86 BACTERIOLOGY 



of approximately 45 c,c. of distilled water, and boiled for a minute, 

 i c.c. of a 0.5 per cent solution of phenolphthalein in 50 per cent 



N 



alcohol is now added and solution of sodium hydroxide run in 



20 



from the burette until the color changes to a faint but distinct 

 and permanent pink color. The burette reading is recorded 

 and the amount of alkali necessary to neutralize the 5 c.c. of 

 medium in respect to phenolphthalein thus ascertained. This 

 titration may well be repeated, especially by beginners. As a 

 result of these titrations we shall have ascertained the amount of 

 alkali necessary to neutralize the remaining broth to either indi- 

 cator. For example suppose that 5 c.c. of the broth titrated as 

 follows : 



0.5 c.c. of alkali with litmus as indicator. 



20 



2. o c.c. of alkali with phenolphthalein as indicator 



In order to neutralize the remaining 980 c.c. of broth to 



litmus would require or 08 c.c. of alkali. A solu- 



5 20 



tion of alkali twenty times as strong as this, namely normal 

 sodium hydroxide, is employed for this purpose, and only f or 

 4.9 c.c. of this are necessary to neutralize the 980 c.c. of broth 

 to litmus. The reaction generally required for pathogenic 

 bacteria is slightly alkaline to litmus and for this reason an excess 

 of 10 c.c. of normal alkali per liter is added to the broth, 9.8 c.c. 

 for the 980 c.c., making altogether 14.7 c.c. to be added. Cal- 

 culation from the result obtained with phenolphthalein in the 

 same way shows that 19.6 c.c. of normal alkali would be required 

 to neutralize the medium to this indicator. The desired final 

 reaction of the medium in respect to phenolphthalein is acid, 

 usually that of 5 to 15 c.c. of normal acid per liter, or 0.5 to 1.5 

 per 100 c.c., or 0.5 to 1.5 per cent, as it is commonly expressed 

 after Fuller. 1 In this instance, therefore, 5 to 15 c.c. per liter, 

 or 4.9 to 14.7 c.c. less than the 19.6 for the 980 c.c., would be 



1 Fuller. Journal of Amer. Public Health Assoc., 1905. 



