26 PHYSIOLOGICAL CHEMISTRY 



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

 amounts of sugar solution the brownish-red precipitate is generally formed. 

 The differences in color of the cuprous oxide precipitates under different con- 

 ditions are apparently due to differences in the size of the particles, the more 

 finely divided precipitates having a yellow color, while the coarser ones are red. 

 In the presence of protective colloidal substances the yellow precipitate is 

 usually formed. 1 



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

 test is not entirely reliable when used to detect sugar in the urine. Such 

 bodies as conjugate glycuronates, uric acid, nulceoprotein, and homogen- 

 tisic acid when present in sufficient amount may produce a result simi- 

 lar to that produced by sugar. Phosphates of the alkaline earths may 

 be precipitated by the alkali of the Fehling's solution and in appearance 

 may be mistaken for cuprous oxide. 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 examina- 

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

 considerable amount of sugar is present. According to Laird, 2 even 

 small amounts of creatinine will retard the reaction velocity of reducing 

 sugars with Fehling's solution. 



In testing urine preserved by chloroform a positive test may be ob- 

 tained in the absence of sugar. This is due to the fact that the hot 

 alkali produces formic acid (a reducing fatty acid) from the chloroform. 



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 reaction 

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



(c) Benedict's Test. 3 Benedict has modified the Fehling solution and has 

 succeeded in obtaining one which does not deteriorate upon long standing. 4 

 The following is the procedure for the detection of glucose in solution. To 5 c.c. 

 of the reagent in a test-tube add 8 (not more) drops of the solution under exami- 



1 Fischer and Hooker: Science, N. S., 45, 505, 1917. 



'Laird: Journal of Pathology and Bacteriology, 16, 398, 1912. 



3 Benedict: Jour. 'Biol. Chem., 5, 485, 1909: Jour. Am. Med. Ass'n, 57, 1193, 1911. 



4 Benedict's solution has the following composition: 



Copper sulphate 17 3 grams. 



Sodium citrate 173 -o grams. 



Sodium carbonate (anhydrous) 100 . o grams. 



Distilled water to make i liter. 



With the aid of heat dissolve the sodium citrate and carbonate in about 800 c.c. of water. 

 Pour (through a folded filter paper if necessary) into a glass graduate and make up to 850 

 c.c. Dissolve the copper sulphate in about 100 c.c. of water. Pour the carbonate-citrate 

 solution into a large beaker or casserole and add the copper sulphate solution slowly, with 

 constant stirring and make up to one liter. The mixed solution is ready for use and does 

 not deteriorate upon long standing. 



