Determination of Lactic Acid. 135 



to chemists as decinormal, or -^o~, and the acidity was 

 reckoned in terms of cubic centimeters of alkali re- 

 quired to neutralize the acid in a given quantity of 

 cream. In the use of this test, an indicator which 

 changes color according as the medium is acid or 

 alkaline, is used. A convenient indicator is phenol- 

 phthalein, which is colorless in acid solutions and 

 pinkish in alkaline. Later, Professor Farrington* 

 devised tablets containing a known amount of alkali 

 for the same purpose, known as Farrington's alkaline 

 tablets. Each tablet contains an amount of alkali 

 equal to 3.8 cubic centimeters of decinormal alkali, 

 and a sufficient amount of phenolphthalein indicator. 

 The cream to be tested is measured, and to it is 

 added a solution of the tablets (one tablet in ten 

 cubic centimeters of water) until the cream retains a 

 pinkish tinge. The tablet solution should always 

 be fresh, not more than ten hours old. The tab- 

 lets themselves will keep indefinitely. The proper 

 degree of acidity is indie tated when 30-35 c. c. of 

 Farrington tablet solution, or 11-13 c. c. of deci- 

 normal alkali, are required to neutralize the acid in 

 20 cubic centimeters of cream. 



Determ f nation of lactic acid in milk or cream. — Far- 

 rington's alkaline tablets may also be conveniently 

 used to determine the percentage of lactic acid in any 

 given sample of milk or cream. In order to do this, 

 it is onl}^ necessary to understand that equal volumes 

 of normal or decinormal acids and alkalies neutral- 



*Bull. 32, 111. Agr. Exp. Sta. Bull. 52, Wis. Agr. Exp. Sta. 



