120 Testing Milk and Its Products. 



The indicator used is a solution of phenolphtalein, a 

 light yellowish 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). 



132. 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 Neutral- 

 izer (or alkali solution) is cautiously dropped in from 

 a burette, the point at which the solution stands before 

 any is drawn off 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 disappear 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 stand- 

 ing, owing to the effect of the carbonic acid of the air, 

 to which phenolphtalein is very sensitive. The amount 

 of the alkali solution 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 predict hy the rmilTher of eo. of the sample 

 tested, the quotient being multiplied by 100. 



