DISINFECTANTS— STANDARDIZATION 11 



placed in their respective test tubes, all is placed in a 

 water bath so that the solutions may be brought to a 

 temperature of 20 degrees Centigrade. A standard so- 

 lution of pure phenol is made and standardized by the 

 United States Pharmacopeia method to contain a five 

 per-cent solution by weight. Dilutions are made fresh 

 from this each day. "When everything is ready, 1-10 

 cubic centimeter of the culture is added by the pipette 

 to each of the dilutions in the seed tubes. 



The solutions are planted from the seed tubes into 

 the culture tubes every two and one-half minutes up 

 to fifteen minutes, and for this a 4-millimeter platinum 

 loop. United States standard, 23-gauge wire is used. 



In adding the culture to the dilution the best method 

 is to tip the test tube at an angle of forty-five degrees, 

 lightly touch the pipette against the side of the tube 

 below the surface line, and then shake gently. The 

 broth tubes are placed in the incubator at 37 degrees 

 Centigrade for forty-eight hours. 



The mean between the strength and time coeflS^cients 

 is used for determining the coefficient. To determine 

 the coefficient, the figure representing the degree of dilu- 

 tion of the weakest strength of the disinfectant that 

 kills within two and one-half minutes is divided by 

 the figure representing the degree of dilution of the 

 weakest strength of the phenol control that kills within 

 the same time. The same is done for the weakest 

 strength that kills in fifteen minutes. "The mean of the 

 two is the coefficient. 



As has been stated before, the coefficient simply rep- 

 resents the germicidal power of the disinfectant tested, 

 relative to the power of phenol on the same organisms 

 under the same conditions, and should be accepted only 

 as such. 



