How do you measure concentration of reducing sugars?

1 Answer
Jul 23, 2015


One method is to do a colorimetric determination using Benedict's solution.


Benedict's solution is a deep-blue alkaline solution of stabilized #"Cu"^(2+)# ions.

It oxidizes aldehyde groups to carboxylate ions, and the #"Cu"^(2+)# ions are reduced to a brick red precipitate of #"Cu"_2"O"#.

#"RCHO(aq)" + underbrace("Cu"^(2+)("aq"))_(color(blue)("deep blue")) + "OH"^(-)("aq") → "RCOO"^(-)("aq") + underbrace("Cu"_2"O"("s"))_(color(red)("brick red"))+ "3H"_2"O"#

Heating the unknown to about 95°C with Benedict's solution leads to the formation of a brick-red precipitate that indicates the presence of a reducing sugar.

You treat your unknown and a standard set of solutions containing known concentrations of glucose with standard Benedict's solution.

You remove the precipitate and measure the intensity of absorption in a colorimeter.

A calibration graph prepared from the glucose solutions enables you to determine the concentration of your unknown.