THE PREPARATION OF CULTURE MEDIA 



117 



6. For acidification C. P. hydrochloric acid in normal solution should 

 be used. 



7. When glycerin is used, this should be of the redistilled variety. 



8. The agar-agar employed should be of the finest grade of commer- 

 cial thread agar. 



9. The gelatin should be the commercial sheet gelatin washed as 

 free as possible of acid and impurities. 



10. Chemicals and carbohydrates which are used should be as 

 nearly chemically pure as possible. 



Titration of Media. — Next in 

 importance to the actual composi- 

 tion of media is the adjustment of 

 their reaction. Bacteria are highly 

 susceptible to variations in the 

 acidity and alkahnity of media, 

 excessive degress of either may 

 completely inhibit development or' 

 moderate variations may lead to 

 marked modifications of cultural 

 characteristics. It is necessary, 

 therefore, to adjust the reaction 

 both for the sake of favoring 

 growth and in order to insure uni- 

 formity of growth characters. This 

 is accompHshed by titration which 

 is best carried out according to the 

 recommendations of the committee 

 mentioned above. 



The color indicator employed for 

 the titration is a five-tenths per cent 

 solution of phenolphthalein in fifty 

 per cent alcohol. The chief advan- 

 tage of this indicator over others is 

 due to the fact that it indicates the 

 presence of organic acid and acid 

 compounds in its reaction. For 



actual titration -^ (^ normal) solutions of sodium hydrate or of hy- 

 drochloric acid are used. Since media in the process of preparation 

 are usually acid, the NaOH solution is the one most frequently needed. 

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



Fig. 19. — Burette for Titrating 

 Media. 



