Appendix. 205 



of the autoclave facilitate chemical reactions and changes 

 which in some cases are undesirable. 



When the latter method is used, media contained in 

 ordinary receptacles shall be sterilized by exposure in an 

 autoclave at a temperature of 120 C. (15 pounds pres- 

 sure) for five minutes. Where media are sterilized in 

 large bulk, the period of heating shall be extended to 12 

 minutes. It is preferable, however, to sterilize media in 

 reasonably small containers (500 to 700 c.c.). 



In intermittent sterilization, media shall be placed on 

 each of three successive days in streaming steam for 30 

 minntes after the steam fills the sterilizer. 



REACTION. 



Phenolphthalein shall be the standard indicator used 

 in obtaining the reaction of all media. Turmeric paper 

 possesses similar properties, and its use is advised where 

 phenolphthalein is not available. 



Titrations and adjustment of reactions shall be made 

 as follows: 



Put 5 c.c. of the medium to be tested into 45 c.c. 

 distilled water. Boil briskly one minute. Add i c.c. 

 of phenolphthalein solution (5 g. of commercial salt in 

 one liter of 50 per cent alcohol.) Titrate while hot 



N 



(preferably while boiling) with caustic soda. A faint 



20 



but distinct pink color marks the true end-point. This 

 distinct pink color may be more precisely described as a 



