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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.
Benedict's reagent is a solution of copper sulfate, sodium hydroxide, and tartaric acid. It can be used to detect reducing sugars. The procedure for this is:
- Dissolve the sample to be tested in water.
- Add the Benedict’s solution to this.
- Heat the mixture.
A change in color indicates the presence of reducing sugars. When there is no sugar in the mixture the color is blue. In the presence of sugar the color changes. The final color depends on the concentration of the sugar. From a low concentration to a higher one the color change progresses from green to yellow to orange, red and finally brown.
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