Benedicts solution contains copper sulfate (CuSO4), sodium
hydroxide (NaOH) and tartaric acid. Copper sulfate when dissolved is a deep blue
solution.
Some sugars, such as glucose and fructose have
aldehyde or ketone groups available to react with the copper in the copper sulfate.
These are called reducing sugars.
When this happens the
copper in the copper sulfate is reduced from a +2 ion to a +1 ion and forms copper oxide
which precipitates as a brown suspension.
The procedure is
to dissolve the sugar you are testing and add several drops of Benedicts reagent. Then
gently heat the solution and observe any color change. If the sugar does not contain
free aldehyde or ketone groups the color will not change. If either of those groups are
present, there will be an observable color change. The change in color is an indication
of how much of the sugar tested contains one or both of the oxidizable
groups.
Depending on the ratio of blue copper sulfate to
brown copper oxide you may see any of the following colors: green, orange, red, or
brown.
So the test can not only give you a qualitative
result (reducing sugar or non-reducing sugar) but a semi-quantitative result depending
on the color observed.
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