558 HANDBOOK OF PHOTOGRAPHY 



Ferric chloride (crystals) 25 g. i^i oz. 



Potassium citrate 75 g. 10 oz. 



Sodium sulphite (desiccated) 30 g. 4 oz. 



Citric acid 20 g. 'ZVs oz. 



Sodium thiosulphate crystals 200 g. 1 lb. 1 1 oz. 



Water to make 11. 1 gal. 



Potassium Permanganate Reduce?: — A dilute solution of potassium permanganate 

 acidified with sulphuric acid may be classified as a subtractive reducer, although like 

 the modified Belitzski's reducer, it shows characteristics intermediate between sub- 

 tractive and proportional reducers. The solution is not stable in contact with the 

 air for more than a few hours when a single-solution reducer is used. Accordingly 

 the reducer may be made up in two stock solutions as follows: 



Solution 1 



Potassium^ permanganate 1 . g. l^i oz. 



Water to make 0.5 1. 32 oz. 



Solution 2 



Sulphuric acid (concentrated) 5.0 cc. 1 oz. 



Water to make 0.51. 32 oz. 



The working solution is made up of one part of each of the stock solutions. 



After the desired reduction has been obtained, the film should be immersed in a 

 1 per cent solution of sodium bisulphite for several minutes, or in an acid fixing bath, 

 and should then be rewashed. This reducer has the characteristics of a subtractive 

 reducer, but by decreasing the amount of sulphuric acid or by using a weak acid, such 

 as acetic acid, the reducer is more nearly proportional in its characteristics. 



Proportional Reducers. Krauss' Ferric Ammonium Sulphate Reducer. — A reducer 

 having little or no action on low values of density but appreciable action on densities 

 greater than about 0.5 may be made of ferric ammonium sulphate in acid solution. 

 The formula, originally suggested by Krauss, but modified by Crabtree and Muehler 

 is 



Ferric ammonium sulphate 15 g. 0.5 oz. 



Sulphuric acid (concentrated) 10 cc. 0.3 oz. 



Water to make 11. 32 oz. 



In order that stains may not result, it is essential that the film to be reduced be 

 freed of hypo and silver salts and that the film be not permitted to come into contact 

 with the air during reduction. A disadvantage of this reducer is that it softens the 

 gelatin during treatment, so that prehardening in a formalin bath is desirable. 



The film is immersed in the above solution until the desired reduction is obtained, 

 but this should not exceed about 8 min., as otherwise the low values of densities are 

 reduced and the property of proportional reduction is no longer maintained. Treat- 

 ment for 6 min. gives about 30 per cent reduction in density. 



Nietz and Huse Reducer. — Subtractive and superproportional reducers may be 

 combined to produce a proportional reducer. Such a proportional reducer may be 

 made by combining potassium permanganate and ammonium persulphate reducers. 



The following, worked out bj' Nietz and Huse, is a satisfactory formula for pro- 

 portional reduction: 



Solution 1 



Potassium permanganate . 25 g. 38.0 gr. 



Sulphuric acid (10 per cent) 15 cc. 0.25 oz. 



Distilled water to make 1 1. .35 oz. 



Solution 2 



Ammonium persulphate 25 g. . 75 oz. 



Distilled water to make 1 1. 35 oz. 



For use one part of solution 1 is added to three parts of solution 2. The keeping 

 properties of the combined solutions is very poor so the solutions should be mixed 



