CONCENTRATION OF DISINFECTANT 



141 



the time necessary for disinfection. This equation represents the relation when 

 one molecule of one substance reacts with an excess of molecules of a seooncl. 

 For purposes of calculation it may be written 



wlog C + log t = a constant, 

 that is, the relation between log C and log t is a linear one. 

 An example will make this clear (Table 16). 



TABLE 16 

 Disinfection of Paratyphoid Bacilli by Phenol at 20° C. 



In this table the value of n is taken as 5-5 ; the method of calculating this we 

 shall consider presently. It will be seen that the values of the constant are closely 

 similar. If the logarithms of the concentrations are plotted against the logarithms 

 of time, the resulting curve is found to be linear (Fig. 25). 



1-0- 



'^0-9 



0-8 



^0-7 



0-6 



05 



10 



1-5 

 Time, in Li 



Z-0 



2-5 



30 



Fig. 25. 



Disinfection of paratyphoid bacilli with varying concentrations of phenol. Both the numbers 



of the bacteria and time are expressed logarithmically. 



(After Watson, from observations by Chick.) 



For dealing with the salts of the heavy metals, a slight modification of the 

 formula is required, due to the fact that these salts are dissociated in solution, 

 and their action depends not on their molecular but on their ionic concentration. 

 If the concentration of Hg-ions is substituted for concentration of HgClj, for 

 example, then the formula holds good (see Table 17). 



