CULTURE MEDIA 85 



Peptone' 10 grams. 



Sodium chloride (common salt) S grams. 



These should be dissolved by stirring at a temperature below 

 60° C. The mixture is then boiled for half ah hour over the 

 direct flame, cooled slightly, and filtered through paper pre- 

 viously wet with warm water. The filtrate should be clear and 

 light yellow in color, and should be diluted to 1000 c.c. with 

 distilled water. Its reaction is acid, a reaction unfavorable to 

 the growth of many bacteria, especially to many pathogenic 

 forms. 



The amount of alkali to be added is ascertained by titration. 

 For this purpose exactly 5 c.c. of the broth is placed in each of 

 three test-tubes. Five-tenths cubic centimeters of a 5 per cent 

 solution of purified litmus (Merck's highest purity) is added to 



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 each tube. An accurately prepared — solution of sodium hydrox- 

 ide^ is then run in drop by drop from a graduated burette, the 

 reading of which has been recorded, into one of the tubes until the 

 red color, just changes to blue. The burette reading is taken 

 and recorded. The alkali is then run into the second tube rather 

 rapidly until the endpoint ascertained by the first test is nearly 

 reached. By comparing the color of this tube with that of the 

 first one and with the third to which no alkali has yet been added, 

 the exact point at which the color is changing from red to blue may 

 be accurately judged. When this point is reached, the burette 

 reading is again recorded and the amount of alkali necessary 

 to neutralize the 5 c.c. of broth ascertained. The third tube 

 should then be titrated to confirm the previous result. The 

 titration of the broth should now be repeated, using phenolphtha- 

 lein as an indicator. For this purpose, 5 c.c. of the medium is 

 transferred to a small porcelain dish, diluted by the addition 



' Commercial peptones are mixtures of albumoses and a small amount of peptone. 

 ' A normal solution of sodium hydroxide contains one gram-molecule of anhy- 



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drous NaOH, or 40 grams, in a liter. The— solution contains — of this amount 



or 2 grams in a liter. 



