ANTISEPTICS 159 



the same, namely about 25 mg. per 100 cc. Evidently, 

 yeast ceases to multiply when the undissociated acid 

 reaches a certain concentration. 



Salicylic acid has a dissociation constant of 1.06X10'^, 

 and consequently, its efficiency increases rapidly by a 

 decrease of pH below 4, while with benzoic acid which 

 is almost completely undissociated at this acidity, the 

 efficiency is changed but little by a further decrease in 

 pH. But the general principle as to the effect of undis- 

 sociated molecules is the same with both acids as may 

 be seen from Table 32. 



The case of sulfurous acid, which is dibasic and has 

 two dissociation constants, is somewhat different. We 

 have 



For each pH there exist corresponding concentrations of 

 HSOs ions, of SO3 ions and of undissociated H0SO3 which 

 are shown in Table 33. 



The experiments with Bact. coli showed very consis- 

 tently that the growth of this organism is inhibited by a 

 concentration of about 10 mg. of HSO3 ions per 100 cc. 

 The growth of yeast, however, was not inhibited by these 

 ions, but only by the undissociated H0SO3, at concentra- 

 tions of 0.4 mg. per 100 cc, or 4 ppm. 



Rahn and Conn measured also the bactericidal effi- 

 ciency of H2SO3 and found that it is due exclusively to 

 the undissociated acid. It is remarkable that Bacterium 

 coli, of which the growth is so readily inhibited by HSO3 

 ions, can tolerate about 10 times as much undissociated 

 acid as yeast. 



Propionic acid has recently been used as a fungicide 

 in the food industries. According to Kulman (1940), it 

 is more efficient than formic or acetic acid with yeast, but 

 not with bacteria. Ingle (1940) found the free acid more 



