SOME PHYSICO-CHEMICAL MEASUREMENTS ON MILK. 183 



were used, and, as carried out, gave similar results, but 

 the E.M.P. method is more sensitive and more accurate. 

 The first experiments on this matter were conducted by the 

 E.M.F. method and then the indicators were used after 

 finding oat that this latter method was sufficiently accurate 

 for the purpose. 



E.M.F. Method. 

 This method was carried out with the usual apparatus 

 described in Ostwald — Luther's " Hand und Hilfsbuch." A 

 platinum electrode saturated with hydrogen and inserted 

 in milk was connected with a deci-normal calomel elec- 

 trode and the E.M.F. of the combination determined. From 

 this the concentration of H ion was calculated. 



Sorensen 1 gives the formula 



TV - 0*3377 . 1QO n 

 - Ph= -^057T- atl8 °* 

 for calculating the concentration of hydrogen ion from 

 the E.M.F. far). This formula was used by the author, 

 after being assured of its accuracy by experimental con- 

 firmation in a considerable number of cases. 



For the purpose of these determinations a number of 

 carefully prepared standard solutions were made according 

 to the manner described by Sorensen. These standard 

 solutions were made up from the following : — 



(1) *1 molecular hydrochloric acid. 



(2) '1 molecular di-sodium-hydrogen citrate. 



(3) *1 molecular sodium hydrate. 



(4) 1/15 molecular potassium-di-hydrogen phosphate. 



(5) 1/15 molecular di-sodium-hydrogen phosphate. 

 The method of expressing the concentration of hydrogen 



ion in the solutions is also the method used by Sorensen. 

 The concentration is expressed in this way as a power of 

 ten and the index alone used to represent the concentration. 



1 Biochem. Zeit., xxi, p. 106, 1909. 



