GLUCOSE 61 



color indicates the presence of unreduced copper. Harrison has 

 recently suggested the following procedure to determine the 

 exact end -point: To about 1 c.c. of a starch iodide solution in a 

 test-tube add 2 to 3 drops of acetic acid and introduce into the 

 acidified mixture 1 to 2 drops of the solution to be tested. Unre- 

 duced copper will be indicated by the production of a purplish-red 

 or blue color due to the liberation of iodine. 



It is ordinarily customary to make at least three determina- 

 tions by Fehling's method before coming to a final conclusion 

 regarding the sugar content of the urine under examination. 



Calculation. Ten c.c. of Fehling's solution is completely 

 reduced by 0.05 gm. of dextrose. If y represents the number 

 of cubic centimeters of undiluted urine (obtained by dividing the 

 burette reading by 10) necessary to reduce the 10 c.c. of Fehling's 

 solution, we have the following proportion: 



y : 0.05 : : 100 : x (percentage of dextrose). 



GLUCOSE 



Benedict's Method l 



Principle. Benedict's reagent for the estimation of reducing 

 sugars contains potassium thiocyanate as well as copper sulphate, 

 and in the presence of the former a white precipitate of cuprous 

 thiocyanate is formed on reduction instead of the usual red 

 precipitate of cuprous oxide. The small amount of potassium 

 ferrocyanide also aids in keeping cuprous oxide in solution. As 

 the precipitate formed is white the loss of all blue tint in the 

 solution, indicating complete reduction of the copper, is readily 

 observed. The alkali used is sodium carbonate, which has the 

 advantage over the hydroxide in that there is less danger of 

 destruction of small amounts of sugar. The solution also has 

 the great advantage of being stable for an indefinite length of 

 time. The method is recommended for simplicity and accuracy. 



Procedure. The urine, 10 c.c. of which should be diluted 



with water to 100 c.c. (unless the sugar content is believed to be 



low, when it may be used undiluted), is poured into a 50 c.c. 



burette up to the zero mark. Twenty-five c.c. of the reagent 



Benedict: Jour. Am. Med. Ass'n., 1911, 57, 1193. 



