Reducing Sugars and the Benedict by ashrafp


									                      Reducing Sugars and the Benedict’s Test

Reducing sugars are the sugars that can REDUCE Benedict’s reagent from its blue colour
to a brick-red precipitate. All monosaccharides and some disaccharides (e.g. maltose and
lactose) are reducing sugars. Sucrose is a disaccharide and it is NOT a reducing sugar.

Benedict’s reagent contains copper sulphate. Reducing sugars reduce soluble blue copper
sulphate, containing copper (ii) ions (Cu2+) to insoluble red-brown copper oxide
containing copper (i). The latter is seen as a precipitate.

So, reducing sugars can reduce the copper ions in the benedict’s reagent from Cu2+ to Cu+

       Ionic equation: Cu2+ + e-            Cu+
                      blue solution         brick-red ppt.

The Benedict’s test is a QUALITATIVE TEST – that is it tests whether a reducing sugar
is present or not.
It can be used as a QUANTITATIVE TEST – that is to assess how much reducing sugar
is present.

When being used as a semi-quantitative test the final precipitate will appear green to
yellow to orange to red-brown with increasing amounts of reducing sugar. (The initial
yellow colour blends with the blue of the copper sulphate solution to give the green
colouration.)         v. small amount of reducing sugar  green ppt.
                      Slightly more reducing sugar  yellow ppt.
                      Even more reducing sugar  orange ppt.
                      Lots of reducing sugar  brick-red ppt.

N.B. The most common disaccharides are:
                   Maltose = glucose + glucose
                   Lactose = glucose + galactose
                   Sucrose = glucose + fructose

To top