DETERMINATION OF SUGAR 95 



solution the sulphite will itself give a blue color with the phos- 

 photungstic acid. 



DETERMINATION OF SUGAR 



Method of Folin and Wu l 



Principle. When sugar and an alkaline solution of copper are 

 heated cuprous oxide is formed. In the presence of a molybdate 

 phosphate solution cuprous oxide gives rise to an intense and 

 stable color reaction. The color so obtained is compared with 

 the color given by a standard solution of sugar. 



Solutions Needed. 1. Standard Sugar Solution. Three stand- 

 ard sugar solutions should be on hand: (1) a stock solution, 1 per 

 cent dextrose or invert sugar, preserved with xylene or toluene; 

 (2) a solution containing 1 mg. of sugar per 10 c.c. (5 c.c. of stock 

 solution diluted to 500 c.c.); (3) a solution containing 2 mg. of 

 sugar per 10 c.c. (5 c.c. of the stock solution diluted to 250 c.c.) 

 The diluted solutions should be preserved with a little added 

 toluene or xylene; it is better not to depend on such diluted solu- 

 tions to keep for more than a month, but the stock solution should 

 keep indefinitely. If pure dextrose is not available, a standard 

 solution of invert sugar may be made from cane sugar and is equally 

 useful. To make this transfer exactly 1 gm. of cane sugar to a 

 100 c.c. volumetric flask; add 20 c.c. of normal hydrochloric acid 

 and let the mixture stand over night at room temperature (or 

 rotate the flask and contents continuously in the water bath at 

 70 C.). Add 1.68 gm. of sodium bicarbonate and about 0.2 gm. 

 of sodium acetate to neutralize the hydrochloric acid. Shake a few 

 minutes to remove most of the carbonic acid and fill to the 100 c.c. 

 mark with water. Then add 5 c.c. more of water (1 gm. of cane 

 sugar yields 1.05 gms. of invert sugar) and mix. Transfer to a 

 bottle; add a few drops of xylene or toluene, shake well, and stopper 

 tightly. 



2. Alkaline Copper Solution. Dissolve 40 gms. of anhydrous 

 sodium carbonate in about 400 c.c. of water and transfer to a liter 

 flask. Add 7.5 gms. of tartaric acid and when the latter has dis- 

 solved add 4.5 gms. of crystallized copper sulphate; mix and make 

 up to a volume of 1 liter. If the carbonate used is impure a sedi- 



1 Folin and Wu: Jour. Biol. Chem., 1920, 41, 367. 



