GENERAL BACTERIOLOGY 
97 
have a “normal” solution of sodium hydroxide; if we 
wish to make one-twentieth normal solution take 1/20 of 
40, i. e., 2 grams, and dissolve it in 1,000 c. c. of dis¬ 
tilled water; to make one-tenth normal solution take 1/10 
of 40, i. e., 4 grams of NaOH in 1,000 c. c. of water, etc. 
In the case of hydrochloric acid the formula is HC1; 
the molecular weight of hydrogen (H) is 1; that of 
chlorine (Cl) is 35.5; therefore, the molecular weight of 
HC1 is 36.5; now HC1 being a liquid, we measure it in 
cubic centimeters (c.c.) and not in grams; take 36.5 c.c. 
of hydrochloric acid and add distilled water up to 1,000 
c.c.; to make one-twentieth normal solution of hydro¬ 
chloric acid, take 1/20 of 36.5, i. e., 1.825 c.c. of hydro¬ 
chloric acid and add distilled water up to 1,000 c.c. 
Titration of Acid Medium.—Into an evaporating dish 
of 100 c.c. capacity pour 5 c.c. of the medium to be 
titrated, and add 45 c.c. of distilled water. Boil for 
three minutes over a free flame (to drive off the carbon 
dioxide). Add one c.c. of the phenolphthalein solution; 
the medium being acid, no color will be present, as ex¬ 
plained above. Now from a special tall tube called a 
“bilrette” (this is a tall, narrow tube, usually of 50 c.c. 
capacity graduated in tenths of a c.c., and provided with 
a stopcock to permit removal of small quantities of fluids), 
a few drops of the “twentieth normal solution of sodium 
hydroxide” (N/20 NaOH) are permitted to drop into the 
evaporating dish containing 5 c.c. of the medium and 45 
c.c: of distilled water; at first no color is present, but 
little by little a pink color will develop, which, however, 
will disappear upon stirring with a glass rod. When a 
permanent pink color—that is, one which will not disap¬ 
pear upon stirring—has been reached, we know that the 
neutral point (that is, neither acid nor alkaline) has been 
obtained. 
