1¢6 
necks of these flasks should have an. internal diameter of at least three- 
fourths of an inch. The reason for this diameter is necessary if the milk 
is neutralized in the flask. This neutralization can be done in the beaker 
into which the milk is weighed, if weights are taken. Third, pipettes, a 
Babcock milk pipette accurately graduated to deliver 17.5 cc. of milk, 
when 17.5 ce. or 18 grams of milk are used. When 22 ce. or 22.6 grams of 
milk are used it will be necessary to have a volume pipette graduated to 
deliver the above amounts or a 25 ec. Mohr pipette graduated into 1/10 ee. 
will be required. Fourth, one 100 cc. pipette or a volumetric flask graduated 
to hold 100 ce. Fifth, beakers of convenient sizes holding at least 200 ce. 
Sixth, if standard solutions are to be made, measuring cylinders or volu- 
metric flasks holding 1,000 ce. are needed. 
In regard to the making of the solutions it is best to prepare both the so- 
dium hydroxide and the acetic acid as tenth normal. The accuracy of the 
succeeding work depends primarily on the correctness of the standard 
alkali and acetic acid. When it is desirable to make dilutions for different 
quantities of milk it can be made from the tenth normal stock solution. 
The phenolpthalein solution is prepared by dissolving one gram of phenol- 
pthalein powder in 100 cc. of 50 per cent. alcohol. This should be neutralized 
by the use of a few drops of a NaOH to a very slight pink color. 
Carrying out the operation. Weigh out 22.66 grams of milk, or measure 
out 22 ce., neutralize in the beaker in which the weighing has been made, 
using only enough alkali to give a very faint pink, then transfer to a 200 ce. 
flask and wash out beaker with 75 to 80 ce. of distilled water, free from 
carbon dioxide, shake and warm to 22° to 25° C. At this point observe 
the color of the diluted milk. Frequently on dilution the pink color becomes 
quite pronounced; if so, add a few drops of N acetic acid to a light pink. 
N 
N’ acetic acid, frequently shaking, for milk 
Run in from a burette 25 ce. of a | 
rich in casein it would require 30 to 40 ce. of acid. Then fill up to the 200 
cc. mark, insert stopper and shake thoroughly. After standing for 5 or 
10 minutes, filter, after filtration pipette or measure 100 ce. of the filtrate 
into a 250 ce. or 300 cc. beaker and titrate to a permanent faint pink color, 
record the ec. used. Since 25 ce. were added to the total volume and only 
one-half titrated, we only take 12.5 ce. into consideration. From what has 
\ acetic acid has been used in forming 
been said a portion of the 25 ce. 
free casein, therefore the difference between 12.5 cc. and the amount of 
N NaOH used to neutralize the acid in the 100 ce. filtrate equals the number 
10 
