CHAPTER IV 



DETERMINATION OF THE GERMICIDAL POWER OF CHEMICAL 



DISINFECTANTS 



To accurately determine the amount of a particular chemical required to 

 kill all pathogenic bacteria, under varying conditions of environment, is prac- 

 tically impossible. The germicidal power of a chemical disinfectant varies in 

 different media and at different temperatures; some which are powerful germi- 

 cides in media free of albumin are nearly inert when associated with albumin; 

 some are powerful germicides when in solution and less germicidal when mixed 

 with a substance that throws them out of solution. 



An estimation of the germicidal power of a given substance may be made 

 which gives valuable information regarding the proper employment of that sub- 

 stance as a disinfectant under various conditions. 



This is done by obtaining typical pure cultures of as many different organ- 

 isms as possible staphylococci, typhoid, colon, diphtheria, tubercle, anthrax 

 and tetanus bacilli, etc. mixing different amounts of the disinfectant with a 

 fixed amount of the bacteria, both with and without the admixture of organic 

 matter, letting the mixture stand at a given temperature for a certain time and 

 then testing the mixture for sterility by making subcultures from it. 



This is a very extensive, time-consuming procedure and at best yields only 

 approximate results. Different strains of the same organism under identical 

 conditions often show variations of several hundred per cent, in their resistance 

 to a chemical disinfectant. 



Within certain limits the following conditions influence the action of chemical 

 disinfectants: 



1. Length of time the disinfectant is in contact with bacteria. 



2. Temperature at which the contact occurs (at low temperatures disin- 

 fectants are much less active than at high, for every rise of 10 the efficiency is 

 increased two- to tenfold. 



3. Physical and chemical properties of the medium in which the infective 

 organisms exist. 



4. The amount of disinfectant and the number and nature of the bacteria. 

 On account of the many factors that complicate and limit approximate 



determinations of the disinfectant power of chemicals, such determinations were 

 largely neglected until recent years with a consequent development of gross 

 misconceptions and inefficient use of chemical disinfectants. 



In 1898 Rideal and Walker introduced a test, known as the " Rideal- Walker 

 Carbolic Acid Coefficient Test," for the determination of the relative disinfect- 

 ant power of chemicals, in comparison with that of carbolic acid. Later this 

 test was improved upon by the introduction of one known as the "Lancet 



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