MATERIALS. 
309 
Normal 
JfCl 
b. Adjust the acidity to 1.5 per cent, as follows: 
Measure out 10 c.c. of the mixture, placing it in an evaporating dish 
with ten times its bulk of water. Bring to a boil and add a few drops of 
phenolphthalein solution.* This solution is colorless so long as it is acid, 
but turns red when alkaline. The agar medium remains red after the 
addition of phenolphthalein, showing it to be acid. 
Place the evaporating dish under a 
burette containing i/io normal NaOH 
(one part normal NaOH diluted with 
nine parts of distilled water) (Fig. 57). 
Take the reading on the burette. Allow 
the solution to fall drop by drop into the 
evaporating dish, stirring between each 
drop. As long as the material remains 
acid no color will appear. Continue 
adding the NaOH until a faint red color 
appears and does not disappear upon 
stirring. When this point is reached 
the contents of the evaporating dish 
are neutral. 
Take the reading upon the burette 
and determine the difference between 
the first and second reading. The 
difference gives the number of cubic 
centimeters of i/io normal NaOH 
needed to neutralize 10 c.c. of the cul- 
ture medium. Divide by 10 to de- 
termine the amount necessary to neu- 
tralize one c.c., and multiply by 990 to 
determine the amount necessary to 
neutralize the rest of the agar medium. 
Add to the agar medium sufficient 
NaOH to neutralize it. Instead of 
adding i /io normal, add normal NaOH, 
using of course only i/io as many 
cubic centimeters as the above calcula- 
tion shows would be needed of i/io 
normal NaOH. This will bring the 
whole to the neutral point. 
Add normal HC1 to the amount of 
15 c.c. per liter. In this case, there being only 990 c.c. left, the amount 
added should be 14.9 c.c. This makes an acidity of 1.5 per cent., the best 
reaction for most bacteria. 
c. Boil vigorously for fifteen minutes and then restore the original 
weight by adding water. Test the reaction again as before to see if it is 
FIG. 57 Two burettes arranged for 
neutralizing culture media. 
* Eight per cent, phenolphthalein in 50 per cent, alcohol. 
