Benedicts Test - Overview, Preparation, Analysis, Limitations, FAQs

Q: How do you prepare Benedict’s solution?

By combining cupric ions (Cu 2+ cations) from copper sulphate pentahydrate with citric acid molecules in a basic environment given by sodium carbonate, Benedict’s solution (or Benedict’s reagent) can be made.

Edited By Team Careers360 | Updated on Jun 20, 2022 10:41 AM IST

What is Benedict's Solution ?

Benedict’s test is a chemical test that can be used to determine whether or not an analyte contains reducing sugars. As a result, this test can identify simple carbohydrates having a free ketone or aldehyde functional group. Benedict’s reagent (also known as Benedicts solution) is a complicated mixture of sodium citrate, sodium carbonate, and the pentahydrate of copper (II) sulphate, which is used in the test.

Reducing Sugar Test

When Benedict’s reagent is subjected to reducing sugars, the reactions it undergoes result in the production of a brick-red precipitate, indicating a positive Benedict test principle. The changes in colour of Benedict’s reagent (from clear blue to brick-red) caused by exposure to reducing sugars are depicted in the figure below.

Benedict's test for sugars

The colour change of reducing sugars on addition of Benedict’s reagent

Benedict's sugar test is a straightforward method for determining the concentration of reducing monosaccharides in a solution. Benedict’s test can detect some disaccharides, however sucrose (table sugar) is an unreactive disaccharide. Benedict’s solution is a reagent, a substance that changes colour when exposed to another substance.

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Benedict Test Reaction in Benedict Test for Urine Sample

Benedict’s test can also be used to determine whether or not there is glucose in a benedict test for urine sample. When benedict’s test for glucose is present in the analyte, the test produces a positive result because it identifies any aldehydes and hydroxy ketones, and glucose is an aldose whose open-chain forms an aldehyde group in reducing substances in urine. The presence of ascorbic acid, homogentisic acid, and other reducing chemicals in benedict test for urine, on the other hand, can cause a positive reaction. As a result, a positive Benedict reaction test does not always mean the person is diabetic.

Principle of Benedict Test (benedict solution formula and benedict’s reagent formula)

Benedict reagent formula: A reducing sugar is transformed to an enediol formation when it is heated in the presence of an alkali (which is a relatively powerful reducing agent). The cupric ions (Cu²⁺) in Benedict’s reagent test are converted to cuprous ions (Cu⁺) when benedict’s test for reducing sugars are present in the analyte. These cuprous ions combine with the reaction mixture to generate copper (I) oxide, which precipitates as a brick-red substance.

An Aldose + Benedict’s reagent (blue) → Carboxylate ion + Brick Red precipitate

RCHO + 2 Cu²⁺ (citrate) → RCOO^- + Cu₂S (s)

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Benedict's Test Procedure

Preparation of Benedict's Reagent

In distilled water, combine 17.3 g of copper sulphate pentahydrate (CuSO₄.5H₂O), 100 g of sodium carbonate (Na₂CO₃), and 173 g of sodium citrate to make one litre of Benedict’s reagent (required quantity). Copper (II) sulphate serves as a source of Cu²⁺ ions, sodium carbonate serves as an alkaline medium, and sodium citrate creates Cu²⁺ ion complexes. As a solvent, distilled water is employed.

Alkalinity is imparted to the reaction media by anhydrous sodium carbonate.
Sodium Citrate - creates a compound with cupric ions, preventing them from decomposing into cuprous ions during storage.
The cupric ions are produced by copper (II) sulphate pentahydrate.
As a solvent, distilled water is employed.

By heating Benedict’s reagent in a test tube, you can determine its purity. The fact that the blue benedict solution colour of the solution does not change when heated indicates that the reagent is pure.

Composition of benedict reagent: CuSO₄.5H₂O + Na₂CO₃ + Na₃C₆H₅

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Benedict's Test for Reducing Sugars

One millilitres of analyte sample must be combined with two millilitres of Benedict’s reagent and heated for 3 to 5 minutes in a bath of boiling water. The presence of reducing sugars in the analyte is confirmed by the formation of a brick-red coloured cuprous oxide precipitate.

Analysis of the Benedict's test Results

Depending on the amount of reducing sugar in the sample, the colour ranges from green to yellow to orange to brick-red; a sample containing 1% glucose produces a brick-red precipitate.

Any mono or disaccharide having a hemiacetal or hemiketal group will change colour in Benedict’s test.
Because sucrose or table sugar lacks these groups, it will not result in a positive test.

NCERT Chemistry Notes :

OBSERVATION OF COLOUR	g% OF REDUCING SUGAR	INTERPRETATION
Blue or no colour change	0%	Absence of reducing sugar
Green	0.5-1	Trace amount of reducing sugar
Yellow	1-1.5	Small amount of reducing sugar
Orange-Red	1.5-2	Moderate amount of reducing sugar
Brick Red	>2	Large amount of reducing sugar

Limitations of Benedict's Test

If some medicines are present, such as salicylates, isoniazid, streptomycin, penicillin, and p-amino salicylic acid, false-positive results in the test can occur. The compounds in concentrated benedict test for urine, such as urate, creatinine, and ascorbic acid, may diminish Benedict's reactivity (the reduction is slight).

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Frequently Asked Questions (FAQs)

1. What exactly does Benedict’s test detect?

Benedict test is used for the detection of the presence of simple benedict’s test for carbohydrates in an unidentified analyte. This test can be used to look for free aldehyde or ketone functional groups in reducing sugars. A monosaccharide or a disaccharide might be used as the reducing sugar.

2. What is the best way to carry out Benedict’s test?

Benedict’s test is performed by mixing one millilitre of analyte solution with two millilitres of Benedict’s reagent in a test tube. Then, for about 3 minutes, this combination must be heated in a hot water bath (or until a visible change in colour occurs).

3. What is Benedict’s reagent?

Benedict’s reagent, also called Benedict’s solution test, is a chemical reagent made composed of a complicated mixture of sodium citrate, sodium carbonate, and copper (II) sulphate pentahydrate. Benedict’s reagent changes colour from clear blue to brick red when exposed to reducing sugars and other reducing chemicals.

4. Give some examples of substances that give positive results for Benedict’s test?

Glucose, Fructose, Ribose are examples of substances that give positive results for Benedict’s test.

5. How do you prepare Benedict’s solution?

By combining cupric ions (Cu²⁺ cations) from copper sulphate pentahydrate with citric acid molecules in a basic environment given by sodium carbonate, Benedict’s solution (or Benedict’s reagent) can be made.