338 HANDBOOK OF PHOTOGRAPHY 



Glass bottles tightly stoppered are suitable for storage of the dry powers when the 

 quantities involved are suitable. Tin cans, lined with good-grade clean paper and 

 taped to hinder the access of air and moisture, are also frequently used. Larger 

 quantities of some reducers, e.g., hydroquinone, may be stored in strong cardboard 

 containers where conditions are dry enough. The small user, wishing to preserve 

 relatively small amounts will probably find the storage bottle best, and the large 

 user will be guided by the shipping containers provided by the manufacturers. 



Table III. — Reduction- Potential Series 

 (Nietz-Tschibissoff) 



Ferrous oxalate 0.3 



p-Phenylenediamine hydrochloride, no alkali 0.3 



p-Phenylenediamine hydrochloride, plus alkali. . 0.4 



Methyl-p-phenylenediamine hydrochloride, no alkali 0.7 



Phenylhydrazine <1.0 



Hydroquinone 1.0 



2-Hydroxyhydroquinone > 1 . 



p-Hydroxyphenyl glycine (glycin) 1.6 



Hydroxylamine 2.0 



Toluhydroquinone 2.2 



Methyl-p-phenylenediamine hydrochloride, plus alkali 3.5 



Benzyl-p-aminophenol <5.0 



Dimethyl-p-phenylenediamine hydrochloride 5.0 



p-Hydroxydiphenylamine <6.0 



p-Aminophenol 6.0 



Chlorohydroquinone ( Adurol-Hauff ) 6 . 0-7 . 



p-Amino-o-cresol 7.0 



Dibroiuohydroquinone ^ 8.0 



p-Amino-OT-cresol 9.0 



Dimethyl-p-aminophenol sulphate 10.0 



Dichlorohydroquinone 11.0 



Pyrogallol 16.0 



Methyl-p-aminophenol sulphate (metol) 20.0 



Bromohydroquinone ( Adurol-Sohering) 21.0 



p-Methylamino-o-cresoI 23 . 



2:4-Diaininophenol (Amidol) plus alkali 30-40 



Thiocarbamide (Thiourea) 50 . 



All developing formulas make provision for the preservation of the reducers, in 

 solution, by presence of a compound or compounds as preservative. The most com- 

 mon and most universally used preservative is sodium sulphite. It also has other 

 effects in the developer which will be noted later. The theoretical description of the 

 action of sulphite as a preservative is not complete and unambiguous, but the fact 

 of its activity remains. From an elementary point of view its activity is probablj- 

 sufficient to consider it as a substance which itself may be oxidized to sodium sulphate 

 and is preferentially oxidized with consequent preservation of the reducer. In most 

 formulas the sulphite is present in three to ten times the concentration of the reducers, 

 and this is probably vital to the prolonged life of the solution, but even small quan- 

 tities of sulphite make relatively large differences to the initial rate of aerial oxidation 

 of reducers. 



In some formulas bisulphite or metabisulphite is used instead of the sulphite 

 itself, but in alkaline solution the result is essentially the same. 



The simple description just suggested of the preservative action of sulphite does 

 not cover adequately all the facts, as hydroquinone, a developing agent, itself preserves 

 a sulphite solution. 



Experience has shown that the presence of two reducers in a developing formula 

 frequently decreases the susceptibility of a developer to aerial oxidation. Metol 

 preserves Amidol when used together in suitable formulas. Hydroquinone preserves 



