g6 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 sodjum 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 

 alkah necessary to neutralize the remaining broth to either indi- 

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



follows : 



N 

 o.K c.c. of — alkali with litmus as. indicator. 

 ^ 20 



N . 



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



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



. would require '— or 98 c.c of — alkali. A solution of 



alkali twenty times, as strong as this, namely normal sodium 



hydroxide, is employed for this purpose, and only — 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 Ijter is added to the broth, 9.8 c.c. for the 98.0 c.c, makr 

 ing altogether 14.7 c.c. to be added. Calculation 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.^ In this instance, 



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



