92 



DISINFECTANTS 



As to permanganate and chromate, their reaction with 

 organic compounds is so ubiquitous that it is impossible 

 to point to any specific reaction as the cause of death. 



7. The Halogens. Chlorine and iodine re- 

 semble permanganate in their very rapid reaction with 

 many different organic compounds, but they are far more 

 powerful disinfectants than permanganate. They are 

 used widely : chlorine in the disinfection of water supplies 

 (which contain practically no organic matter to weaken 

 its action) and iodine externally, in medical practice, 

 mostly as a skin disinfectant. Their reaction with organic 

 matter is so great that the phenol coefficient, which is 

 about 200 when determined in the ordinary way, with % 

 ml. of culture, rises to 2,000 when determined with wash- 

 ed bacteria (see p. 110). 



Chlorine disinfection has several unusual features. One 

 is its great sensitivity to pH. The data by Tilley and 

 Chapin, 1930 (Table 17) will illustrate this. According to 

 Holwerda (1928) the sensitivity to pH can be explained 

 by the reaction of chlorine with water: 



CI2 + H,0 — > HOCl + HCl 

 and 



HOCl — > H+ + OCl- 



TABLE 17 



