60 METHODS FOR THE ANALYSIS OF URINE 



method has several disadvantages over Benedict's method. 

 The end-point is difficult to determine and the mixed solution is 

 unstable. It gives less accurate results. 



Procedure. Place 10 c.c. of the urine under examination in 

 a 100 c.c. volumetric flask and make the volume up to 100 c.c. 

 with distilled water. (If the urine contains less than 0.5 per cent 

 of sugar it may be used without dilution. A concentration of 

 about 0.5 per cent is the most satisfactory for this titration.) 

 Thoroughly mix this diluted urine by pouring it into a beaker 

 and stirring with a glass rod, then transfer a portion of it to a 

 burette which is properly supported in a clamp. 



Now place 10 c.c. of Fehling's solution in a small beaker, 

 dilute it with approximately 40 c.c. of distilled water, heat to 

 boiling, and observe whether decomposition of the Fehling's 

 solution itself has occurred as indicated by the production of a 

 turbidity. If such turbidity is produced the Fehling's solution 

 is unfit for use. Clamp the burette containing the dilute urine 

 immediately over the beaker and carefully allow from 0.5 to 1 c.c. 

 of the diluted urine to flow into the boiling Fehling's solution. 

 Bring the solution to the boiling-point after each addition of urine 

 and continue running the urine from the burette, 0.5 to 1 c.c. 

 at a time, as indicated, until the Fehling's solution is completely 

 reduced, i.e., until all the cupric oxide in solution has been pre- 

 cipitated 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 process 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. Several means of accu- 

 rately fixing this point have been suggested, but they are practi- 

 cally 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 fil- 

 trate 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 



