Barfoed’s Test

 

Definition: What is Barfoed’s Test?

Barfoed’s test is a chemical test used for detecting the presence of monosaccharides. It is based on the reduction of cupric (II) acetate to cuprous (I) oxide (Cu2O), which forms a brick-red precipitate. Disaccharides may react, but the reaction is much slower because they have to get hydrolyzed first and then react with the reagent cupric acetate to produce cuprous oxide.

This test is named after the Danish chemist Christen Thomsen Barfoed, who reported it in a journal in 1873.

Barfoed’s Test Principle

Aldoses and ketoses can reduce cupric ions, even in acidic conditions. It is based on the reduction of copper(II) acetate to copper(I) oxide (Cu2O), which forms a brick-red precipitate.  Disaccharides may also react, but the reaction is much slower. This test is used to distinguished reducing monosaccharides from disaccharides by controlling pH and the time of heating.

Barfoed’s Reagent Preparation

Barfoed Reagent: Copper(II) acetate in acetic acid. Barfoed’s reagent is the reagent used in this test. It is prepared by adding 0.33 molar solution of neutral cupric (II) acetate to 1% acetic acid solution.

• Barfoed’s reagent is prepared by dissolving 24 gm of copper acetate in 400 ml of boiling water. 
• To this add 25 ml of 8.5% acetic acid solution. 
• Stir cool the solution and dilute to 500 ml.
• The reagent does not keep well and it is therefore advisable to make it up when it is actually required.

Barfoed’s Test Procedure

• One ml of a sample solution is placed in a test tube.
• Three ml of Barfoed’s reagent is added.
• The solution is then heated in a boiling water bath for two minutes and allowed to cool.
• Record color and record the time required to develop a red precipitate.

Barfoed’s Test Results

If a red precipitate is formed within two minutes, a monosaccharide is present. If the red precipitate is formed after ten minutes of heating, a disaccharide is present.

Interpretation:

• The positive reaction indicates the presence of a reducing mono saccharide. 
• On prolonged heating disaccharides can also give this test positive. 
• Hence, the solution should be boiled for 3 minutes only.

Difference between Barfoed’s Test and Benedict’s Test

• The Barfoed’s reagent is similar to Benedict’s reagent except that the pH is lower (around 4.5), and heating time is reduced to two minutes.

• Benedict’s test would determine if the sample is a reducing sugar, and Barfoed’s test would determine if it is a monosaccharide or disaccharide.