Benedict's Test

Qualitative or quantitative test for reducing sugars

Safety

Methods

Results

Benedict’s solution reacts with reducing sugars on heating and reduces the Cu(II) ion to Cu(I) producing a precipitate of red copper oxide. The resulting colour change depends on the type and concentration of sugar, so this test has been used semi-quantitatively to indicate approximate concentrations.

An alternative version of Benedict's reagent for quantitative testing (QBS) contains potassium thyocyanate and does not form red copper oxide. Instead the presence of reducing sugar is measured by the loss of the blue colour of copper sulphate and a white precipitate is formed which will settle out or can be removed by filtration before colorimetric determination of the filrate.

Using a colorimeter and either version of Benedict's reagent you can obtain accurate, fully quantitative determinations of concentration down to 0.001M, (180µg of glucose/cm³). This is about 5 times lower than the concentrations detectable with test strips.

Lower concentrations can be detected rather more easily and in smaller volumes using DNSA

WEAR EYE PROTECTION TAKE CARE WITH BOILING WATER

Benedict's reagent

• Add about 5cm³ of the reagent to a small amount of sample in a test tube.
• Stand the test tube in boiling water for a few minutes.
• A colour change through green to yellow, brown and finally to red indicates the presence of reducing sugar.

You can use Benedict's reagent in a quantitative test....

• Add 3 cm³ of Benedict’s solution to 3cm³ of the sample to be tested.
• Stand the test tube in boiling water for 5 minutes.
• Filter the contents of the tube through a fine grade filter paper such as Whatman Grade 6.

At low concentrations of reducing sugar there will be unreacted copper sulphate left. Read absorbance using red light.

At higher concentrations there will be increasing amounts of red copper oxide in the filtrate. Read absorbance using blue light

Quantitative Benedict's reagent

This solution contains potassium thiocyanate and does not give a red precipitate on boiling. The amount of reducing sugar present is measured by the disappearance of the blue colour of copper sulphate.

• Add 2cm³ of QBS to 4cm³ of sample in a test tube.
• Stand the test tube in boiling water for 5 minutes
• Allow the tubes to stand until the precipitate settles, or filter to remove the precipitate.
• Measure the absorbance using red light

Benedict's solution



Quantitative Benedict's solution

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