Chemistry

Benedict’s test – Principle, Procedure, and Uses

Benedict’s test is used to detect the presence of reducing sugars in a solution. The reducing sugars generally contain free aldehyde or ketone groups. The color of the benedict’s solution changes from blue to brick red in a solution containing reducing sugars. The principle behind this reaction is that Benedict’s solution contains copper ions which react with the free aldehyde or ketone group to give a brick-red precipitate of cuprous oxide. In this article, we discuss the principle, procedure, and uses of Benedict’s solution.

What is Benedict’s test?

It is a type of test done to detect the reducing sugar(they are carbohydrates that have aldehyde or ketone functional group in molecular structure, which include monosaccharides like glucose and fructose and disaccharides like maltose and lactose)

Glucose is called reducing sugar as it has aldehyde groups and they are capable of transferring electrons to other compounds, this process is reduction. Fructose is also a reducing sugar as they has ketone groups.

Benedict’s solution composition

The bright blue solution is prepared by mixing CuSO₄.5H₂O (copper sulfate pentahydrate), sodium carbonate, and sodium citrate in distilled water. The presence of copper ions in the solution gives its characteristic blue color.

Benedict's test solution chemical composition — Chemical composition of benedict’s solution

What is Benedict’s quantitative reagent?

Using color change, we can only qualitatively determine the presence of reducing sugars. But, using the quantitative reagent, we can determine the amount of reducing sugars present. It contains potassium thiocyanate, which is used for determining how much reducing sugar is present by titration.

Principle of benedict’s test

The reducing sugar is heated with Benedict’s reagent.
Due to the presence of alkaline sodium, the sugar is converted into enediols strong powerful reducing agent).
During this reduction reaction, the mixture changes its color blue to brick-red precipitate because of the formation of cuprous oxide.
Copper(II) form is reduced to copper(I).
Red cuprous is insoluble in water and can be easily removed.
The basic reaction can be represented as: Cupric ions + aldehyde>>>>carboxylic acid + cuprous oxide (brick-red precipitate).

Benedict's test chemical reaction — Chemical reaction for Benedict’s solution

The objective of the benedict’s test

To detect the presence or absence of reduced sugar.
To determine the sugar concentration by using visual aids (not very accurate, but can be used for comparing solutions),.

Benedict’s test procedure

A reducing sugar sample (like blood or urine) is mixed with Benedict’s solution(5ml).
The solution is heated in a test tube for about two minutes and is cooled down.
The color of the mixture changes accordingly resulting in a precipitate indicating a positive result.

Results of Benedict’s test

Color	Concentration of reducing sugar	Interpretation
Blue (no change in color)	0	No reducing sugar
Green precipitate	0.5-1	Traceable amount of reducing sugar
Yellow precipitate	1-1.5	A small amount of reduced sugar
Orange-red precipitate	1.5-2	A moderate amount of reduced sugar
Brick-red precipitate	>2	A large amount of reduced sugar

Results of benedict’s test

Benedict's test results and colours shown based on concentration of reducing sugars — Benedict’s test results and colors are shown based on the concentration of reducing sugars

Uses and Application of benedict’s test

To detect the presence of glucose in blood and urine, where detection of excess glucose means diabetes. (Note: A positive test can also be given due to the presence of ascorbic acid, homogentisic acid, or other reducing substances; therefore, it does not actually imply diabetes).

What is the role of each of the components of Benedict’s solution?

The Benedict’s solution is a blue-colored solution. Since the testing is based on color change, hence the color of Benedict’s solution is very critical for the test. The CuSO4.5H2O(copper sulfate pentahydrate) salt is the source of the blue color due to the presence of copper(II) ions. Copper (II) can sometimes degrade to copper(I). So, sodium citrate is used to stabilize the copper ions by complexing with them. Sodium carbonate is used to maintain an alkaline medium required for the redox reaction during the test. Water is just added as a solvent for the reaction.

Why heating is required in Benedict’s test?

Complex carbohydrate like starch does not react positively until and unless they are broken down through heating or by digestion. Complex sugars need to be decomposed into glucose and fructose then it will show a positive reaction.

What are the examples of sugars that can be detected by Benedict’s test?

All reducing sugars can be detected using this Benedict’s test. All monosaccharides are reducing sugars. Examples include lactose, maltose, glucose, fructose, galactose, ribose, xylose, etc.