28 PHYSIOLOGICAL CHEMISTRY. 



oxide indicates that reduction has taken place. The yellow pre- 

 cipitate is more likely to occur if the sugar solution is added rap- 

 idly and in large amount, whereas with a less rapid addition of 

 smaller amounts of sugar solution the brownish-red precipitate is 

 generally formed. 



This is a much more satisfactory test than Trammer's, but even 

 this test is not entirely reliable when used to detect sugar in the 

 urine. Such bodies as conjugate glycuronates, uric acid, nucleo- 

 protein and homogentisic acid when present in sufficient amount 

 may produce a result similar to that produced by sugar. Phos- 

 phates of the alkaline earths may be precipitated by the alkali of 

 the Fehling's solution and in appearance may be mistaken for 

 cuprous hydroxide. Cupric hydroxide may also be reduced to 

 cuprous oxide and this in turn be dissolved by creatinine, a normal 

 urinary constituent. This will give the urine under examination 

 a greenish tinge and may obscure the sugar reaction even when a 

 considerable amount of sugar is present. 



(c) Benedict's Modifications of Fehling's Test. First Modifi- 

 cation. 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 sec- 

 onds and permit it to cool to room temperature spontaneously. (If 

 desired this process may be repeated, although it is ordinarily un- 

 necessary.) 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 that, 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. 



Benedict's modified Fehling solution consists of two definite solutions a 

 cupric sulphate solution and an alkaline tartrate solution, which may be pre- 

 pared as follows : 



Cupric sulphate solution = 34.65 grams of cupric 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. 



