Benedict’s Test- Principle, Composition, Preparation, Procedure and Result Interpretation 4.35/5 (271)

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Benedict’s Test- Principle, Composition, Preparation, Procedure and Result Interpretation

Benedict’s Test 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.

Benedict's Test

Benedict’s quantitative reagent contains potassium thiocyanate and is used to determine how much reducing sugar is present. This solution forms a copper thiocyanate precipitate which is white and can be used in a titration. The titration should be repeated with 1% glucose solution instead of the sample for calibration

Principle of Benedict’s Test

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.

The red copper(I) oxide formed is insoluble in water and is precipitated out of solution. This accounts for the precipitate formed. As the concentration of reducing sugar increases, the nearer the final colour is to brick-red and the greater the precipitate formed. Sometimes a brick red solid, copper oxide, precipitates out of the solution and collects at the bottom of the test tube.

Sodium carbonate provides the alkaline conditions which are required for the redox reaction. Sodium citrate complexes with the copper (II) ions so that they do not deteriorate to copper(I) ions during storage.

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.

Composition and Preparation of Benedict’s Solution

Benedict’s solution is a deep-blue alkaline solution used to test for the presence of the aldehyde functional group, – CHO.

Anhydrous sodium carbonate = 100 gm
Sodium citrate – 173 gm
Copper(II) sulfate pentahydrate = 17.3 gm

One litre of Benedict’s solution can be prepared from 100 g of anhydrous sodium carbonate, 173 g of sodium citrate and 17.3 g of copper(II) sulfate pentahydrate.

Procedure of Benedict’s Test

  1. Approximately 1 ml of sample is placed into a clean test tube.
  2. 2 ml (10 drops) of Benedict’s reagent (CuSO4) is placed in the test tube.
  3. The solution is then heated in a boiling water bath for 3-5 minutes.
  4. Observe for color change in the solution of test tubes or precipitate formation.

Result Interpretation of Benedict’s Test

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.

Result Interpretation of Benedict's Test

Positive Benedict’s Test: Formation of a reddish precipitate within three minutes. Reducing sugars present. Example: Glucose
Negative Benedict’s Test: No color change (Remains Blue). Reducing sugars absent. Example: Sucrose.

References

  1. National Institutes of Health, Testing for Lipids, Proteins and Carbohydrates- Benedict’s solution.
  2. Fayetteville State University- Biological Molecules: Carbohydrates, Lipids, Proteins.
  3. Harper College- Benedict’s Test.
  4. National Biochemicals Corp.- BENEDICT’S SOLUTION (MB4755).
  5. Science Olympiad- Use of Benedict’s Solution.
  6. Brilliant Biology Student 2015- Food Tests- Benedict’s Test for Reducing Sugars.
  7. BBC Bitesize- Chemistry- Carbohydrates.
  8. University of Manitoba- The Molecules of Life: Biochemistry- Carbohydrates.
  9. Northern Kentucky University- Benedict’s Reagent: A Test for Reducing Sugars.
  10. KNUST Open Educational Resources, Benedict’s Test – Qualitative Test in Carbohydrates.
  11. Mark Rothery’s Biology Web Site- Biochemical Tests.
  12. All Medical Stuff- Benedict’s test for reducing sugar.
  13. Hendrix College- Benedicts Test for Glucose.
  14. Info Please- Benedict’s solution.
  15. Mystrica- Benedict’s Test.
  16. Amrita Virtual Lab Collaborative Platform- Qualitative Analysis of Carbohydrates.
  17. Wikipedia.

Benedict’s Test- Principle, Composition, Preparation, Procedure and Result Interpretation

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Benedict’s Test- Principle, Composition, Preparation, Procedure and Result Interpretation

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The Author

Sagar Aryal

Sagar Aryal

I am Sagar Aryal, a passionate Microbiologist and the Scientific Blogger. I did my Master's Degree in Medical Microbiology and currently working as a Teaching Assistant at St. Xavier's College, Kathmandu, Nepal. I am particularly interested in research related to Medical Microbiology and Virology. Find me on Facebook, Twitter or Linkedin !!!

13 Comments

Add a Comment
  1. Au faites cette solution n’est pas totalement indiquée pour le diagnostic du diabète,car elle peut aussi chercher d’autres sucres reducteurs(le saccharose par exemple,…) dans les urines et on peut se tromper que le patient souffre du diabète.Le meilleur test de dépistage reste la spectrophotométrie DU GLUCOSE.

  2. This helped me alot

  3. Hello my name is John David Buttle and I have no life. I live with my mom…still and I am a sicopath.

  4. Thank you this is very helpful because I am doing a science fair project

  5. Wow! I will never order for benedict’s solution again.Thanks a lot for your generous and helpful article.

  6. Thank you very much sir,these really helped me,as I was writting a lab report.

  7. Reaction and mechanism for preparation of benidicts reagent needed

  8. thanku so much sir…you r inspirable for all researcher student…

  9. Thanks Mr. Sagar, this is very useful post.

  10. Thank you Dr for your help

  11. This has Been very informative !
    Thank you very much.
    🙂

  12. This is very helpful because it summarizes all that is to be known.Thank You

  13. Thank You Sir, I Do Really Appreciate This Blog

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