CARBOHYDRATES AND RELATED BODIES. 2Q 



This Fehling's solution is commonly employed as a quali- 

 tative test for reducing sugars in general, but the reaction 

 may be first illustrated by a simpler method : 



Ex. TROMMER'S TEST. Add to a dilute solution of glucose a consid- 

 erable excess of strong potassium hydroxide solution and then a very few 

 drops of a dilute copper sulphate solution. This produces no precipitation 

 but imparts a deep blue color. On warming the solution a yellowish pre- 

 cipitate forms, which grows bright red by boiling. This is cuprous oxide, 

 and the test is known as " Trommer's " test. It is frequently employed 

 to detect the presence of sugar in liquids, especially in urine, but on the 

 whole is not as satisfactory as the next one. 



Ex. FEHLIXG'S TEST. To a very weak glucose solution add an equal 

 volume of diluted Fehling's solution. The mixture remains deep blue in 

 the cold, but on heating a yellowish precipitate turning to red is soon pro- 

 duced. This precipitate of cuprous oxide comes from the reduction of the 

 cupric compound held in solution in the test reagent. In the first or Trom- 

 mer test the first indication of the presence of a sugar is the formation of 

 a deep blue clear solution rather than a greenish blue precipitate of cupric 

 hydroxide which would result from the action of the copper sulphate and 

 alkali alone. But cupric hydroxide dissolves in solutions of sugars and 

 other polyhydric alcohols, the solution being deep blue, and stable in the 

 cold. In the case of glucose and other reducing (aldehyde or ketone) 

 sugars this stability is only temporary, since reduction follows on boiling. 

 If the polyhydric alcohol employed to produce the deep blue solution is 

 not a reducing substance the liquid remains clear and stable, even on boil- 

 ing. This is the case with the Fehling's solution, in which the cupric 

 hydroxide is held dissolved through the alcoholic behavior of the Rochelle 

 salt. Similar solutions are made by the aid of glycerol (trihydric) and 

 mannitol (hexahydric). The Fehling test has this advantage over the 

 Trommer test, that in the latter if too much copper sulphate is used, and 

 little sugar is present the precipitate on boiling may be mainly black cupric 

 oxide instead of the red cuprous oxide. With the Fehling liquid no black 

 precipitate can form. 



Ex. BISMUTH REDUCTION TEST. Add to a glucose solution some strong 

 potassium hydroxide solution and then a very small amount of bismuth 

 subnitrate. For an ordinary test a few milligrams will be enough. On 

 boiling, a black precipitate appears, which frequently forms a bright mir- 

 ror on the walls of the test-tube. This precipitate seems to be a mixture 

 of metallic bismuth with some oxide, and shows the strong reducing power 

 of the sugar. Similar reductions may be obtained from alkaline solutions 

 of several heavy metals, but these tests illustrate the general principle. 



Another test which serves for the recognition of even minute 

 traces of glucose and other sugars, is the following, proposed 

 bv Molisch : 



