METHODS OF EXAMINING MILK 255 
10 grammes of powdered phenolphthalein in 300 c.c. of 
90 per cent. alcohol. The one-tenth normal ("/:) solu- 
tion of sodium hydroxide (NaOH) should be prepared 
and standardized as directed by the U. S. Pharmacopceia. 
Each cubic centimetre of this solution contains sufficient 
sodium hydroxide (0.004 gramme) to neutralize 0.009 
gramme of lactic acid. 
After thoroughly mixing the milk or cream to be 
tested, 50 ¢.c. are measured into the beaker or cup with 
the pipette. The pipette is then filled to the 50 c.c. mark 
with water, preferably distilled, which is added to the 
milk or cream. This is done for the purpose of rinsing 
all milk or cream from the pipette. Five to ten drops of 
the phenolphthalein solution are then placed in the beaker 
or cup and the fluid is mixed by stirring with a glass rod. 
The tenth normal sodium hydroxide solution is run into 
the beaker or cup drop by drop, stirring frequently, until 
a pink color appears and remains permanently. The 
amount of sodium hydroxide solution used is then read 
off from the burette. 
The result of the test may be expressed in degrees 
or in per cent of acidity. Degrees of acidity correspond 
to the number of ¢.c. of tenth-normal sodium hydroxide 
solution used to the hundred c.c. of milk or cream. The 
per cent. of acidity may be obtained by the following 
calculation: 
No. e.c. 8/10 NaOH used X 0.009 
No. c.c. milk or cream used X 100 = per cent. acidity. 
For example, if 9 c.c. of the one-tenth normal solu- 
tion are used in testing 50 c.c. of milk or cream, then 
9 X 0.009 
7x 100 = 9 X 0.018 = 0.16 per cent. acidity. 
