68 DISINFECTANTS 



where K is the deathrate constant at the concentration 

 c = l. 



The value of n varies greatly with different disin- 

 fectants. It may be as low as V2 (which means that the 

 deathrate constant is proportional to the square root of 

 the concentration) as Paul and associates (1910) found 

 for staphylococci exposed to hydrochloric acid. With 

 many common disinfectants it is near 1, as, e.g., with 

 formaldehyde, hydrogen peroxide, metal salts, and some 

 acids. In this case only, the death rate is directly pro- 

 portional to the concentration. The highest coefficients 

 recorded (see below. Table 15) were obtained with alcohols 

 at concentrations of 26 to 40% ; but these high values may 

 be partly due to osmotic effects or to partial dehydration 

 of the protoplasm of the cells. Otherwise the highest 

 values, which are in the neighborhood of 6, were obtained 

 with phenol and its homologues. This makes the phenol 

 group quite different from other disinfectants. 



We can calculate n from the deathrate constants Ki and 

 ATo at two different concentrations Ci and C2. The rela- 

 tion 



K, 



K, 



i-^j 



gives 



n = 



log Ki - log K2 



log Ci - log C2 



Using the numerical values of Table 14, we have 



log 0.22 - log 0.08 ^ Q ^3 

 log 64 - log 16 



The concentration exponent n can also be determined 

 from the death times, since the death times in experi- 

 ments with identical inocula are inversely proportional to 

 the deathrate constants : 



