CARBOHYDRATES 25 



OH 

 Cu - Cu = 0+H 2 O. 



\ Cupric oxide 



\ (black). 



OH 



Cupric hydroxide 

 (whitish-blue). 



Reaction in absence of a reducing agent. 



OH 



2Cu - Cu 2 O+2H 2 O+O. 



\Cuprous oxide 

 (yellow to red). 



OH 



Cupric hydroxide 



Reaction in presence of a reducing agent. 



The chemical equations here discussed are exemplified in Trommer's 

 and Fehling's tests. 



(a) Trammer's Test. To 5 c.c. of sugar solution in a test-tube add one-half its 

 volume of KOH or NaOH. Mix thoroughly and add, drop by drop, a very dilute 

 solution of copper sulphate. Continue the addition until there is a slight permanent 

 precipitate of cupric hydroxide and in consequence the solution is slightly turbid. 

 Heat, and the cupric hydroxide is reduced to yellow or brownish-red cuprous oxide. 



If the solution of copper sulphate used is too strong a small brownish-red pre- 

 cipitate produced in a weak sugar solution may be entirely masked. On the other 

 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 oxide. 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 

 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 



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 = 34.65 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. 



