50 PHARMACEUTICAL BACTERIOLOGY. 



calculate the volume of distilled water to be added in order that i c.c. of 

 sodium hydroxide solution will neutralize i c.c. of normal oxalic acid solution. 

 Having a normal solution of sodium hydroxide, it is now possible to 

 prepare a normal solution of hydrochloric acid, etc. A tenth- (N/io), 

 twentieth- (N/2o), fiftieth- (N/5o) normal solution is a normal solution 

 diluted ten, twenty, and fifty times. 



An acid reaction is indicated by + , and an alkaline by . The degree 

 of acidity of any culture medium in preparation may be indicated by the 

 amount of normal sodium hydroxide solution required to render it neutral 

 to phenolphthalein. Neutralization by titration is done as follows: Place 

 5 c.c. of the medium to be neutralized in a dish, add 45 c.c. of distilled 

 water, stir, and bring to a boil. Add i c.c. of phenolphthalein solution 

 (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 twentieth- 

 normal sodium hydroxide necessary to neutralize the 5 c.c. of medium, and 

 from this calculate the amount of normal sodium hydroxide solution neces- 

 sary 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 c.c. 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 example, 

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

 would require 100 c.c. 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 stir- 

 ring, 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 + 1 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 



