Benedict's Test for Reducing Sugars
The Benedicts test – A test for reducing sugars (goes red)
Reducing Sugars. All monosaccharides and most disaccharides will reduce copper (II) sulphate, producing a precipitate of copper (I) oxide on heating, so they are called reducing sugars. Benedict’s reagent is an aqueous solution of copper (II) sulphate, sodium carbonate and sodium citrate.
Collect : Bunsen burner, large beaker, boiling tubes, gauze, tripod, solutions to test (sucrose, glucose, fructose), heat proof mat, benedicts reagent, measuring cylinder
Add 2 cm³ of test solution add an equal quantity of Benedict’s reagent.
Shake, and heat for 5 minutes at 95°C in a water bath.
A precipitate indicates reducing sugar. The colour and density of the precipitate gives an indication of the amount of reducing sugar present, so this test is semi-quantitative. The original pale blue colour means no reducing sugar, a green precipitate means relatively little sugar; a brown or red precipitate means progressively more sugar is present. Record your results in your book.
Non-reducing Sugars. _________ is called a non-reducing sugar because it does not reduce copper sulphate. This is because the reducing groups (aldehyde and ketone groups) on its constituent monosaccharides are removed during the formation of the _______ bond. However, if it is first hydrolysed (broken down) to its constituent monosaccharides (_________ and _________), it will then give a positive Benedict's test. So _________ is the only sugar that will give a negative Benedict's test before hydrolysis and a positive test afterwards.
Hydrolysis is achieved by the following technique:
Using a separate sample, boil 2cm3 of the test solution with 1cm3 dilute hydrochloric acid for 3 minutes to hydrolyse the glycosidic bond.
Neutralise the solution by gently adding small amounts of solid sodium hydrogen carbonate, check with pH paper. Then test as before for reducing sugars.
Reducing Sugars. All monosaccharides and most disaccharides will reduce copper (II) sulphate, producing a precipitate of copper (I) oxide on heating, so they are called reducing sugars. Benedict’s reagent is an aqueous solution of copper (II) sulphate, sodium carbonate and sodium citrate.
Collect : Bunsen burner, large beaker, boiling tubes, gauze, tripod, solutions to test (sucrose, glucose, fructose), heat proof mat, benedicts reagent, measuring cylinder
Add 2 cm³ of test solution add an equal quantity of Benedict’s reagent.
Shake, and heat for 5 minutes at 95°C in a water bath.
A precipitate indicates reducing sugar. The colour and density of the precipitate gives an indication of the amount of reducing sugar present, so this test is semi-quantitative. The original pale blue colour means no reducing sugar, a green precipitate means relatively little sugar; a brown or red precipitate means progressively more sugar is present. Record your results in your book.
Non-reducing Sugars. _________ is called a non-reducing sugar because it does not reduce copper sulphate. This is because the reducing groups (aldehyde and ketone groups) on its constituent monosaccharides are removed during the formation of the _______ bond. However, if it is first hydrolysed (broken down) to its constituent monosaccharides (_________ and _________), it will then give a positive Benedict's test. So _________ is the only sugar that will give a negative Benedict's test before hydrolysis and a positive test afterwards.
Hydrolysis is achieved by the following technique:
Using a separate sample, boil 2cm3 of the test solution with 1cm3 dilute hydrochloric acid for 3 minutes to hydrolyse the glycosidic bond.
Neutralise the solution by gently adding small amounts of solid sodium hydrogen carbonate, check with pH paper. Then test as before for reducing sugars.