DETERMINATION OF ACIDITY. 287 



To determine the per cent, of acid in the milk the following 

 calculation is necessary : The number of c.c. of the alkaline 

 used is multiplied by .009 (the amount of acid neutralized 

 by i c.c. of this one tenth normal solution) and divided by 

 the number of c.c. of milk tested, the whole multiplied by 100 

 to give the per cent, of acid, thus : 



If 50 c.c. of milk are neutralized by 30 c.c. of the one tenth 

 normal solution the result is : 



30 X .009 ^, 0/ 



- X 100 = .54% acid. 

 So 



This method is quite accurate, but a more convenient one, 

 which is fairly accurate, has been devised by which the 

 phenolphthalein and the alkaline are preserved in dried tab- 

 lets, the Farrington's alkaline tablets. These are prepared 

 with a measured quantity of material such that their use 

 requires no calculation. They are used as follows : 



Place in a cylinder 97 c.c. of water and five alkaline tablets. 

 (These may be procured of dealers in dairy supplies.) Cork 

 tightly with a rubber cork and lay the cylinder on the table 

 to allow the tablets to dissolve (Fig. 43). They dissolve 



FIG. 43. 



Showing method of dissolving alkaline tablets. 



slowly and it is best to set them dissolving the night before 

 they are wanted. The solution will not keep, however, more 

 than twenty-four hours and must be used fresh. 



17.6 c.c. of milk to be tested is placed in a porcelain dish. 

 The dissolved tablet solution is placed in a burette and drawn 

 off into the milk as in the previous method, until a faint pink 



