DISINFECTANTS IQI 



VII. STANDARDIZING OF DISINFECTANTS 



The thorough quantitative studies of disinfectants be- 

 tween 1890 and 1900 made it possible to compare their 

 eniciency, and Rideal and Walker in 1903 proposed to use 

 phenol as the standard of comparison because it is a 

 reliable, not very selective disinfectant that can be ob- 

 tained chemically pure at reasonable cost anywhere. 

 Their method consisted in inoculating various dilutions 

 of phenol, as well as of the disinfectant under test, with 

 equal amounts of bacteria and determining the death 

 times after transfer into broth. Dilutions of equal death 

 times were compared and their ratio was believed to rep- 

 resent a reliable index of the relative disinfectant actions. 

 Thus a '^ phenol coefficient" was obtained which told how 

 many times more efficient the substance investigated was 

 than phenol. 



This method received a severe jolt when Chick and 

 Martin (1908) showed that the phenol coefficient of sev- 

 eral common disinfectants varied very much when the 

 death time was changed (see Table 19). Watson (1908) 

 explained this variation quite simply. Chemical reac- 

 tion rates are not always proportional to the concen- 

 tration, but to the concentration raised to a certain 

 power, and, with phenol, the concentration exponent is 

 about 6. No other disinfectant except the homologues 

 of phenoP has such a high exponent (a list of concentra- 

 tion exponents is given in Table 15). The action of most 

 disinfectants is approximately proportional to their con- 

 centration, and the decision to use phenol, with its ab- 

 normal concentration exponent, as a standard, was very 

 unfortunate. 



Phelps (1911) pointed out that to characterize a dis- 

 infectant, we need only to know the death rate and the 



1. Concerning the exceptional values for alcohols see p. 



68. 



