METHODS OF TESTING THE ACIDITY OF MILK 139 



truth. Several forms of bottle have been devised 

 which reduce this danger to a minimum ; some of these 

 are illustrated in Figs. 39, 3ga and 



OPERATION OF TESTING FOR ACIDITY 



Before describing details of any of the methods in 

 use, we will briefly consider, some of the principal 

 points that apply to all methods. From the material 

 to be tested measure the amount desired into a cup 

 or beaker; nothing is better than a white china tea- 

 cup. The pipette is rinsed by filling it with water, 

 which is added to the material in the cup. The addi- 

 tion of water to the sample to the extent of three or 

 four volumes will enable one to see the pink coloration 

 at the end more sharply. Then add 5 to 10 drops of the 

 phenolphthalein solution, and from the burette filled 

 with alkali run small portions into the mixture in the 

 cup, stirring thoroughly after each addition. A pink 

 color soon appears but disappears on stirring. The 

 addition of alkali in small amounts at a time is con? 

 tinued with care; sooner or later, according to the 

 amount of acid present, it will be noticed after each 

 addition of alkali, that the pink color disappears more 

 slowly showing that the acid is becoming nearly neu- 

 tralized. When the color disappears quite slowly, add 

 the alkali not more than one drop at a time. Finally, 

 a point is reached when, after the addition of one drop 

 of alkali, the pink color does not disappear even after 

 stirring 20 to 30 seconds. This indicates that the acid 

 is completely neutralized. Add no more alkali. The 

 pink color will disappear after standing some time, 

 even when the solution is alkaline. Some experience 



