Benedicts Test

Benedict’s test is a simple chemistry test which is used to detect the reducing sugars. Reducing sugars are the carbohydrates that have free aldehyde or ketone functional group in its molecular structure. These include monosaccharides such as glucose and fructose and disaccharides such as lactose and maltose. Benedict’s reagent, also known as benedict’s solution is used in Benedict’s test for detecting simple sugars such as glucose. It is a bright blue solution that is prepared by mixing copper sulfate pentahydrate, sodium carbonate, and sodium citrate in distilled water. In this article, we will learn about the benedict’s test in detail that includes the benedict test principle, benedict’s test procedure, benedict test reaction, and the benedict’s solution formula.

What is Benedict’s Test?

Benedict’s test is a chemical test which is used to check for the presence of reducing sugars in an analyte. Hence, simple carbohydrates that contain a free ketone or aldehyde functional group can be identified using this test. The benedict’s test for reducing sugars is based on the benedict’s reagent, which is also known as Benedict’s solution and is a complex mixture of sodium citrate, pentahydrate of copper(II) sulfate, and sodium carbonate.

When it exposed to the reducing sugars, the reactions undergone through Benedict’s reagent form a brick-red precipitate that indicates a positive reducing sugar test. Take a look at the image of the benedict’s test colours of Benedict’s reagent that change from clear blue to brick-red and which are triggered by the exposure to reducing sugars.

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Benedict’s test can also be used for checking the presence of glucose in a sample of urine. Since this benedict test for urine detects the presence of any aldehydes and α-hydroxy ketones and given that glucose is an aldose that has open-chain that forms an aldehyde group, the test gives a positive result if glucose is present in the analyte. However, a positive reaction is also given in the presence of homogentisic acid, ascorbic acid and several other reducing substances. Hence, a positive Benedict’s test does not always mean that the test subject is having diabetes

Principle of Benedict Test

Benedict’s test is performed when the reducing sugar is heated with Benedict‘s reagent. The alkaline sodium carbonate present converts the sugar into a strong reducing agent called as enediols. During the occurrence of the reduction reaction, the mixture changes its colour from blue to brick-red precipitate because of the formation of cuprous oxide Cu2o. Copper which is in its cupric Cu2+ or copper (I) form is then reduced to cuprous Cu+ or copper (II) form. Since the red-coloured cuprous oxide is insoluble in water, it is separated. If the concentration of the sugar is high, the colour of the resultant solution becomes more reddish, and, in turn, the volume of the precipitate increases.

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Benedict’s Test Procedure

A mixture which contains the reducing sugar (about 8 drops of urine) and Benedict’s solution (approximately 5 ml) is heated in a test tube for around two minutes and is then allowed to cool. The colour of the mixture changes accordingly and results in precipitates. This indicates positive results. The results of the benedict’s reagent test are summarized in the table below.

Benedict’s Reagent Test Result

Observation of Color

The Concentration of Reducing Sugar in g %

Interpretation

Blue, no change in colour

0

No presence of reducing sugar

Green precipitate

0.5 - 1

Presence of traceable amount of reducing sugar

Yellow precipitate

1 - 1.5

Presence of a small amount of reducing sugar

Orange-red precipitate

1.5 - 2

Presence of a moderate amount of reducing sugar

Brick-red precipitate

> 2

Presence of a large amount of reducing sugar

Benedict’s Test Results

The test results for benedict’s test for glucose is shown below.

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Limitation of Benedict’s Test

The limitations of Benedict’s test are as follows:

  1. False-positive reactions in the test can also be obtained if there are certain drugs present for example, salicylates, isoniazid, streptomycin, penicillin, and p-aminosalicylic acid.

  2. The chemicals present in the concentrated urine may reduce Benedict’s reaction which includes urate, creatinine, and ascorbic acid (the reduction is slight).

FAQ (Frequently Asked Questions)

1. What is the Chemical Formula of Benedict’s Solution?

The chemical formula of Benedict’s solution is not present since the benedict’s solution is a mixture and not a chemical compound. Benedict’s solution is made by dissolving 173 g monosodium citrate, 17.3 g cupric sulphate pentahydrate and 100 g anhydrous sodium carbonate in distilled water. The final volume of the solution is one litre.

2. Do Complex Carbohydrates Give You Positive Results in Benedict’s Test?

Complex carbohydrates such as starch and cellulose do not give a positive result in Benedict’s test. They only give you a positive test if they are broken down into simpler sugar through heating.

3. How is Benedict’s Test and Barfoed’s Test Different?

Benedict’s test is used for determining the presence of reducing sugar. On the other hand, Barfoed’s test is used for finding if the sugar is a monosaccharide or a disaccharide.