CARBOHYDRATES 27 



Ammonium salts also interfere with Fehling's test. If present in 

 excess the solution (e.g., urine) should be made alkaline and boiled in 

 order to decompose the ammonium salts. 



If the solution under examination by Fehling's test is acid in re- 

 action it must be neutralized or made alkaline before applying the test. 



(c) Benedict's Modifications of Fehling's Test. First Modification. To 2 c.c. 

 of Benedict's solution 1 in a test-tube add 6 c.c. of distilled water and 7-9 drops 

 (not more) of the solution under examination. Boil the mixture vigorously for 

 about 15-30 seconds and permit it to cool to room temperature spontaneously. 



If sugar is present in the solution a precipitate will form which is 

 often bluish-green or green at first, especially if the percentage of sugar is 

 low, and which usually becomes yellowish upon standing. If the sugar 

 present exceeds 0.06 per cent this precipitate generally forms at or below 

 the boiling-point, whereas if less than 0.06 per cent of sugar is present the 

 precipitate forms more slowly and generally only after the solution has 

 cooled. 



Benedict claims, whereas the original Fehling test will not serve 

 to detect sugar when present in a concentration of less than o.i per 

 cent, that the above modification will serve to detect sugar when 

 present in as small quantity as 0.015-0.02 per cent. Corroboration 

 of this claim of increased delicacy has recently been submitted by 

 Harrison. 2 



The modified Fehling solution used in the above test differs from 

 the original Fehling solution in that 100 grams of sodium carbonate is 

 substituted for the 125 grams of potassium hydroxide ordinarily used, 

 thus forming a Fehling solution which is considerably less alkaline 

 than the original. This alteration in the composition of the Fehling 

 solution is of advantage in the detection of sugar in the urine inasmuch 

 as the strong alkalinity of the ordinary Fehling solution has a tendency, 

 when the reagent is boiled with a urine containing a small amount of 

 dextrose, to decompose sufficient of the sugar to render the detection of 

 the remaining portion exceedingly difficult by the usual technic. Bene- 

 dict claims that for this reason the use of this modified solution permits 

 the detection of much smaller amounts of sugar than does the use of the 

 ordinarily Fehling solution. 



1 Benedict's modified Fehling solution consists of two definite solutions a copper sul- 

 phate solution and an alkaline tartrate solution, which may be prepared as follows: 



Copper sulphate solution = 34.6 5 grams of copper sulphate dissolved in water and made 

 up to 500 c.c. 



Alkaline tartrate solution = ioo grams of anhydrous sodium carbonate and 173 grams of 

 Rochelle salt dissolved in water and made up to 500 c.c. 



These solutions should be preserved separately in rubber-stoppered bottles and mixed 

 in equal volumes when needed for use. This is done to prevent deterioration. 



2 Harrison: Pharm. Jour., 87, 746, 1911. 



