Reducing sugars can donate electrons, reducing another chemical. All monosaccharides and most disaccharides (maltose, lactose) are reducing sugars. Sucrose is a non-reducing sugar.Test for reducing sugars (Benedict's test).
1. Add an equal volume of Benedict's reagent (blue, containing copper(II) sulfate) to the sample in solution.
2. Heat in a boiling water bath for about 5 minutes.
3. A reducing sugar reduces blue Cu2+ ions to copper(I) oxide, which forms a brick-red precipitate.If no reducing sugar is present, the solution stays blue. The colour depends on how much reducing sugar there is:blue → green → yellow → orange → brick red so the test is semi-quantitative. To compare samples, use the same volumes and heating time and compare colours with standards of known concentration. For a more precise result, filter off and weigh the precipitate, or use a colorimeter to measure how much blue colour remains in the solution.Test for non-reducing sugars. If the Benedict's test is negative:
1. Boil a fresh sample with dilute hydrochloric acid. This hydrolyses the glycosidic bond, releasing monosaccharides.
2. Neutralise with sodium hydrogencarbonate, because Benedict's reagent does not work in acid.
3. Repeat the Benedict's test. A brick-red precipitate now shows a non-reducing sugar was present.
Worked example: a sample stays blue in the first test but turns orange after acid hydrolysis. It contains a non-reducing sugar, such as sucrose, but no reducing sugar.
1. Add an equal volume of Benedict's reagent (blue, containing copper(II) sulfate) to the sample in solution.
2. Heat in a boiling water bath for about 5 minutes.
3. A reducing sugar reduces blue Cu2+ ions to copper(I) oxide, which forms a brick-red precipitate.If no reducing sugar is present, the solution stays blue. The colour depends on how much reducing sugar there is:
1. Boil a fresh sample with dilute hydrochloric acid. This hydrolyses the glycosidic bond, releasing monosaccharides.
2. Neutralise with sodium hydrogencarbonate, because Benedict's reagent does not work in acid.
3. Repeat the Benedict's test. A brick-red precipitate now shows a non-reducing sugar was present.
Worked example: a sample stays blue in the first test but turns orange after acid hydrolysis. It contains a non-reducing sugar, such as sucrose, but no reducing sugar.