Benedict's Test for Non-reducing Sugars
The Benedict's Test for Non-reducing Sugars determines the presence of non-reducing sugars - sugars which do not have an aldehyde functional group.Many students confuse the Benedict's Test for Non-Reducing Sugars with the Benedict's Test for Reducing Sugars, perhaps because some of the steps in the procedure are similar.
Benedict's solution is the principle reagent in both tests. The test for non-reducing sugars is often conducted on a food sample which tested negative for reducing sugar. If reducing sugars have been shown to be present, a heavier precipitate is often observed when the test for non-reducing sugar is conducted.
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.
What is a Non-reducing Sugar?
Sugars can be classified as either reducing or non-reducing based on their ability to reduce copper(II) ions to copper (I) ions during the Benedict's Test.
Non-reducing sugars do not contain an aldehyde group - the reducing species. Reducing sugars are simple, disaccharide sugars. Sucrose is the most common disaccharide non-reducing sugar.
Observation and Interpretation
Principle of the Benedict's Test for Non-reducing Sugar
Disaccharides are hydrolyzed to their constituent monosaccharides when boiled in dilute hydrochloric acid. The monosaccharides are reducing sugars are they contain the aldehyde group which is the reducing species.
Neutralization of the mixture by sodium hydrogen carbonate is necessary as the reduction of the copper(II) ions will not take place in acidic conditions - of excess acid is present.
Benedict's Solution contains copper(II) sulphate, sodium carbonate and sodium citrate.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.
Sodium carbonate provides the alkaline conditions which are required for the redox reaction above.
Sodium citrate complexes with the copper (II) ions so that they do not deteriorate to copper(I) ions during storage.