FIXING, WASHING, AND DRYING 381 



The first term on the right side of the equation represents the metalUc silver which 

 remains on the photographic film and is unaffected (for the most partO by the sodium 

 thiosulphate. The two middle terms represent compounds in solution in the hypo 

 bath, while the last term shows how much hypo remains. Several silver-sodium 

 thiosulphate compounds beside that shown are possible, and it is probable that a 

 number of different ones coexist in equilibrium in the solution. Which particular 

 compounds are present depends upon the concentration of silver or upon the exhaus- 

 tion of the solution. A highly exhausted hypo bath may contain an appreciable quan- 

 tity of NaAgS203, which is only slightly soluble and somewhat unstable. Because 

 of the low solubility, this compound will be difficult to wash out from the film. It is 

 also unstable and may break down to form silver sulphide (Ag2S), which produces 

 stains. It is important to observe that the amount of fresh hypo solution has been 

 reduced from XNa2S20s to (X — 4)Na2S203, so that it will be less effective in subse- 

 quent fixations than in the original bath. 



If only metallic silver and silver halide were brought over into the fixing solution, as 

 indicated in the equation above, sodium thiosulphate would be a satisfactory solvent 

 for the halide and would be a suitable fixing solution. However, the film carries over 

 into the fixing solution a certain amount of the developer solution. The developer 

 solution in the hypo oxidizes, and the oxidation products produce stains. This 

 situation may be prevented through the use of a stop bath or, if this is not convenient, 

 by adding. to the fixing bath some substance which prevents the oxidation of the 

 developer solution in the fixing bath, as well as an acid to neutralize the alkalinity 

 of the developer. The addition of the acid (which is usually sodium sulphite) in the 

 fixing bath also aids in preventing oxidation of the developing agent. 



While fixing baths containing the silver halide solvent (hypo), an oxidation 

 preventive (sodium sulphite) and an acid or acid sulphite (such as acetic acid or 

 sodium bisulphite) produce a suitable fixing solution, a hardening agent is also usually 

 added. This is desirable because the hardened film is not so easily damaged in 

 handling and in subsequent operations as a nonhardened film. The three most impor- 

 tant gelatin hardeners are: (1) potassium alum [K2S04-A1(S04)3-24H20]; (2) chrome 

 alum [K2S04-Cr2(S04)3-24H20]; and (3) formalin, which is a 40 per cent solution of 

 formaldehyde (HCHO) in water. 



Unless a stop bath is employed, free acid in a fixing bath is a necessity to prevent 

 stains due to oxidation of the developer in the fixing bath. So far as the neutraliza- 

 tion of the alkalinity of the developer is concerned, any acid might be used. Prac- 

 tically, however, the use of a strong acid results in the liberation of sulphur which in 

 turn reacts with metallic silver to produce stains of silver sulphide, according to the 

 equations : 



Na2S203 + 2HC1 = 2NaCl + H2SO3 + S (2) 



2Ag + S = Ag2S (3) 



Mechanism of Fixing. — In a given period of time and with active agitation of a 

 fresh fixing bath whose volume is large compared to that of the material being fixed, 

 a certain portion of the silver bromide originally present in the photosensitive material 

 will be dissolved. In the next unit of time the same fraction of the silver which 

 remains after the end of the first time interval will be dissolved, and so on. In timc^ 

 intervals of the same duration the same fraction of the remaining silver will be dis- 

 solved. The amount of sih^er dissolved in each time interval will decrease with time. 

 The dissolving of the silver bromide is therefore a geometric or exponential process 



1 See Russell, H. D., and J. I. Crabtree, The Reducing Action of Fixing Baths on the Silver 

 Image, J. Soc. Motion Picture Engrs., 18 (No. 3), 371-397 (1932), for a study of deviations from this 

 general statement. 



