consequently requires the greatest care, 

 but suitable storage and mixing procedures must be provided for all the reducers. 



