Testing the Acidity of Milk and Cream. Ill 



The indicator used is a solution of phenolplitalein, a yel- 

 lowish light powder; its compounds with alkalies are 

 red, in weak alkaline solutions pink colored, while its 

 acid compounds are colorless. The phenolphtalein solu- 

 tion used is prepared by dissolving 10 grams in 300 cc. 

 of 90 per cent, alcohol (Mohr). 



129. In testing the acidity of either milk or cream it 

 is necessary to measure out with exactness the quantity 

 of liquid to be tested ; Manns recommended using a 50 cc. 

 pipette. This amount of milk or cream is measured into 

 a clean tin, porcelain or glass cup, a few drops of the 

 phenolphtalein solution are added, and the Neutralizer 

 (or alkali solution) is cautiously dropped in from a bur- 

 ette, the point at which the solution stands before any is 

 drawn out being noted. By constant stirring during this 

 operation it will be noticed that the pink color formed 

 by the addition of even a drop of alkali solution will at 

 first entirely disappear, but as more and more of the acid 

 in the sample becomes neutralized, the color will disap- 

 pear more slowly, until finally a point is reached when 

 the pink color remains permanent for a time. No more 

 alkali should be added after the first appearance of a 

 uniform pink color in the sample. This color will fade 

 and gradually disappear again on standing, owing to the 

 effect of the carbonic acid of the air, to which phenol- 

 phtalein is very sensitive. The amount of the alkali solu- 

 tion used for the test is then obtained from the reading 

 on the scale of the burette. The per cent, of acid in the 

 sample is calculated by multiplying the number of cc. of 

 alkali solution used, by .009 and dividing the product 

 by the number of cc. of the sample tested, the quotient 

 being multiplied by 100. 



