GLUCOSE. 17 



QUANTITATIVE TEST FOR GLUCOSE. 



Fehling's Method. The solutions used are: (A) 34.64 

 grammes of cupric sulphate (CuS0 4 , 5H 2 0), dissolved in 

 enough water to make the volume 500 cubic centimeters. 

 The crystals used must be dark blue and not effloresced; 

 (B) 187 grammes of pure Eochelle salt and 68 grammes of 

 sodium hydrate in water enough to make the volume 500 

 cubic centimeters. These solutions must be kept separate. 



33. For each determination mix 5 cubic centimeters 

 of A with 5 cubic centimeters of B, measuring carefully 

 with a pipette. Add about 40 cubic centimeters of water, 

 and heat to boiling in a beaker or porcelain dish. If the 

 solution is good there will be no red precipitate. 



The best results are obtained when the solution con- 

 tains from 0.5 per cent, to 1.0 per cent, of sugar; that is, 

 when from 5 to 10 cubic centimeters are necessary to 

 destroy the blue color of the Fehling solution. If it con- 

 tains more than this, it must be diluted with water to 5 or 

 10 times its volume, measuring accurately the water added 

 and mixing thoroughly. 



The Fehling solution after dilution is heated to boil- 

 ing, and the sugar solution run in from a burette until the 

 blue color has been destroyed, leaving the liquid colorless 

 above the red precipitate. If too much sugar has been 

 added it begins to turn yellowish. The amount of sugar 

 is ascertained most quickly by making two determinations: 

 first, a rough one, then one which is made more carefully. 

 Make the first by running in the sugar solution, 2 or 3 

 cubic centimeters at a time, as long as the blue color is 

 well marked, then 1 cubic centimeter at a time, heating to 

 boiling after each addition. It can be learned by this first 

 test within 1 or 2 cubic centimeters how much will be re- 



