29 



It is to be observed yet, that the decolorized solution, on stand- 

 ing over the cuprous oxide, absorbs oxygen from the air, and be- 

 comes again blue. For this reason each tritration must be carried 

 to an end in the shortest time, and a subsequent coloration is not 

 to be taken into account. We, therefore, determine the exact 

 quantity by two to three titrations. The first titration is only used 

 to determine the reducing qualities roughly. 



1 c.c. Fehling's solution = 0.00676 grs. milk sugar. 



It is not advisable, as some propose, to take a drop of the fluid 

 out by means of a glass rod, and to test this on a porcelain plate 

 for copper, as described above. Finely divided cuprous oxide re^ 

 mains frequently for a long time suspended in the fluid, and the 

 least trace of it in the fluid is apt to deceive. 



The calculation is as follows : 



Provided we used 18.2 c.c. of the sugar solution to reduce 5 c.c. 

 copper solution, we have : 



18.2 : 250 = 0.0338 : X 

 X = 0.464 milk sugar. 



Occasionally the sugar of milk is determined by difference. 



According to the method of Soxhlet, 25 c.c. milk are diluted 

 with 400 c.c. water, and coagulated by means of a solution of sul- 

 phate of copper; the precipitate is filtered off, and the fluid is 

 brought to 500 c.c. 



100 c.c. of this solution are then transferred to a beaker, and, 

 after adding 50 c.c. of Fehling's solution, are boiled over double 

 wire netting, while the glass is kept covered to prevent loss. After 

 six minutes' boiling, the sub-oxide of copper is collected on an 

 asbestos filter, dried, and reduced by heating in a current of hydro- 

 gen gas. The method is said to give quick (?) and accurate re- 

 sults. According to Soxhlet'* investigations, 



mgrs. of copper weighed correspond to mgrs. milk sugar : 



393.7 300 

 363.6 275 

 333.0 250 



300.8 225 

 269.6 200 

 237.5 175 

 204.0 150 

 171.4 125 

 138.3 100 



