FIXING, WASHING, AND DRYING 385 



of the hypo bath by the acid carried from the stop bath into the fixing bath. If these 

 precautions are not taken and the photographic materials are delivered to the fixing 

 bath directly from the developer solution, the fixing bath soon becomes discolored 

 from the oxidized developer, and these oxidized products stain the negatives and 

 prints. With a warm fixing bath a tendency exists for the gelatin to swell and to 

 become soft, which may easily result in frilling, reticulation, melting of the emulsion 

 from the gelatin base or other difficulties. 



The only problem likely to arise in the determination of the composition of a plain 

 fixing bath is that of determining optimum concentration. Various investigations 

 indicate that the maximum speed of fixing occurs when the hypo concentration is 

 about 30 to 40 per cent. 



Add Hardening Fixing Baths. — -A good summary of the properties and character- 

 istics of acid hardening fixing baths has been given by Crabtree and Hartt^ from which 

 the following material is taken. 



An acid hardening fixing bath contains the following ingredients: (1) a silver 

 halide solvent, (2) an antistaining agent, (3) a preservative or sulphurization inhibitor, 

 and (4) a hardening agent. 



Although many solvents of silver halides are available, the most satisfactory for 

 fixing purposes are sodium and ammonium thiosulphates. A solution of ammonium 

 thiosulphate of given concentration fixes more rapidly than a corresponding solution 

 of sodium thiosulphate, but ammonium thiosulphate is more expensive, and commer- 

 cial grades are apt to contain free sulphur. For most purposes sodium thiosulphate 

 fixes sufficiently rapidly and is entirely satisfactory. 



Practically any acid will function as an antistaining agent because it has merely to 

 neutralize the alkali in the developer carried over by the films and thereby retard 

 oxidation of the developing agent. Organic acids are more suitable than inorganic 

 acids because they are less dissociated and therefore have a correspondingly smaller 

 tendency to precipitate sulphur from the fixing solution. Acetic acid has been found 

 to be the most generally efficient for use in fixing baths, but much is still desired, from 

 the standpoint of a perfect acid, for this purpose. 



It is imperative, when compounding an acid fixing bath, to have a quantity of 

 free acid present to prevent discoloration of the bath by developer oxidation products 

 and to prevent the precipitation of the hardening agents by the alkali in the developer. 

 This free acid tends to cause precipitation of sulphur from the hypo especially at 

 temperatures above 70°F. It is therefore necessary to add some substance which will 

 prevent the precipitation of sulphur without impairing the antistaining properties of 

 the acid. 



Two types of substances act in this manner. The first of these consists of the 

 alkaline sulphites of which sodium sulphite is the most common. Since hypo decom- 

 poses in the presence of acid to form sodium sulphite and sulphur, it is considered that 

 the addition of sulphite to an acid fixing bath retards the decomposition of the hj^po 

 because it tends to reverse the action by virtue of its mass action. 



The second type of preservative consists of the alkali salts of organic acids which 

 are commonly referred to as buffer salts; sodium acetate is a typical example. The 

 effect of this type of substance is to buffer or reduce the hj^drogen-ion concentration 

 of the acid employed below the limits at which sulphur is precipitated from hj^po by 

 acids. 



The hardening of gelatin may be either temporarj^ or permanent. Temporary 

 hardening agents raise the melting point and prevent the swelling of the gelatin only 

 while the gelatin is in contact with the hardening solution. A concentrated solution 



1 Crabtree, J. I., and H. A. Haktt, Some Properties of Fixing Baths, J. Soc. Motion Picture Engrs., 

 13 (No. 38), 364-405 (1929). 



