

CARBOHYDRATES. 2/ 



Cu - OH \ 



-*- + H 2 0. 



Cu - OH / 



Cu 



Cuprous hydroxide Cuprous oxide 



(yellow). (brownish-red). 



The chemical equations here discussed are exemplified in Trom- 

 mer'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 thor- 

 oughly and add, drop by drop, a very dilute solution of cupric 

 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 cuprous hydroxide or to brownish-red cuprous oxide. If 

 the solution of cupric sulphate used is too strong a small brownish- 

 red precipitate 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 cupric sulphate is used a light-colored pre- 

 cipitate 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 satis- 

 factory. 



(&) Fehling's Test. To about i c.c. of Fehling's solution 1 

 in a test-tube add about 4 c.c. of water, and boil. This is done 

 to determine whether the solution will of itself cause the forma- 

 tion 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 hydroxide or brownish-red cuprous 



1 Fehling's solution is composed of two definite solutions a cupric sulphate 

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



Cupric sulphate solution = 34.65 grams of cupric 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. 



