Question

What is Barfoed’s test?

Answer 1

Barfoed’s test includes the reaction of copper sulphate with monosaccharide or disaccharide for the formation of copper oxide. The copper oxide appears as red or brown coloured precipitates at the bottom of the test vessel.

Complete answer:
The Barfoed’s test is used to distinguish between monosaccharides and disaccharides. Monosaccharides are single sugar molecules and disaccharides are sugar molecules that contain two monosaccharide molecules.
In this test, copper sulphate is taken with acetic acid in a test tube and is heated with the test object, i.e., monosaccharide or disaccharide.
Now, as heating begins copper in copper sulphate starts turning into copper ions due to the acidic medium provided by acetic acid. These copper ions with two positive charges are then reduced to copper oxide precipitates which have a red or brown colour.
Now, this reaction occurs fast if the test object is monosaccharide and the reaction occurs slowly if the test object is disaccharide. Precipitates of copper oxide start appearing in around five minutes in the case of monosaccharides and the precipitates start appearing in around ten minutes in case off disaccharides.
Thus, in this way monosaccharides and disaccharides are easily distinguished. This test was invented by Danish chemist Christen Thomsen Barfoed and thus is named after him.

Note:
Copper acetate can also be used in place of copper sulphate to obtain copper ions in the solution. The mixture of copper sulphate and acetic acid is called Barfoed’s reagent. This reagent does not keep well and is thus prepared at the time of the experiment.