158 



ANTISEPTICS 



and are given in Table 32. The preventive dose va- 

 ried from 50 mg. of sodium benzoate per 100 cc. of 

 medium, at pH 3.5, to over 2,500 mg., at pH 6.5. These 

 values, multiplied by 0.847\ give the amounts of free 

 benzoic acid, and the figures thus obtained multiplied by 



TABLE 32 



Inhibition of growth of Saccharomyces ellipsoideus by benzoic and 

 salicylic acid. (From Rahn and Conn, 1944. Courtesy of "Industrial 

 and Engineering Chemistry.") 



pH 



Undissociated 

 fraction 

 1 — 



Inhibitory concentration, in mg. per 100 cc. 



Sodium salt 

 Growth No growth 



Undissociated acid 

 Growth No growth 



Sodium Benzoate 



Sodium Salicylate 



the undissociated fraction (1-a), which is characteristic 

 for a given pH, furnish the concentrations of undisso- 

 ciated acid. The last column of Table 32 shows that, 

 even with great differences in the amount of added ben- 

 zoate, the amount of undissociated acid remains nearly 



1. Ratio of molecular weight of benzoic acid to that of sodium 

 benzoate. 



