ACIDS AND ALKALIES 119 



It will be noticed that to cause a 99 per cent, reduction, the strength of HCl 

 required was 0-0077 normal, whereas that of H2SO4 was rather greater, 0-0096 

 normal. But as the degree of .dissociation was greater with HCl than with H2SO4, 

 the final concentration of H-ions in the two solutions was practically identical. 



From this experiment we may, therefore, conclude that the disinfectant action 

 of mineral acids in high dilution is a function of their degree of dissociation, and 

 hence of their resulting H-ion concentration. Incidentally, we may notice that 

 a considerably higher concentration of acid is necessary to sterilize a bacterial 

 suspension completely than to reduce its numbers by 99 per cent. This point 

 will be dealt with under the section dealing with the physical factors concerned 

 in disinfection. 



The effect of the H-ion concentration of the medium on bacteria suspended in 

 it is rather complex. There is, first of all, an optimum concentration for growth ; 

 for Bad. coli this is about pH 7-6. There is, secondly, an optimum concentration 

 for survival ; for Bad. coli this is about pH 6-0. Thirdly, there is a point at which 

 the acid-tolerance of the organism fails ; this for Bad. coli is about pH 4-6. During 

 growth in a medium containing a fermentable carbohydrate, Bad. coli produces 

 acid, which raises the H-ion concentration of the medium to about pH 5-0, This 

 degree of acidity can be well tolerated, but if the acidity is increased beyond this 

 point, instead of continuing to grow, the organisms cease multiplying and rapidly 

 die. And lastly, there is evidence that the H-ion concentration most suitable 

 for certain fermentative processes is different from the optimum pH for growth 

 (Cohen and Clark 1919, Gale 1940). 



Here, however, we are dealing with the acid-tolerance of micro-organisms, 

 and this limit varies with different species. In Winslow and Lochridge's experi- 

 ments, already referred to, the parts per million of dissociated hydrogen necessary 

 to sterihze a suspension of Bad. coli in 40 minutes were 12-80 ; to sterilize a 

 suspension of Salm. typhi only 4-85 were required. 



Apart, however, from the action of their free H-ions, certain acids have another 

 disinfectant action on bacteria, which appears to be dependent on the nature of 

 the molecule. To produce a 99 per cent, reduction in the number of Bad. coli 

 in 40 minutes, Winslow and Lochridge (1906) found that a 0-0812 N solution 

 of acetic acid, or an 0-0097 N solution of benzoic acid was required. The degree 

 of dissociation of each acid at its respective concentration is only about 1 per cent., 

 so that the amount of dissociated hydrogen in the acetic acid was 1-2 parts per 

 million, and in the benzoic acid 0-1 parts per million. It will be remembered, 

 however, that when HCl was used, 7-49 parts per million were necessary. From 

 this it is evident that the toxic action of acetic and of benzoic acid depends on 

 some other factor than their H-ion concentration. This other factor must be 

 either the anion or the undissociated molecule. There is some evidence that the 

 bactericidal activity of the monobasic series of organic acids increases with increase 

 in molecular weight and decrease in surface tension, while with the dibasic organic 

 acids the reverse holds true (Keid 1932). Halogenation of the fatty acids is said 

 to increase their germicidal power (Tetsumoto 1937). The subject, however, is 

 complex, and no general statement can yet be made ; it will be discussed further 

 in the section dealing with salt action. 



Summary. — (1) The disinfectant action of mineral acids is proportional, not 

 to their normal strength, but to the number of free H-ions per unit volume. 



(2) The organic acids are only slightly dissociated, so that their H-ion con- 



