78 PHARMACEUTICAL BACTERIOLOGY 



(0.5 per cent, of phenolphthalein in 50 per cent, alcohol). Add enough 

 of twentieth-normal sodium hydroxide solution (in a burette) , with constant 

 stirring, to give a faint but distinct pink color. Read the amount of twen- 

 tieth-normal sodium hydroxide necessary to neutralize the 5 cc. of medium 

 and from this calculate the amount of normal sodium hydroxide solution 

 necessary to neutralize the entire quantity of culture medium. Now boil 

 the medium and again titrate, when it will be found that there is a slight 

 acid reaction. A third titration is rarely necessary. 



Another method is to take 10 cc. of the culture medium, add a few drops 

 of the phenolphthalein solution. From a burette add, drop by drop, with 

 constant stirring, a normal sodium hydroxide solution (0.4 per cent.) until 

 a faint pink color appears, which indicates the beginning of the alkaline reac- 

 tion. Repeat this with two more samples. Note the amount of sodium 

 hydroxide solution required in each case, and take the average and calcu- 

 late the amount required for the entire quantity of medium. If, for exam- 

 ple, the average was i cc. for each 10 cc. of medium, then 1000 cc. of bouillon 

 would require 100 cc. of the sodium hydroxide solution; a concentrated 

 solution being used, in order to avoid the dilution of the medium with the 

 water of the caustic-soda solution. Flocculency of the medium usually 

 indicates excessive alkalinity. 



The old, crude, rough-and-ready method is to add, from a beaker, 

 drop by drop, a tenth-normal sodium hydroxide solution, with constant 

 stirring, until red litmus paper just begins to turn blue. In practice it is 

 found that when a culture medium is neutral or slightly alkaline to litmus 

 it is still acid to phenolphthalein. In fact, it is claimed that most bacteria 

 develop best in a medium having a reaction indicated by -f i or +0.5 

 that is, it is sufficiently acid to phenolphthalein to require i per cent, or 0.5 

 per cent, of normal sodium hydroxide solution to render it neutral to 

 phenolphthalein. 



D. Suggestions on the Preparation of Culture Media for Physicians. — First 

 of all, the pharmacist must have the necessary laboratory equipment and 

 necessary skill and experience to prepare culture media. He should ex- 

 plain to a few representative physicians that he is ready to prepare such 

 media as the busy physician may require. The physicians will in all proba- 

 bility indicate that media are likely to be needed in the course of their 

 practice. Allow yourself to be guided by these several suggestions and 

 prepare the media accordingly. 



Make sure that the culture media are clear. There must be no sedi- 

 ment and no flocculency. Not infrequently the medium fails to become 

 sufficiently clear, even though every precaution has been taken. In such 

 cases clarification may be tried, rather than to discard it. Add the white 

 of an egg, thoroughly beaten, to a liter of the medium in the hquid state 



