Benedict’s Test
Last reviewed by Editorial Team on February 27th, 2019.
What is Benedict’s test?
It is a procedure used to test for simple carbohydrates. It identifies reducing sugars that contain free ketone or aldehyde functional groups.
Examples of reducing sugar are glucose, fructose, and galactose.
It also includes disaccharides such as maltose and lactose. The solution is also used to detect glucose in the urine. A positive glucose in urine is one of the indicators of diabetes mellitus. (1, 2)
Image 1: A Benedict’s solution is blue in color which changes to a varying degree of color when brought to heat and a reducing sugar is added.
Picture Source: www.sciencecompany.com
Why some sugars are called reducing sugar?
Some sugars are called reducing because they have the ability to transfer hydrogen to other compounds through a reduction process.
If the reducing sugar is mixed with Benedict’s reagent and bring to heat, you will notice a significant change in the color of the reagent. The color of the reagent depends on the type and amount of sugar. (1, 2, 3)
What is the principle of Benedict’s test?
A Benedict’s solution and a simple carbohydrate are brought to heat and changes in the reagent’s color take place. It usually changes to brick red or orange-red.
The one responsible for changing of color is the reducing ability of simple carbohydrates.
The copper (II) is reduced to Copper (I) ion. Copper (I) ion is water-insoluble and it will precipitate out of the solution. The reducing sugar increases in concentration and its color will change to brick-red.
As the sugar is nearing its final color, the more precipitate is formed and settle at the bottom of the test tube. (2, 3, 4)
Do complex carbohydrates test positive for Benedict’s test?
A complex carbohydrate like starch does not test positive for Benedict’s test for the reason that it is too large. It should be broken down first through digestion or heating.
Benedict’s reagent composition
- Anhydrous sodium carbonate
- Sodium citrate
- Copper(II) sulfate pentahydrate (4, 5)
With a 100 grams of anhydrous sodium carbonate, 17.3 grams of Copper (II) and 173 grams of sodium citrate, you can create a liter of Benedict’s solution.
The Benedict’s solution is also called Benedict’s qualitative solution and Benedict’s reagent. It was named after Stanley Rossiter Benedict, an American chemist.
The Benedict’s reagent is a mixture of sodium citrate, sodium carbonate, and copper (II) sulphate pentahydrate. It detects the presence of reducing sugar.
The solution is deep blue in color but the color will eventually change if a substance with reducing sugar is present and bring to heat. The color changes to brick red if the amount of reducing sugar is high.
There will also be a noticeable formation of a precipitate at the bottom of the test tube. In some tests, the changes in color is a bit orange or pale orange, which indicates that the amount of reducing sugar is not that high.
The brick red color is the indicator that the amount of reducing sugar is too high. (4, 5, 6, 7)
Picture 2: The test tube on the left is negative in reducing sugar while the test tube on the right contains a certain amount of reducing sugar.
Photo Source: image.slidesharecdn.com
How does Benedict’s test work?
Photo 3: The image shows the step by step process of Benedict’s test.
Image Source: www.researchgate.net
The Benedict’s test procedure includes the following:
- Get a clean test tube. Place inside the test tube 1 ml of sample.
- Put about 10 drops of Benedict’s reagent in the test tube.
- Bring the solution to heat in a boiling water bath for approximately five minutes.
- Observe for changes in color and watch out for precipitate formation. (4, 8, 9)
Benedict’s test results
Image 4: The image shows the varying results of Benedict’s test.
Picture Source: microbiologyinfo.com
Benedict’s test colors are important in result interpretation. The color of the reagent could change at a varying degree, which indicates a certain amount of sugar present in the solution. Results are as follows:
- Green – The amount of sugar in the solution is between 0.1% and 0.5%.
- Yellow – The amount of sugar in the solution is between 0.5% and 1%.
- Orange – The amount of sugar in the solution is between 1% and 1.5%.
- Red – The amount of sugar in the solution is between 1.5% and 2%.
- Brick red – The amount of sugar in the solution is more than 2%. (2, 9, 10)
A Benedict’s test is positive if a reddish precipitate is formed within three minutes.
On the other hand, the test is negative if there is no changes in the color. The color of the reagent stays the same (blue).
Are there any limitations?
If a certain drug is present in the solution, the outcome could be false positive.
If the urine is tested for the presence of simple sugar using Benedict’s test, the result may not be accurate as some components of the urine could alter the result such as creatinine, ascorbic acid, and urate. (3, 6, 8)
References:
- https://microbiologyinfo.com/benedicts-test-principle-composition-preparation-procedure-and-result-interpretation/
- https://en.wikipedia.org/wiki/Benedict%27s_reagent
- http://brilliantbiologystudent.weebly.com/benedicts-test-for-reducing-sugars.html
- http://www.onlinebiologynotes.com/benedicts-test-objective-principle-reagents-procedure-and-result/
- https://medium.com/@worldofchemical/benedicts-reagent-test-for-monosaccharides-8259e446606a
- https://www.nku.edu/~whitsonma/Bio150LSite/Lab%203%20Organic/Bio150LRevMolec.html
- http://dept.harpercollege.edu/chemistry/chm/100/dgodambe/thedisk/carbo/bened/benedict.htm
- https://laboratoryinfo.com/benedicts-test-principle-reagent-preparation-procedure-interpretation/
- https://sciencing.com/test-reducing-sugars-5529759.html
- https://www.revolvy.com/page/Benedict%27s-reagent
