134 MODERN METHODS OF TESTING MILK 



we run from a graduated cylinder or burette some 

 standard solution of caustic soda, prepared by dis- 

 solving 4 parts by weight of pure caustic soda in 1,000 

 parts of distilled water. This solution of caustic soda 

 we add, a little at a time, to the solution of lactic 

 acid, stirring or otherwise agitating the mixture thor- 

 oughly after each addition. The pink color that ap- 

 pears when the caustic soda solution is added disap- 

 pears on stirring. After the alkali has been added 

 several times, the color disappears less rapidly each 

 time. The gradual addition of the alkali is continued 

 until finally the pink color does not disappear readily 

 on continued agitation but remains for some moments. 

 The neutralization of the acid by the alkali is complete, 

 and the addition of alkali stops at this point. The ap- 

 pearance of the pink color throughout the body of the 

 liquid means that enough alkali has been added to 

 combine with the lactic acid, and a little more, one 

 drop of the alkali solution being enough to produce 

 the pink color with the indicator after the acid is neu- 

 tralized. The liquid in the cup contained at the start 

 only a solution of lactic acid. As soon as we added 

 alkali, it combined with the lactic acid, forming the 

 neutral compound, sodium lactate. We then had a 

 mixture of lactic acid and sodium lactate. As we con- 

 tinued to add alkali, the amount of sodium lactate in- 

 creased, while the amount of lactic acid decreased. 

 Finally, a point is reached when the solution in the 

 cup contains no free lactic acid, but only sodium lac- 

 tate, and the addition of one more drop of alkali turns 

 the indicator pink, producing a more lasting coloration 

 throughout the solution and showing that the acid 



