DETERMINATION OF LACTOSE. 51 



For use in the analysis equal volumes of solutions one 

 and two are mixed together. Since the mixture of the 

 two solutions suffers decomposition by standing, they 

 should be kept separate until required for use. They 

 should be kept in well stoppered bottles. 



The following are the details of the process : lo c. c. 

 of each of the two reagents are mixed in a small porcelain 

 evaporator, diluted with 50 c. c. of water and heated to 

 boiling. Into the solution a few cubic centimeters of the 

 filtrate containing the sugar are run in and heated for a 

 few moments. It is then allowed to stand till the red 

 cuprous oxide settles out. The color of the supernatant 

 liquid is then noted. If it be still blue a few more c. c. of 

 the sugar solution are run in from the burette and again 

 heated and allowed to settle. This process is continued 

 until the solution becomes colorless. Another sample of 

 the Fehling's solution is then measured out and titrated 

 with the sugar solution as before. The previous deter- 

 mination will serve as a guide so that nearly the requireil 

 amount of sugar solution may be run in at once. The 

 exact end point may then be determined by careful titra- 

 tion drop by drop. At least three tests of this sort should 

 be made. The first observation is usually discarded. 



The exact point of complete discoloration of the fluid 

 is often difficult to detect because of the incomplete sep- 

 aration of the precipitate. The use of side tests of potas- 

 sium ferroc>anide for the determination of the end re- 

 action, as formerly recommended, is considered unsatis- 

 factory. The solution should not be allowed to stand too 

 long in order to permit the precipitate to settle com- 

 pletely because the cuprous oxide easily oxidizes to cu- 

 pric oxide by the action of the oxygen of the air and dis- 

 solves again in the alkaline solution producing a blue 

 color. If the end reaction be difficult to determine with 

 precision, a small sample of the solution may be filtered 



