Objective
Barfoed’s test is a biochemical test devised by the Swedish physician C.T Barfoed (1815-1899). The main objective of the test is to distinguish between monosaccharides and reducing disaccharides.
Principle
Barfoed’s test reaction is based on the reduction of cupric acetate by reducing monosaccharides and reducing disaccharides. Reduction of cupric acetate produces cuprous oxide which gives a brick red precipitate.
The reaction is conducted in a slightly acidic medium. A mixture of ethanoic (acetic) acid copper (II) acetate is added to the test solution and boiled.
RCHO +2 CU2+ +2H2O―> RCOOH + Cu2O + 4H+
Reducing monosaccharides react with Bedford’s reagent much faster than disaccharides and produce a copious amount of red precipitate of copper (I) oxide within three minutes. Disaccharide sugars as they are weaker reducing agents, react at a slower rate and so do not form red precipitate even for 10 or 12 minutes. The reaction with disaccharides is slower because disaccharides have to get hydrolyzed first and then react with the reagent cupric acetate to produce cuprous oxide.
Reagents For Barfoed’s Test
- Copper acetate: Provides an acidic medium
- Acetic acid: Provide cupric ions
- Test solution: 5% Glucose, 5% Sucrose, 5% Maltose, 5% Lactose, 5% Starch
- Water bath
- Barfoed’s reagent: Dissolve 13.3g of copper acetate in 200 ml of distilled water and add 1.8 ml of glacial acetic acid to it.
- Dry test tubes
- Pipettes
Barfoed’s Test Procedure
- Take 1ml of test sample in dry test tube
- Take 1ml of distilled water in another tube as control.
- Add 2 ml of Barfoed’s reagent to all test tubes
- Keep in boiling water bath
- Look for the development of brick red precipitate
- Take note of the time taken to develop the color.
Result Interpretation
- Positive Barfoed’s Test: Brick red precipitate forms within 5 minutes of boiling in the case of monosaccharide while for disaccharides, the precipitate forms between 7-12 minutes.
- Negative Barfoed’s Test: Absence of red color.