18- Benedicts test

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Test for sugars using Benedict's reagent

Two different test depending on whether you have a reducing sugar or non-reducing sugar

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Benedict's test for reducing sugars

Reducing sugar has an aldehyde group (CHO)

Reducing sugars can reduce copper(II) ions in reagent into copper (I) ions

Forms brick-red precipitate

All monosaccharides are reducing sugars

Some common disaccharides e.g. lactose and maltose are also reducing sugars

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Benedict's test for non-reducing sugars

Non-reducing sugar does not contain a free aldehyde group

These sugars can't reduce copper (II) ions in Benedict's reagent

To test for non-reducing sugar must first break glycosidic bond

Then Benedict's reagent can be used

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How to carry out Benedict's test

1- Add Benedict's reagent and heat at 80°C for 3 mins

2- Brick-red precipitate indicates high conc of sugar

Green, yellow and orange indicate lower conc of sugar

Blue colour indicates no sugar present

Non-reducing sugars

1- Add dilute HCl to non-reducing sugar and boil for 5 mins

Neutralise with NaHCO3

2- Add Benedict's reagent and heat at 80°C for 3 mins

3- Green, yellow, orange and red indicate presence of sugar

Reagent test strips

Commercially produced test strips can be used to test for glucose

Strip dipped in solution

Changes colour

Colour can be compared to chart that comes with test

Tells you amount of glucose present

Often used to test for glucose in urine of diabetic patients

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