CARBOHYDRATES 41 



kalies on the carbohydrates, but the above equations serve to 

 show the nature of the part played by the copper compound in 

 the reaction. 



The above test would work very well if the solution to be 

 tested contained much sugar. If this were not the case, how- 

 ever, much black cupric oxide would be formed which might 

 easily obscure any small amount of red cuprous oxide resulting 

 from the reducing action of a small amount of sugar. Accord- 

 ingly it is more satisfactory to use Fehling 's solution for the test. 

 This is made up in two parts, A and B, which are mixed in equal 

 quantities immediately before using. A contains copper sul- 

 phate; B contains sodium hydrate and sodium potassium tar- 

 trate. On mixing these two solutions a deep blue liquid results. 

 The two solutions are kept separate, as otherwise the tartrate 

 will slowly reduce the copper. The advantage in Fehling 's rea- 

 gent lies in the fact that the sodium potassium tartrate unites 

 with the cupric hydroxide to form a complex ion ; thus the cupric 

 hydroxide does not precipitate and does not decompose into the 

 black cupric oxide. On boiling, the liquid remains clear and 

 blue. The combined cupric hydroxide is in equilibrium with a 

 very small amount of this compound in solution so that as fast 

 as the free cupric hydroxide is reduced by the sugar solution, 

 more of the copper-hydrate-tartrate compound dissociates. This 

 complex compound thus furnishes a ready supply of copper 

 hydroxide, and if sufficient sugar is present, all of the copper 

 will be reduced. At this point the blue color will have disap- 

 peared from the liquid. The following equation illustrates the 

 formation of the complex compound : 



COO COO 



! I 



CHOH+2 Cu(OH) 2 ^ CHO CuOH+2H 2 



CHOH CHO CuOH 



COO COO 



