What is Benedict’s Test?
Benedict’s test is a chemical test that can be used to check for the presence of reducing sugars in a given analyte. Therefore, simple carbohydrates containing a free ketone or aldehyde functional group can be identified with this test. The test is based on Benedict’s reagent (also known as Benedict’s solution), which is a complex mixture of sodium citrate, sodium carbonate, and the pentahydrate of copper(II) sulfate.
When exposed to reducing sugars, the reactions undergone by Benedict’s reagent result in the formation of a brick-red precipitate, which indicates a positive Benedict’s test. An image detailing the changes in the colour of Benedict’s reagent (from clear blue to brick-red) that are triggered by exposure to reducing sugars is provided below.
It can be noted that Benedict’s test can also be used to check for the presence of glucose in a urine sample. Since this test detects any aldehydes and α-hydroxy ketones and glucose is an aldose whose open-chain forms an aldehyde group, the test yields a positive result when glucose is present in the analyte. However, a positive reaction can also be given by the presence of ascorbic acid, homogentisic acid, and other reducing substances urine. Therefore, a positive Benedict’s test does not necessarily imply that the test subject is diabetic.
Benedict’s Test Principle
When a reducing sugar is subjected to heat in the presence of an alkali, it gets converted into an enediol (which is a relatively powerful reducing agent). Therefore, when reducing sugars are present in the analyte, the cupric ions (Cu2+) in Benedict’s reagent are reduced to cuprous ions (Cu+). These cuprous ions form copper(I) oxide with the reaction mixture and precipitate out as a brick-red coloured compound.
An illustration detailing the reaction between an aldose and Benedict’s reagent that results in the formation of cuprous oxide is provided above.
Benedict’s Test Procedure
Preparation of Benedict’s Reagent
One litre of Benedict’s reagent can be prepared by mixing 17.3 grams of copper sulfate pentahydrate (CuSO4.5H2O), 100 grams of sodium carbonate (Na2CO3), and 173 grams of sodium citrate in distilled water (required quantity). Here, the copper(II) sulfate acts as a source of Cu2+ ions, the sodium carbonate provides an alkaline medium, and the sodium citrate forms complexes with the Cu2+ ions. Distilled water is used as a solvent.
The purity of Benedict’s reagent can be checked by heating it in a test tube. No changes in the blue colour of the solution upon heating is an implication that the reagent is pure.
Testing for Reducing Sugars
One millilitre of the analyte sample must be mixed with 2 millilitres of Benedict’s reagent and heated in a bath of boiling water for 3 to 5 minutes. The development of a brick-red coloured precipitate of cuprous oxide confirms the presence of reducing sugars in the analyte.
Interpreting the Results of Benedict’s Test
| Colour of the Precipitate | g% of Reducing Sugar |
| Green | 0.5% |
| Yellow | 1% |
| Orange | 1.5% |
| Red | 2% |
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Frequently Asked Questions – FAQs
What does Benedict’s test detect?
The primary application of Benedict’s test is to detect the presence of simple carbohydrates in an unidentified analyte. This test can be used to check for reducing sugars that hold free aldehyde or ketone functional groups. The reducing sugar can be either a monosaccharide or a disaccharide.
What is Benedict’s reagent?
Benedict’s reagent, also known as Benedict’s solution, is a chemical reagent which is made up of a complex mixture of sodium citrate, sodium carbonate, and the pentahydrate of copper(II) sulfate. When exposed to reducing sugars and other reducing substances, Benedict’s reagent changes its colour from clear blue to brick red.
How can Benedict’s test be performed?
Benedict’s test can be performed by taking one millilitre of the analyte solution in a test tube and mixing it with two millilitres of Benedict’s reagent. Then, this mixture must be heated in a hot water bath for approximately 3 minutes (or until a visible change in colour occurs).
How is Benedict’s solution prepared?
Benedict’s solution (or Benedict’s reagent) can be prepared by complexing cupric ions (Cu2+ cations) from the copper sulfate pentahydrate with citric acid molecules in a basic environment provided by sodium carbonate. The final product can be employed to check for the presence of reducing sugars.
List some substances that give positive results for Benedict’s test.
Some examples of substances that yield positive results for Benedict’s test are listed below.
- Glucose
- Fructose
- Ribose
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