28 QUESTIONS AND ANSWERS 



Publow's, Mann's, Farrington's, and Marshall's, 

 but the principle is the same in all of them. An 

 alkaline solution of known strength and an indicator 

 called phenol-phthalein are the reagents used. The 

 purpose of the indicator is to indicate the reaction 

 of the milk or cream ; i. e., it shows whether the 

 milk or cream is acid, alkaline, or neutral. The in- 

 dicator has no color effect in an acid solution, but it 

 turns an alkaline substance red. When the neutral 

 point is reached, the faintly pink color is barely 

 distinguishable. 



To make the test : 



A known quantity of the milk or cream to be 

 tested is placed in a white cup. To it a few drops 

 of indicator are added. An alkaline solution of 

 known strength is then allowed to run in drop by 

 drop from a graduated burette until the milk or 

 cream assumes a faintly pink color, which signifies 

 that all the acid in the milk or cream has been 

 neutralized by the alkali used. The amount of 

 alkali used can be read on the burette, and as its 

 strength is known it is an easy matter to figure the 

 amount of acid in the milk or cream. One c. c. of 

 %o normal alkali neutralizes .009 grams lactic acid. 



In the test used at Cornell university the alkali 

 used is a 34o normal solution of caustic soda. This 

 makes a very convenient strength, because when 9 

 grams of milk or cream are taken, .1 c. c. of it 

 neutralizes or represents .01 per cent of lactic acid. 

 Thus, if in testing the cream 4 c. c. of the alkali 

 was used, the cream contained .40 per cent of acid. 



In Farrington's test the alkali is made %o normal 

 by adding 5 tablets in 97 c. c. water. Then, when 



