18 THE CARBOHYDRATES. 



quired. Then rinse out the beaker, take again 10 cubic 

 centimeters of the Fehling solution, diluted as before; heat 

 to boiling and run in at once within 1 cubic centimeter of 

 the necessary amount of the sugar solution. Bring it to a 

 boil. Then add the sugar solution a few drops at a time, 

 heating after each addition, until the blue color has just 

 been decolorized. 



Since 10 cubic centimeters of the Fehling solution is 

 decolorized by 0.05 gramme of glucose, the amount of the 

 sugar solution or urine which has been used from the 

 burette must have contained 0.05 gramme of glucose. Eead 

 the volume which has been poured from the burette, and 

 calculate the percentage of sugar in the original solution. 

 If this has been diluted with water the amount in the dilute 

 solution must be multiplied by the number of times it was 

 diluted. Eemember that however much of the sugar solu- 

 tion may have been used to destroy the blue color, it con- 

 tained 0.05 gramme of sugar. For example, if the amount 

 used was 10 cubic centimeters, there would be 0.005 

 gramme of glucose in 1 cubic centimeter; that is, in 1 

 gramme of solution. In 100 grammes there would be 0.5 

 gramme of glucose, or 0.5 per cent. 



The floating red precipitate of cuprous oxid obscures the encl 

 reaction and makes the titration slow if time is allowed for this 

 to settle. Purdy's modification of Fehling's reagent consists in 

 the addition of a large excess of ammonium hydrate which pre- 

 vents the precipitation of the cuprous oxid. The reagent con- 

 tains: 



Cupric sulphate 4.742 grm. 



Glycerin, pure 38 cc. 



Dissolve in about 200 cc. of water. 



Potassium hydrate 23.5 grm. 



Dissolve in about 200 cc. of water. 



