46 DAIRY CHEMISTRY 



definite and known amount of an alkali for purposes 

 of combination or neutralization. In determining the 

 acidity of milk, it is necessary to determine simply 

 the amount of alkali required to combine with and 

 neutralize all of the acid present, changing it to the 

 class of compounds known as salts. For this pur- 

 pose a standard alkali solution is required. A tenth 

 normal solution of sodium hydrate is usually em- 

 ployed for the purpose. The following pieces of 

 apparatus are required : one 50 cc. burette gradu- 

 ated to .1 cc. ; a burette holder or support; glass 

 beakers or earthenware cups ; one 25 or 50 cc. pipette. 

 Phenolphthalein is used as an indicator. In an acid 

 solution phenolphthalein is colorless, while in a weak 

 alkaline solution it gives a pink color and in a 

 stronger alkaline solution a red color. This indicator 

 is prepared by dissolving 10 gins, of the powder in 

 250 cc. of 90 per cent alcohol. 



The acidity is determined in the following way : 

 measure with the pipette 50 cc. of the milk into a 

 glass beaker or porcelain cup. In case distilled or 

 soft water is available, an equal amount is added to 

 the measured milk ; 5 to 7 drops of the indicator 

 are added, and then the standard alkaii from a 

 burette a little at a time, until the color of the milk 

 upon stirring is changed to a faint pink. Before 

 adding the alkali the height of the liquid in the 

 burette should be recorded, and at the close of the 

 operation it should be read again in order to deter- 

 mine the number of cubic centimeters required to 



