Barfoed’s Test:

 Barfoed’s Test:

                Take 2ml of the given solution and 2ml of the Barfoed’s reagent. Mix and keep in boiling water bath for 3 minutes. A red precipitate of cuprous oxide indicates the presence of monosaccharide. If the precipitate does not appear after boiling allow the tube to stand for about 15 minutes and examine.

 

Explanation:

                Barfoed’s reagent is a solution of copper acetate and Glacial acetic acid. Reduction of cupric salt to cuprous oxide in the acid midium of the reagent gives red precipitate Monosaccharides react very fast where as the rection with the reducing disaccharides is slow. This test is used to distinguish monosaccharides from reducing diaccharides, but prolonged boiling produce hydrolysis of disaccharides and thus will also answer this test.

 

Benedict’s Test:

                Take 5 ml of Benedict’s reagent add 8 drops of the given solution, boil vigorously for 2 minutes and allow to stand till the precipitate settles, Greenish, Yellow, Yellow red or reddish brown precipitate of cuprous oxide is obtained depending upon the concentration of reducing sugar.

 

Explanation:

                Benedict’s qualitative reagent contains copper sulphate, sodium citrate and mild alkali sodium carbonate all in one solution. Thus it has an advantage over the Fehling’s reagent which has two solutions.

               

                This test like the Fehlings test is based on the reducing property of carbohydrates. Reducing sugars under alkaline conditions tantomerise to form enodiols. The enodiols are unstable and decompose to yield a variety of products. The chain reaction continues to produce short chain aldehydes which are powerfull reducing agents. They can reduce cupric ions to form cuprous ion which precipitates as yellow cuprous hydroxide or red cuprous oxide this is the basis for the Benedict’s & Fehling’s reaction. The cupric hydroxide formed is not easily soluble. In order to keep the hydroxide in solution a metal chelator like citrate (or titrate) is include in the solution. This test is employed as a routine for examination of urine for sugar. Detection of glucose in urine by this test is of clinical significance in the diagnosis and control of diabetes mellitus. Depending upon the amount of sugar present the following coloured precipitate will be obtained.