368 PHYSIOLOGICAL CHEMISTRY. 



meters of diluted urine used in the process and calculate the per- 

 centage of dextrose present, in the sample of urine analyzed, ac- 

 cording to the method given below. 



This is a very satisfactory method, the main objection to its use 

 being the uncertainty attending the determination of the end-reac- 

 tion, i. e., the difficulty with which the exact point where the blue 

 color finally disappears is noted. Several means of accurately fix- 

 ing this point have been suggested but they are practically all open 

 to objection. As good a " check " as any, perhaps, is to filter a few 

 drops of the solution, through a double paper, after the blue color 

 has apparently disappeared, acidify the filtrate with acetic acid and 

 add potassium ferrocyanide. If the copper of the Fehling's solu- 

 tion has been completely reduced, there will be no color reaction, 

 whereas the production of a brown color indicates the presence of 

 unreduced copper. Harrison has recently suggested the follow- 

 ing procedure to determine the exact end-point: To about I c.c. 

 of a starch iodide solution 1 in a test-tube add 2-3 drops of acetic 

 acid and introduce into the acidified mixture 1-2 drops of the 

 solution to be tested. Unreduced copper will be indicated by the 

 production of a purplish-red or blue color due to the liberation of 

 iodine. 



It is ordinarily customary to make at least three determinations 

 by Fehling's method before coming to a final conclusion regarding 

 the sugar content of the urine under examination. 



Calculation. Ten c.c. of Fehling's solution is completely reduced 

 by 0.05 gram of dextrose. 2 If y represents the number of cubic 

 centimeters of undiluted urine (obtained by dividing the burette 

 reading by 10) necessary to reduce the 10 c.c. of Fehling's solu- 

 tion, we have the following proportion : 



y : 0.05 : : TOO : x (percentage of dextrose) . 

 2. Benedict's Method. To 30 c.c. of Benedict's solution 3 in 



1 The starch-iodide solution may be prepared as follows : Mix o.i gram of 

 starch with cold water in a mortar and pour the suspended starch granules into 

 75-100 c.c. of boiling water, stirring continuously. Cool the starch paste, add 

 20-25 grams of potassium iodide and dilute the mixture to 250 c.c. This solution 

 deteriorates upon standing, and therefore must be freshly prepared as needed. 



2 The values for certain other sugars are as follows : 



Lactose 0.0676 gram. 



Maltose 0.074 gram. 



Invert sugar 0.0475 gram. 



3 Benedict's solution used in the quantitative determination of sugar consists of 

 three separate solutions. The cupric sulphate solution and the alkaline tartrate 



