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Benedicts Solution or Reagent is used to test for simple carbohydrates. The Benedict’s test identifies reducing sugars (monosaccharide’s and some disaccharides), which have free ketone or aldehyde functional groups. Benedict’s solution can be used to test for the presence of glucose in urine.
Some sugars such as glucose are called reducing sugars because they are capable of transferring hydrogens electrons to other compounds, a process called reduction. When reducing sugars are mixed with Benedicts reagent and heated, a reduction reaction causes the Benedicts reagent to change color. The color varies from green to dark red (brick) or rusty-brown, depending on the amount of and type of sugar.
When Benedict’s solution and simple carbohydrates are heated, the solution changes to orange red/ brick red. This reaction is caused by the reducing property of simple carbohydrates. The copper (II) ions in the Benedict’s solution are reduced to Copper (I) ions, which causes the color change.
Complex carbohydrates such as starches DO NOT react positive with the Benedict’s test unless they are broken down through heating or digestion (try chewing crackers and then doing the test). Table sugar (disaccharide) is a non-reducing sugar and does also not react with the iodine or with the Benedict Reagent. Sugar needs to be decomposed into its components glucose and fructose then the glucose test would be positive but the starch test would still be negative.
Procedure
Pour 5 mL of your test substance into a test tube.
Add 10 drops of Benedict’s solution to your test tube. (Caution: use care when handling Benedict’s to avoid staining hands or clothing!) Gently shake/swirl the contents of the tube.
Place your test tube in the hot water bath for 3-5 minutes. When heated, Benedict’s solution will change color from blue to yellow/orange/red/brown in the presence of monosaccharides (simple sugars).
Once cool, clean your test tubes thoroughly.
Analysis
If the color upon boiling is changed into green, then there would be 0.1 to 0.5 percent sugar in solution.
If it changes color to yellow, then 0.5 to 1 percent sugar is present.
If it changes to orange, then it means that 1 to 1.5 percent sugar is present.
If color changes to red,then 1.5 to 2.0 percent sugar is present.
And if color changes to brick red,it means that more than 2 percent sugar is present in solution.
Sources:
Benedict's Test- Principle, Preparation, Procedure and Result Interpretation
Aryal et al.
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