340 HANDBOOK OF PHOTOGRAPHY 



greater than 7.0) it would be unwise to secure it bj- use of a small quantity of strong 

 alkali such as sodium or potassium hydroxide because relatively small quantities of 

 other chemicals, such as carbon dioxide from the air, could react with the hydroxide 

 and change the pH considerably. It is better to secure the low alkalinity by higher 

 concentration of weaker alkali. Of course, where high alkalinity, high pH, is desired, 

 the stronger alkalies must be used, but sufficient concentration is then used to avoid 

 rapid changes from introduction of small quantities of other agents. In some for- 

 mulas the solutions are actually buffered in the chemical sense of that term, to insure 

 maintenance of constant pH in the face of any factors which otherwise might normally 

 be expected to affect it. In many other cases true buffering action is not obtained, 

 liut a concentration of alkaU sufficient to prevent pH change from minor influences 

 is used. 



Ammonia is iised as a developer alkali sometimes but its volatility militates against 

 its use where stability in solution is necessary. Ammonia is a solvent for silver 

 halides, particularly silver chloride, and this factor also limits its utility as an alkali. 

 Its use in developers is not widespread, though occasional!}^ special advantages have 

 been claimed from it. 



A second factor sometimes determining the choice of alkali is the effect of its intro- 

 duction into the fixing bath. Sodium carbonate is the alkali most frequently used in 

 developing formulas, but at times bhsters are produced in enaulsions when transferred 

 from the alkaline developer to the acid fixer. This blistering is most apt to occur when 

 the processing is being conducted at adversely high temperatures and is due to the 

 formation of carbon dioxide bubbles from the reaction of the carbonate with the acid 

 of the hypo. Borates, phosphates, and compounds of that nature do not form 

 gaseous compounds under similar conditions and hence do not cause blistering. 

 Boron compounds, particularly^ in the form of borate ions, have been used as additions 

 to fixing baths to reduce sludging tendencies and increase the hardening life of the 

 bath. Accordingly, the choice of borates as alkalies for developers is a particularly 

 happy one, where suitable alkalinity can be obtained with them, not because of any 

 specific effect of borates on development per se, but as an advantage to the fixing bath. 



One specific combination used in so-called "buffered borax" formulas should be 

 mentioned. That is the use of both borax and boric acid to secure the desired alka- 

 linity. The pH of the bath can then be varied by varymg the ratio of the two, giving 

 good control of developer activity without resorting to low concentration for low pH. 

 As borax alone does not give a very high pH, this combination cannot be used to 

 .secure rapid-acting contrasty formulas but is available for the slower softer ones fre- 

 quently needed in development of negatives. 



Sodium sulphite itself is weakly alkaline, and hence a developing fornmla as simple 

 as sulphite and metol has been used upon occasion. It develops film but is rather 

 quickly exhausted. 



In addition to the direct effect on reducer activity, the pH of the solution markedly 

 influences the swelling of the gelatin. Excessively high pH softens and swells the 

 gelatin to the point of damaging it. Specially hardened emulsions are required where 

 processing necessitates the use of the most extremely alkaline formulas. With ordi- 

 nary emulsions this softening sets a limit to the alkahnity which may practically 

 be used. 



The pH of developing formulas cannot be judged accurately from the kind ami 

 quantity of alkah alone, but as a guide to the alkalinity which can be- obtained with 

 various alkalies, the following table has been prepared. Some of the compounds Hsted 

 are not alkalies in the chemical definition of the term, but in solution with the other 

 compounds, particularly .sulphite, reactions take place producing the necessary 

 alkalinit3\ 



