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power. They all have the same chemical formula C1H, (OH)s, i. e. the same chemical composition, but differ in constitution, and their usefulness as developers is seen to depend on the position of the substitution groups. It may be added that the para compounds are usually more powerful or more energetic as developers than the ortho compounds. Pursuing the substitution idea further, Adurol Hauff replaced the hydrogen atom (H) in the second position (Fig. 3) in hydro-OH quinone (Fig. 8) by chlorine (Cl), thus: with the formula CaHaCl (OH)s, a halogen /'substitution product, mono-chlor-hydroquinone to which he gave the name AdurolHauff. Schering introduced or substituted '-/ bromine in the same way, obtaining monobromo-hydroquinone with the formula <toj1 oJ CaHaBr(OH)s, these modifications being . ur said ~o give adurol definite advantages as compared with hydroquinone as a developer, the mono-chlor compound being preferable to the mono-brom product. Di d Trl Pyrocatechin, hydroquinone and 0z;~;henol; adurol are grouped ~ogether as bi-oxybenzenes. Pyro, wh1ch has been used as a developer since x8sx, is a tri-oxy-benzene, the hydrogen (H) atoms in positions I :2 :3 (Fig. 3) being replaced by three hydroxyl groups thus:

I 1-CI



-OH +COa

' Pyro -QH (11)

which shows how closely related it is to hydroquinone and pyrocatechin. Am"d d What has been said of the substitution Othe~ gr-:pa or introduction of two hydroxyl (OH) groups in obtaining di-hydroxy-benzene derivatives, holds good in the use of two amido (NHa) 路 groups, which gives us a similar series of ortho, para,

Modern Photographic Developers  
Modern Photographic Developers  

A Practical Handbook to the New Developers Telling What They Are and How to Use Them with Reliable Formulae by The Photo Miniature, January...