9/24/13 Benedict
Benedict's Test for Reducing Sugars
The Benedict's test allows us to detect the presence of reducing sugars (sugars with
a free aldehyde or ketone group). All monosaccharides are reducing sugars; they
all have a free reactive carbonyl group. Some disaccharides have exposed
carbonyl groups and are also reducing sugars. Other disaccharides such as
sucrose are non-reducing sugars and will not react with Benedict's solution.
Starches are also non-reducing sugars. The copper sulfate (CuSO4) present in
Benedict's solution reacts with electrons from the aldehyde or ketone group of the
reducing sugar to form cuprous oxide (Cu2O), a red-brown precipitate.
CuSO4 Cu++ + SO4--
2 Cu++ + Reducing Sugar Cu+
(electron donor)
Cu+ Cu2O (precipitate)
The final color of the solution depends on how much of this precipitate was
formed, and therefore the color gives an indication of how much reducing sugar
was present.
Increasing amounts of reducing sugar
green orange red brown
Return to Lab 3 Home Page
Return to 170 Course Outline Page
Return to Welcome Page
www.biosci.ohiou.edu/introbioslab/Bios170/170_2/benedict.htm 1/1
, 9/24/13 Carbohydrates - Barfoed's Test
Barfoed's Test
Shows positive test for:
Reducing monosaccharides
Reactions:
Reducing monosaccharides are oxidized by the copper ion in solution to form a carboxylic acid and a
reddish precipitate of copper (I) oxide within three minutes. Reducing disaccharides undergo the same
reaction, but do so at a slower rate.
How to perform the test:
One ml of a sample solution is placed in a test tube. Three ml of Barfoed's reagent (a solution of cupric
acetate and acetic acid) is added. The solution is then heated in a boiling water bath for three minutes.
A positive test is indicated by:
The formation of a reddish precipitate within three minutes.
a negative test (left) and a positive test (right)
www.harpercollege.edu/tm-ps/chm/100/dgodambe/thedisk/carbo/barf/barfoed.htm 1/2
Benedict's Test for Reducing Sugars
The Benedict's test allows us to detect the presence of reducing sugars (sugars with
a free aldehyde or ketone group). All monosaccharides are reducing sugars; they
all have a free reactive carbonyl group. Some disaccharides have exposed
carbonyl groups and are also reducing sugars. Other disaccharides such as
sucrose are non-reducing sugars and will not react with Benedict's solution.
Starches are also non-reducing sugars. The copper sulfate (CuSO4) present in
Benedict's solution reacts with electrons from the aldehyde or ketone group of the
reducing sugar to form cuprous oxide (Cu2O), a red-brown precipitate.
CuSO4 Cu++ + SO4--
2 Cu++ + Reducing Sugar Cu+
(electron donor)
Cu+ Cu2O (precipitate)
The final color of the solution depends on how much of this precipitate was
formed, and therefore the color gives an indication of how much reducing sugar
was present.
Increasing amounts of reducing sugar
green orange red brown
Return to Lab 3 Home Page
Return to 170 Course Outline Page
Return to Welcome Page
www.biosci.ohiou.edu/introbioslab/Bios170/170_2/benedict.htm 1/1
, 9/24/13 Carbohydrates - Barfoed's Test
Barfoed's Test
Shows positive test for:
Reducing monosaccharides
Reactions:
Reducing monosaccharides are oxidized by the copper ion in solution to form a carboxylic acid and a
reddish precipitate of copper (I) oxide within three minutes. Reducing disaccharides undergo the same
reaction, but do so at a slower rate.
How to perform the test:
One ml of a sample solution is placed in a test tube. Three ml of Barfoed's reagent (a solution of cupric
acetate and acetic acid) is added. The solution is then heated in a boiling water bath for three minutes.
A positive test is indicated by:
The formation of a reddish precipitate within three minutes.
a negative test (left) and a positive test (right)
www.harpercollege.edu/tm-ps/chm/100/dgodambe/thedisk/carbo/barf/barfoed.htm 1/2
