ELEMENTAEY PHYSIOLOGICAL CHEMISTKY 



275 



EXPERIMENT VIII. Dilute 5 c.c. of diabetic urine with 95 c.c. of 

 distilled water, and place the solution in a burette. Place 10 c.c. of 

 Fehling's solution, diluted with four or five times its bulk of water, in a 

 porcelain basin, and bring it to the boil. Now run the diluted urine 

 into the Fehling's solution, stirring 

 all the while, until the blue colour 

 has quite disappeared. To tell the 

 exact moment at which this occurs, 

 allow the basin to stand for a minute 

 or so with the flame removed ; the 

 precipitate of cuprous oxide settles 

 to the bottom, and it can easily 

 be seen, by tilting the basin slightly, 

 whether any blue tint remains in 

 the supernatant fluid. When re- 

 duction is complete read off the 

 number of c.c. of diluted urine re- 

 quired, and divide by 20. The 

 result gives the number of c.c. 

 of undiluted urine which contain 

 0'05 grm. dextrose. How much 

 Avill 100 c.c. contain ? 



It requires several trials before 

 .anything like constant results can 

 be obtained by this method, the 

 difficulty being to hit the exact 

 moment when the blue colour dis- 

 .appears. It is desirable always to 

 apply a preliminary determination 

 in order to find out approximately, 

 where special care will be required 

 in a second titration for the exact 

 result. 



The chief difficulty in the deter- 

 mination of the end reaction is due to 

 the presence of the red precipitate, suspended in the pale blue solution. 

 In order to obviate this difficulty Pavy adds to the Fehling's solution 

 strong ammonia, which has the power of dissolving the cuprous oxide 

 as it forms, the resulting solution being colourless. The end reaction 

 is then easily seen, but the method has the disadvantage that fumes of 

 ammonia are evolved, and the solution very easily absorbs oxygen from 

 the air, and reforms the cupric salt. On account of these facts the 



FIG. 165. Apparatus for the estimation of 

 sugar in urine. 



