142 LABORATORY BACTERIOLOGY 
addition of alkali is a very faint darkening of the fluid, which on 
the addition of more alkali becomes a more evident color, and 
develops into what may be described as an Italian pink. A still 
further addition of alkali suddenly develops a clear and bright pink 
color, and this is the reaction always to be obtained. 
All titrations should be made quickly and in hot solutions, to 
avoid complications arising from the presence of carbon dioxid. 
When this manipulation is carried out uniformly, as here sug- 
gested, and the end point having the same intensity of color is 
always reached, very satisfactory and closely-agreeing results may 
be obtained. 
Neutralization of Media. — The next step in the process is to add 
to the bulk of the medium the calculated amount of reagent, either 
alkali or acid, as may be determined. For the purpose of neutraliza- 
tion a normal solution of sodium hydroxid or of hydric chlorid is 
used, and after being thoroughly stirred the fluid thus neutralized is 
again tested in the same manner as at first to insure the proper 
reaction of the medium being attained. When neutralization is to 
be effected by the addition of alkali, it not infrequently happens that 
after the calculated amount of normal solution of sodium hydroxid 
has been added the second test by titration will show that the 
medium is still acid to phenolphthalein, to the extent sometimes of 
from 0.5 to 1%. This discrepancy is perhaps due to side reactions, 
which are not understood; the reaction of the medium, however, 
must be brought to the desired point by the further addition of 
sodium hydroxid, and the titrations and additions of alkali must be 
repeated until the medium has the desired reaction (7.2. 0.0 % — 
0.005 %, see below). 
After the prescribed period of heating it is frequently found that 
the medium is again slightly acid, usually about 0.5 %. Without 
correcting this the fluid is to be filtered and the calculated amount 
of acid or alkali is to be added to change the reaction to the one 
desired. 
A still further change in reaction is not infrequently to be observed 
after sterilization, the degree of acidity varying apparently with the 
composition of the media and the degree and continuance of the 
heat. 
Manner of expressing the Degree of Reaction of Culture Media. — 
Since at the time the reaction is first determined culture media 
