524 PHYSIOLOGICAL CHEMISTRY 



oxide in solution has been precipitated as cuprous oxide. This point will be 

 indicated by the absolute disappearance of all blue color. When this end point 

 is reached note the number of cubic centimeters of diluted urine used in the proc- 

 ess and calculate the percentage of dextrose present, in the sample of urine 

 analyzed, according to the method given below. 



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

 uncertainty attending the determination of the end-reaction, i.e., the difficulty 

 with which the exact point where the blue color finally disappears is noted. Sev- 

 eral means of accurately fixing 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 solution 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 following 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 solution, we have the following proportion. 



y : 0.05 : : 100 : x (percentage of dextrose). 



Interpretation. See page 523. 



3. Bang's Method. 3 Principle. The solution to be tested is boiled 

 with alkaline cupric chloride solution containing thiocyanate and potas- 

 sium chloride. The cupric salt under these conditions is reduced to 

 the cuprous form without any precipitation taking place. The re- 

 duced copper is .titrated with standard iodine solution using starch as 

 an indicator. The titration reaction is as follows : 



CuCl + I + K 2 C0 3 = CuC0 3 + KC1 + KL 



Procedure. A 100 c.c. Jena flask with a straight neck the flange of which has 

 been cut off is used for the boiling procedure. Over the neck of the flask place 



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 o . 0676 gram. 



Maltose o. 074 gram. 



Invert sugar o . 0475 gram. 



3 Bang: Biochem. Zeit., 49, i, 1913. 



