96 
PRINCIPLES OF BACTERIOLOGY 
3. The peptone should be Witte’s peptone. 
4. The meat should be lean, fresh beef. 
Titration of Media. —Titration of media is a very im¬ 
portant step. By titration of media is understood the 
adjustment of the reaction of the media, that is, whether 
they are acid or alkaline; the bacteria are, as a rule, very 
sensitive to this, and for this reason titration should be 
most carefully carried out in all cases. 
The color indicator used is 0.5 per cent solution of 
phenolphthalein in 50 per cent alcohol. # 
To determine whether the medium is acid or alkaline, 
take a few c.c. of the medium and drop one or two 
drops of the phenolphthalein solution; if the medium is 
acid, no color develops; if alkaline—a pink color appears. 
Most of the media are acid because of the presence of 
meat acids. 
For the actual titration of acid medium the so-called 
“one-twentieth normal solution of sodium hydroxide” is 
used, which is expressed as follows: N/20 NaOH, while 
for the titration of the alkaline medium “one-twentieth 
of normal solution of hydrochloric acid” is used, which 
is expressed as follows: N/20 HC1. By a “normal” 
chemical solution we understand “the molecular weight 
of a substance, expressed in grams, dissolved in a liter 
(1,000 c.c.) of distilled water; in the case of sodium 
hydroxide the formula is NaOH, and its molecular weight 
is that of sodium (Na) which is 23 plus that of oxygen 
(O) which is 16 plus that of hydrogen (H) which is 
one; that is, 23—[—16—[-1=40; the molecular weight of 
sodium hydroxide is, therefore, 40; now, if we dissolve 
40 grams of it in 1,000 c. c. of distilled water, we will 
*Any per cent solution of alcohol can be prepared from the stand¬ 
ard 95 per cent alcohol by taking as many cubic centimeters of 95 
per cent alcohol as the percentage desired and filling it with water 
up to 95 c. c.; in this case, for example, take 50 c. c. of 95 per cent 
alcohol and add 45 c. c. of water. 
