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Benedict's Test for reducing sugar

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Is the Benedict's Test for reducing sugars qualitative or quantitative?

The test may be qualitative, or it may be quantitative.

The qualitative test produces a colour change from blue to green to yellow to orange to brick red.  The qualitative test is also regarded as semi-quantitative as the colour obtained correlates to the concentration of reducing sugars in the solution ( see observations below). This allows for a rough estimation of the amount of reducing sugar present.  The qualitative test is discussed here.

The quantitative test involves the use of potassium thicyanate and the production of copper thiocyanate as white or pale green precipitate. This precipitate can then be titrated.

PROCEDURE

What is the procedure for the Benedict's Test for reducing sugars?

A liquid food sample does not need prior preparation except dilution if viscous or concentrated.

For a solid sample prepare a test solution by crushing the food and adding a moderate amount of distilled water. Decant the suspension to remove large particles. Use the decanted liquid as the test solution.

  • Add 2 cm3 of the sample solution to a test tube. 
  • Add an equal volume of Benedict's solution to the test tube and swirl or vortex the mixture.
  • Leave the test tube in a boiling water bath for about 5 minutes, or until the colour of the mixture does not change.
  • Observe the colour changes during that time as well as the final colour.
  • To prepare a control, repeat the steps above using 2 cm3 of distilled water instead of sample solution


OBSERVATIONS/RESULTS

What are the expected observations for the Benedict's Test for reducing sugars?

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Observations

No Colour Change (Blue)

Green


Yellow


Orange

Brick-red
Interpretations

No reducing sugars present


Trace amounts of reducing sugars present


Low amounts of reducing sugars present


Moderate amounts of reducing sugars present

Large amounts of non-reducing sugars present

DISCUSSION

What is the principle of the Benedict's Test for reducing sugars?

Reducing Sugars  have an aldehyde functional group which can reduce soluble copper (II) ions  - in copper (II) sulphate -  to insoluble copper (I) ions - in copper (i)oxide. The copper (I) oxide is seen as a precipitate.

State the role of copper sulphate in Benedict's Solution.

Reduced Species. The blue copper(II) ions from copper(II) sulphate are reduced to red copper(I) ions by the aldehyde groups in the reducing sugars. This accounts for the colour changes observed.

The red copper(I) oxide formed is insoluble in water and is precipitated out of solution. This accounts for the precipitate formed.

As the concentration of reducing sugar increases, the nearer the final colour is to brick-red and the greater the precipitate formed.

State the role of sodium carbonate in Benedict's Solution.

Alkalinization. Sodium carbonate provides the alkaline conditions which are required for the redox reaction above.

State the role of sodium citrate in Benedict's Solution.

Stabilization. Sodium citrate complexes with the copper (II) ions so that they do not deteriorate to copper(I) ions during storage.

Are there alternative tests to Benedict's Test for Reducing  Sugars?

The Fehling's Test for non-reducing sugar is an alternative to the Benedict's Test. However it is less popular as it less sensitive and requires that the reagents - Fehling's solutions A and B - be kept separate until the experiment is carried out.

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