DETERMINATION OF ACIDITY. 27 



For the quantitative estimation of acidity of milk, a 

 measured quantity of the sample is titrated with dilute 

 sodium hydrate of known strength ; lo c. c. of milk is di- 

 luted with 90 c.c.of distilled water and thoroughly shaken. 

 Add one or two drops of a strong alcoholic solution of 

 phenolphthalein for an indicator. From a burette (gradu- 

 ated in one-tenth c. c), a one-tenth or one-fifth normal 

 solution of sodium hydrate is run in drop by drop with 

 constant stirring, until the bright red color remains per- 

 manent. (Phenolphthalein is colored red with alkalies.) 

 Tlie operation is repeated with a second portion of the 

 milk and the mean of the two titrations taken. The re- 

 sults should not differ by more than one-tenth c. c. The 

 acidity is expressed in terms of cubic centimeters of so- 

 dium hydrate required. (One should not accept the re- 

 sults of one titration.) According to Pfeiffer, the quantity 

 of acid in different kinds of milk differs according to the 

 time of year, the climate, and other conditions, so that no 

 limits can be fixed. He therefore recommends that every 

 observer find the average acidity coefficient for milk from 

 his locality, by making numerous tests of strictly pure 

 samples, "preferably 'stall samples.' " 



The age of milk may be approximately judged 1)\' de- 

 termining the acidity in the following manner: The 

 method is not considered very reliable. 25 c. c. of milk are 

 placed in the flask and a few drops of phenolphthalein 

 added and titrated, as above, witii barium hydroxide solu- 

 tion until a faint red tint remains ; i c.c. of baryta solu- 

 tion should be equivalent to 0.005 grams SO;, that is, 

 one liter of the solution must contain 10.703 grams of 

 barium hydrate. With fresh milk the red color appears on 

 adding 17 c.c. of the barium hydrate solution. This acid- 

 ity remains constant at 10 degrees Centigrade for 48 

 hours, at 15 degi-ees for 20 hours and at ^j degrees for 5 

 hours. If milk kept for one hour in a culture oven 



