APPENDIX 267 



the gelatin and sugar, but also assists in the actual sterili- 

 zation. It is also advisable in the use of the autoclave 

 to keep the small pet cock at the bottom partially open 

 so that steam is escaping during sterilization. This insures 

 the removal of all air. If practicable, store media at room 

 temperature for two days to see that sterilization is complete. 

 In intermittent sterilization, media shall be placed on 

 each of three successive days in streaming steam for 30 

 minutes 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. dis- 

 tilled water. Boil briskly one minute. Add i c.c. of 

 phenolphthalein solution (5 gm. of commercial salt in i 

 liter of 50 per cent alcohol). Titrate while hot (preferably 

 while boiling) with N/20 caustic soda. A faint but distinct 

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

 color may be more precisely described as a combination 

 of 25 per cent of red (wave length approximately 658) 

 with 75 per cent of white as shown by the disks of the 

 color top (described under Record of Tints and Shades 

 of Apparent Color, p. 10 of Standard Methods Report). 

 In practice, titration is continued until the pink color of 

 alkaline phenolphthalein matches that of the fused disks. 



All reactions shall be expressed with reference to the 

 phenolphthalein neutral point and shall be stated in per- 

 centages of normal acid or alkaline solutions required to 

 neutralize them. Alkaline media shall be recorded with 

 the minus ( ) sign before the percentage of normal acid 



