44 A DAIRY LABORATORY GUIDE 



It is a principle of chemistry that an alkali will 

 neutralize an acid. In order, therefore, to find the 

 acid in the milk we take a known quantity of the 

 milk and measure into it an alkali whose strength 

 we know. The instrument used to measure the 

 amount of alkali used is called a burette, and the unit 

 of measure is the cubic centimeter. It is commonly 

 graduated as fine as tenths of a cubic cen- 

 timeter. One can tell when all of the acid is neu- 

 tralized by means of an indicator. The indicator 

 used most in dairy work is phenolphthalein, which is 

 colorless in acid and pink in alkali. If two or three 

 drops of the indicator are put in milk the color will 

 not change, because the milk is acid in reaction. 

 The instant that just enough alkali is added to the 

 milk to neutralize all of the acid the solution will 

 turn pink. 



It is a chemical fact that equal volumes of acids 

 and alkalies of the same chemical strength will ex- 

 actly neutralize one another. In i cc. of a normal 

 solution of lactic acid there are .09 grams of lactic 

 acid. According to the above rule i cc. of any 

 normal alkali solution would just neutralize .09 

 grams of lactic acid. 



In actual practice a solution weaker than a normal 

 solution is usually employed, because a normal solu- 

 tion is so strong that any small variation in the 

 amount used makes a big variation in results. A 

 common solution used is i^io normal (expressed 

 n/io). One cc. of an n/io alkali solution would 

 neutralize .009 grams of lactic acid. An example 

 will illustrate how the per .cent of acid in milk is 

 calculated. Suppose it took 6 cc. of n/io alkaH 

 solution to neutraHze the acid in 20 grams of milk. 

 What is the per cent of acid? One cc. of n/io alkali 

 will neutralize .009 grams of lactic acid. Six cc. will 



