Testing the Acidity of Milk and Cream. 121 



worked out by Soxhlet and Henkel has since been in 

 general use by European chemists. They measured out 

 50 cc. of milk to which was added 2 cc. of a 2 per cent, 

 alcoholic solution of phenolphtalein, and this was ti- 

 trated with a one-fourth normal soda solution 1 (see 

 below). In this country, Dr. A. G. Manns in 1890 pub- 

 lished the results of work done in the line of testing 

 the acidity of milk and cream, 2 and the method of pro- 

 cedure and apparatus proposed by him has become 

 known under the name of Manns' test, and is being 

 advertised as such by dealers in dairy supplies, 

 j 131. Manns' test. The acid in milk or cream is 

 measured by using an alkali solution of a certain strength, 

 with an indicator which shows by a change of color in 

 the milk when all its acid has been neutralized. Any of 

 the alkalies, soda, potash, ammonia, or lime can be used 

 for making the standard solution, but it requires the 

 skill and apparatus of a chemist to prepare it of the 

 proper strength. A tenth-normal solution 3 of caustic 

 soda is the alkali solution used most frequently in de- 



1 Fleischmann, Lehrb. d. Milch wirtschaft, 3rd ed., p. 57. 



2 Illinois experiment station, bulletin 9. 



9 Normal solutions, as a general rule, are prepared so that one liter 

 shall contain the hydrogen equivalent of the active reagent weighed 

 in grams (Sutton). Caustic soda (NaOil; is made up of an atom each 

 of sodium (Na), oxygen (O), and hydrogen (H) ; its molecular weight 

 is therefore 



23+16+1=40 

 NaOH 



A normal soda solution then is made by dissolving 40 grams of soda 

 in water, making up the volume to 1000 c~. : a one-tenth normal solu- 

 tion will contain one-tenth of this amount of soda, or 4 grcitns dissolved 

 in one liter. One cubic centimeter of the latter solution will contain 

 .004 gram of soda, and will neutralize .009 gram of lactic acid. The 

 formula for lactic acid is C,H R O, (see p. 16), and its molecular weight 

 is therefore 3x12+6x1+3x16=90. A tenth-normal solution of lactic 

 acid contains 9 grams per liter, and .009 gram per cubic centimeter. 



