100 
PRINCIPLES OF BACTERIOLOGY 
acid and acetic acid are both acids, yet the acetic acid 
is much “weaker” (scientifically, it means that the hy¬ 
drogen-ion concentration of acetic acid is less than that 
of hydrochloric acid); in spite of this, when titrated 
with an alkaline solution, such as sodium hydroxide 
(NaOH), using litmus or phenolphthalein as an indica¬ 
tor, it will be found that both hydrochloric and acetic 
acids will require the same amount of the alkali to be¬ 
come neutral to either litmus or phenolphthalein indica¬ 
tors. On the other hand, if the hydrogen-ion concentra¬ 
tion of each acid were determined, it would be found 
that that of hydrochloric acid is very much greater 
than that of acetic acid, and thus would show us that 
hydrochloric acid is very much “more acid” than the 
acetic acid. 
The hydrogen-ion concentration is expressed by P H , 
attached to which is a number expressing the logarithm 
of the hydrogen-ion concentration, e.g., P H 6, or P H 6.2, 
or P h 8, and so forth. 
Another reason for the inaccuracy of the older method 
of titration is the fact that in our culture media there 
are such substances as peptones, protein, phosphates, 
etc., which are known as “buffer” solutions, because of 
their property to resist changes in reaction. 
Clark and Lubes who have done so much for the ap¬ 
plication of hydrogen-ion concentration to bacteriolog¬ 
ical work have suggested the various indicators which 
express the true acidity by changing their colors within 
a certain range. 
In the actual practice of colorimetric titration of 
media, a colorimetric set is purchased which consists of 
indicators, buffer solutions, and colorimetric scale. 
Into a thoroughly cleaned test tube, rinsed out with re- 
