26 PHYSIOLOGICAL CHEMISTRY 



hand, particularly in testing for sugar in the urine, if too little copper sulphate is 

 used a light-colored precipitate formed by uric acid and purine bases may obscure 

 the brownish-red precipitate of cuprous oxide. The action of KOH or NaOH in the 

 presence of an excess of sugar and insufficient copper will produce a brownish color. 

 Phosphates of the alkaline earths may also be precipitated in the alkaline solution 

 and be mistaken for cuprous hydroxide. Trommer's test is not very satisfactory. 



Salkowski 1 has proposed a modification of the Trommer procedure which he 

 claims is a very accurate sugar test. 



(b) Fehling's Test. To about i c.c. of Fehling's solution 2 in a test-tube add 

 about 4 c.c. of water, and boil. 3 [The cupric hydroxide is held in solution by the 

 sodium potassium tartrate (Rochelle salt).] This is done to determine whether 

 the solution will of itself cause the formation of a precipitate of brownish-red 

 cuprous oxide. If such a precipitate forms, the Fehling's solution must not be 

 used. Add sugar solution to the warm Fehling's solution a few drops at a time 

 and heat the mixture after each addition. The production of yellow cuprous hy- 

 droxide or brownish-red cuprous oxide indicates that reduction has taken place. 

 The yellowish precipitate is more likely to occur if the sugar solution is added 

 rapidly 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 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, nucleoprotein 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 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 examina- 

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

 considerable amount of sugar is present. According to Laird, 4 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. 



1 Salkowski: Zeit. physiol. Chem., 79, 164, 1912. 



2 Fehling's solution is composed of two definite solutions a copper sulphate 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 = 125 grams of potassium hydroxide 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. 



^ 3 More dilute Fehling's solution should be used in testing very dilute sugar solutions. 

 In'case of concentrated sugar solutions it may sometimes be desirable to use a larger volume 

 of the Fehling's solution. 



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



