120 



DISINFECTION 



centration is relatively low. As, however, they have a markedly germicidal effect, 

 it must be concluded that this is a property of the whole molecule or of the anion 

 and is specific for each acid ; acetic acid has, for example, only 10-20 per cent, 

 of the toxicity of benzoic acid. 



(3) Certain other acids, such as fluoric acid and nitric acid, have a specific 

 action, which is probably due to the anion. 



Alkalies.- — By similar experiments to those described in the section on acids, 

 Kronig and Paul (1897) showed that the disinfectant action of alkalies was dependent 

 on their degree of dissociation, and hence on their concentration of OH-ions. 



Thus, of the bases KOH, NaOH, LiOH, and NH4OH, KOH shows the highest 

 degree of dissociation, and is hence the most actively germicidal ; NH4OH is 

 dissociated the least and is the least actively germicidal (Table 6). 



TABLE 6. (Kronig and Paul 1897.) 

 Disinfection of Anthrax Spores by Alkalies. Initial number of spores was about 6,800 



When we turn to other bases, we find exceptions. Thus, Ba(0H)2 is less dis- 

 sociated than KOH, but is very much more toxic ; similarly with the hydroxides 

 of the other alkaline earths. The reason for this, as we shall see in the section 

 on salts, is that the metallic ion is frequently highly toxic, and assists the hydroxyl- 

 ion in its germicidal activities. 



Summarizing, we may say that tmless a toxic metallic ion is present, the dis- 

 infectant action of an alkali is proportional to its degree of dissociation, and hence 

 to its concentration of hydroxyl-ions. 



Just as bacteria possess a limit of acid-tolerance, so they possess a limit of 

 alkali-tolerance. Cohen (1922) found that for Salm. typhi this was about pH 8-7. 

 It is of interest to note that H-ions appear to be more toxic than OH-ions in similar 

 concentration. 



Salt Action.— Though this chapter primarily concerns the bactericidal action 

 of various physical and chemical agencies, it is convenient to introduce here the 

 subject of salt action in general. 



We have seen that distilled water cannot be considered a satisfactory men- 

 struum for bacteria. Many of the vegetative organisms die rapidly in it, and 

 few survive for long. Ficker (1898) was the first to make direct observations on 

 the action of physiological saline on bacteria. His results showed that instead 

 of being harmless, it was actively bactericidal. Subsequent workers have con- 

 firmed his observations, and have demonstrated that the bactericidal effect is 

 due to the toxicity of the sodium ion. 



Pelepine and Greenwood (1914), working with a number of heavy metals — 



