THE PREPARATION OF CULTURE MEDIA 137 



needed. Five c.c. of the medium to be tested is measured accurately in a 

 carefully washed pipette and transferred into a porcelain evaporating' dish. 

 To this are added 45 c.c. of distilled water. The mixture is thoroughly boiled 

 for three minutes over a free flame. The boiling drives off. C0 2 , giving the 

 true neutral point, and approximates the conditions prevailing during the 

 further sterilization of the medium from which the 5 c.c. have been taken. 

 After boiling, 1 c.c. of the phenolphthalein is added. If the medium is acid, 

 no color is present; if alkaline, a pink or red color appears. The ^~ alkali 

 or acid solution is allowed to drop into the dish from a graduated burette. 

 When the neutral point is approached in an acid solution, each drop of 

 sodium hydrate added brings forth at first a deep red, which, however, upon 

 slight stirring with a clean rod, completely disappears. 3 The end reaction 

 is reached when a faint but clear and distinct pink color remains in the fluid 

 after stirring. 



When titrating alkaline media, the addition of the phenolphthalein 

 produces a red color in the hot medium which gradually fades upon the 

 addition of ^ HC1, becoming colorless at the end point of titration. Titra- 

 tion should be done quickly and in a hot solution. From the result of the 

 titration the computation for the neutralization of the entire bulk of the 

 medium can be made by a simple arithmetical process as illustrated in the 

 following example: 



Let us suppose that we have used: 



2.5 c.c. of^L_NaOH to neutralize 5 c.c, of the medium, 



then 2.5 c.c. of * NaOH will neutralize 100 c.c. " " 



and 25 c.c. of ^ NaOH will neutralize 1,000 c.c., or one liter. 



The adjustment of the reaction of media is largely determined 

 by the particular uses for which the media are designed. For 

 examinations in the practice of sanitation, such as analyses of water, 

 ice, and milk, etc., the American Public Health Association recom- 

 mends a standard reaction of -(- 1 per cent (the plus sign is used 

 to indicate acidity, the minus alkalinity ; + 1 per cent is the expres- 

 sion used to indicate that one per cent of ^ sodium hydrate solution 

 would be required to neutralize the medium or 10 c.c. to the liter). 

 For general work with pathogenic bacteria, the most favorable 

 reaction for routine, media is slight alkalinity, neutrality, or an 

 acidity not exceeding -f- 1 per cent. 



3 See standard textbooks on volumetric analysis. 



