144 



DISINFECTION 



To find the value of d 



V 



_ _ (T- T) 



A; 



610 = 



0-7 



0-27 



= 2-592 



d = 2-5920-' 



= M 



6 = M for r C. 

 = Mio or 2-6 for 10" C. 



A similar experiment is shown diagrammatically in Fig. 26. 



Most observers have obtained 

 higher values of for phenol — 

 generally about 7 or 8 for each 

 rise in temperature of 10° C. 

 For HgClj the temperature co- 

 eflficient is lower, generally 2 to 

 4 for 10° C. 



An alternative method of 

 estimating the value of is to 

 start with a suspension of organ- 

 isms the number of which need 

 not be estimated, and determine 

 how long it takes for complete 

 sterilization at two or more 

 different temperatures. Since 

 the time taken for the com- 

 pletion of a reaction may be 



^^'iSf 2"„?! !it^i'":f„f"i*"',:£P!:^^7„?°!il'!!!i'" considered as inversely propor- 



tional to the velocity, there is 

 no need to estimate the value 

 of k. Thus : 



700 



600 



r 500 



ra400 



200 



100' 



25 30 35 

 n Minutes 



Fig. 26. 



with 0-6 per cent, phenol at different temperatures. 



Continuous curve = 11° C. 



Interrupted curve = 21° C. 



(After Chick.) 



Phenol 6 per 1,000. 

 11° C. 

 21° C. 



Paratyphoid bacilli. 

 Time elapsing in minutes. 



2-2 



1-0 



Then 6 



2-2 



= 2-2 for 10° C, or 2-2"i = 1-08 for 1° C. 



In disinfection by hot water a very much higher value is obtained for d. Thus 

 Chick (1910), working with Salm. typhi at temperatures of 49° C. and 54-1° C, 

 found that the velocity constant of the reaction was increased 13-1 times for the 

 5° C. rise in temperature, i.e. d = 1-67 for 1° C, or about 170 for 10° C. 



The consistent effect of rise of temperature on the velocity of disinfection 

 points to a close analogy with an ordinary chemical reaction. Arrhenius elabor- 

 ated a formula, which has been found to be applicable to many chemical re- 



