GENERAL BACTERIOLOGY 
99 
normal twentieth solution, i. e., instead of 400 c.c. of 
N/20 NaOH we will use 20 c.c. of N/l NaOH. 
The calculations of the alkaline titration do not differ 
in any way from the above. 
For ordinary purposes we use a medium which is not 
neutral but is about 1 per cent; i. e., requires one c.c. of 
N/l NaOH per 100 c.c. of the medium, or 10 c.c. per 1,000 
c.c. (liter). In the above calculation, therefore, we 
should, instead of adding 20 c.c. of N/l NaOH to the liter 
of medium, have added only 10 c.c. of it, thus leaving 
the medium still 10 c.c. of N/l NaOH short for complete 
neutralization; that is, leaving it 1 per cent acid. 
The acidity of the medium is expressed by the sign -f- 
and the alkalinity by that of —. 
The Colorimetric Titration of Media. —This—the mod¬ 
ern method of titrating culture media (as well as many 
other fluids)—is based on the so-called “Hydrogen-Ion 
Concentration,” and it is necessary to explain the mean¬ 
ing of this expression before proceeding to describe its 
application to the titration of culture media. 
The acidity of any solution depends on its contents 
of hydrogen ions (H) and the alkalinity on hydroxyl 
ions (OH) : thus, hydrochloric acid (HC1) is acid because 
of its hydrogen (H) contents, while sodium hydroxide 
(NaOH) is alkaline because of its hydroxyl contents (OH). 
The inaccuracy of the old method of titrating culture 
media is due to the fact that in using the phenolphthalein 
as an indicator, and in using sodium hydroxide and hydro¬ 
chloric acid for the correction of either acidity or al¬ 
kalinity, we do not actually correct them, because 
phenolphthalein is not sensitive enough to determine 
the hydrogen-ion concentration (which is actual acid¬ 
ity) ; we all know that while, for example, hydrochloric 
