98 
PRINCIPLES OF BACTERIOLOGY 
Titration of Alkaline Media. —Titration of alkaline 
media is carried out in the same manner as that of the 
acid medium, except that a N/20 HC1 (one-twentieth nor¬ 
mal solution of hydrochloric acid) is used. When 1 c.c. 
phenolphthalein solution is added, every drop of the 
hydrochloric acid solution added will cause a pink color, 
as explained above, but as more and more of the N/20 
HC1 is added the color will gradually fade away, and 
when the color has completely disappeared, the neutral 
point has been reached. 
This is, in other words, just the opposite of the acid 
titration: there we work from no color to a permanent 
pink, and here from pink to no color. 
The Calculation. —Let us suppose that in titrating an 
acid medium we had to use 2 c. c. of N/20 NaOH to 
make our medium in an evaporating dish (5 c.c. of 
medium, not 50 c.c., as 45 c.c. was distilled water). The 
calculation is then easy: 
If 5 c.c. of the medium requires 2 c.c. of N/20 NaOH 
for neutralization, then 100 c.c. of the medium requires 
20 times as much (because 100 c.c. is twenty times as 
great as 5 c. c.) ; that is, 2x20=40 c. c. of NaOH. 
If we had prepared 1,000 c.c. of the medium (a liter), 
this 1,000 c.c. of the medium will require 10 times as 
much of N/20 NaOH as 100 c.c. did; i. e., 40X10=400 
c.c. of N/20 NaOH. Thus, to our 1,000 c.c. of the medium 
we must add 400 c.c. to the N/20 NaOH to make it neu¬ 
tral ; but adding 400 c. c. will make our medium too 
bulky; for this reason instead of adding one-twentieth of 
NaOH we will add the normal solution of NaOH; how 
much? Since the normal solution of sodium hydroxide 
is twenty times as strong as the normal twentieth solu¬ 
tion, it is evident that we must use one-twentieth of the 
