DISINFECTANTS 103 



If such fluctuations occurred in the same laboratory, the 

 results from different laboratories might be expected to 

 vary still more. Accordingly, the deathrate constant was 

 generally considered unreliable, and the concentration 

 exponent never became popular. Standardization was 

 continued by the phenol coefficient. The method was 

 ''improved" by detailed specifications as to medium, 

 time, temperature, bacterial species and strain to be 

 used, and as to the size of the loop employed for its 

 transfers. 



These specifications limit the usefulness of the phenol 

 coefficient because the values thus obtained are correct 

 only in this narrowly limited set of specified conditions 

 and may be of little use with changed conditions. But the 

 limitations made the phenol coefficients more uniform. 



The present Standard Method in the United States is 

 that outlined by the Federal Drug Administration, usually 

 referred to as the F.D.A. method. In order to compute 

 the phenol coefficient of an unknown disinfectant, the low- 

 est concentration of the latter is determined which will 

 kill the test culture in 10, but not in 5 minutes. At the 

 same time, the lowest phenol concentration is ascertained 

 which kills the bacteria in 10, but not in 5 minutes. The 

 ratio of these two concentrations is the phenol coeffi- 

 cient, which tells how much stronger the unknown dis- 

 infectant is than phenol. 



In the above example. Table 19, the concentrations which 

 kill in 10 minutes are 1.04% for phenol and 0.006 for 

 HgCL. The phenol coefficient is therefore 1.04/0.006 = 

 173. The other two values of Table 19, which show the 

 effect of concentration, are disregarded in the definition 

 of the phenol coefficient which is considered correct only 

 for 10-minute exposures, and not for 2.5 or 30 minutes. 



While the very important role of concentration is com- 

 pletely neglected by the F.D.A. method, attention is given 

 to the variability of the test culture. The ''Hopkins 



