Testing for reducing sugars
Reducing sugars can donate electrons. Examples include the monosaccharides glucose, fructose and galactose; some disaccharides, including maltose, are also reducing sugars.
Benedict's reagent can be used to detect reducing sugars. During a positive reaction, the reducing sugar donates electrons to copper(II) ions in the blue reagent, producing insoluble copper(I) oxide.
- Add Benedict's reagent to the sample solution in a test tube. Use the reagent in excess so that sufficient copper(II) ions are present to react with any reducing sugar.
- Place the tube in a hot or boiling water bath for a few minutes.
- Record the colour and whether a precipitate forms.
| Observation | Interpretation |
|---|---|
| Blue | No reducing sugar detected |
| Green | Low concentration of reducing sugar |
| Yellow | Increasing concentration of reducing sugar |
| Orange | Higher concentration of reducing sugar |
| Brown / brick-red precipitate | High concentration of reducing sugar |
The progression from blue through green, yellow and orange towards brick-red means the test is semi-quantitative: the result gives an indication of reducing-sugar concentration, but not a precise numerical value.
Testing for a non-reducing sugar
A non-reducing sugar does not react directly with Benedict's reagent. Sucrose is a common example. To show that a non-reducing sugar is present, first establish that the untreated sample does not already contain detectable reducing sugar.
- Carry out Benedict's test on a portion of the original sample.
- If the reagent remains blue, take a fresh portion of the sample and add dilute hydrochloric acid.
- Heat this mixture in a water bath that has been brought to the boil. The acid hydrolyses glycosidic bonds, releasing monosaccharides.
- Add sodium hydrogencarbonate to neutralise the acid. Use a suitable indicator, such as red litmus paper, to identify when neutralisation has been reached.
- Add a little further sodium hydrogencarbonate so the solution is slightly alkaline, then carry out Benedict's test on the hydrolysed sample.
- If this second Benedict's test is positive, reducing sugars have been produced by hydrolysis, providing evidence that a non-reducing sugar was present in the original sample.
Exam Tip: Distinguish the two tests clearly: first show that the untreated sample gives a negative Benedict's test, then hydrolyse → neutralise and make slightly alkaline → repeat Benedict's test.