PREPARATION OF MEMA. 69 



APPENDIX A. 



PREPARATION OF MEDIA. 



I. Bouillon. 



1. To 1 liter (1,000 cc.) of water add the following sub- 

 stances : 



10 gms. peptone. 

 5 gms. NaCl. 

 5 gms. Liebig's beef extract. 



2. Boil until dissolved and restore to the original volume 

 with water. 



3. Titrate thus: By means of a pipette, transfer 5 cc. of 

 the solution to a porcelain dish. Add 45 cc. of distilled water 

 and heat to boiling. Titrate while hot with -^ NaOH using 

 phenolphthalein as an indicator. A faint pink color indicates 

 the end point. The reaction of the medium is expressed in 

 degrees of Puller's scale, which represent the number of cc. 



of acid or alkali present in 1,000 cc. of the medium. A minus 

 ( — ) sign indicates alkalinity, and a plus (-|-) sign acidity; 

 e. g., — 20° would indicate that the medium was twenty de- 

 grees alkaline or that 20 cc. of normal acid must be added 

 per liter to neutralize it. 



The reaction of the bouillon is to be -|- 10° Puller's scale or 

 Ifo acid and it must be adjusted to that point by adding 

 normal NaOH or HCl as required; e. g., suppose 30 cc. of 

 Jij- ISTaOH were required to neutralize 5 cc. of the medium, 

 then 100 cc. would require 3.0 cc. of normal NaOH, or a liter 

 would require 30 cc. of normal NaOH. The bouillon in this 

 ease would be 30° acid. The optimum reaction, however, 

 being 10° acid, 20 cc. of normal FaOH must be added per 

 liter of bouillon. 



