Fehling Test - Meaning, Definition, Purpose & Principles, FAQs

Edited By Team Careers360 | Updated on Jun 29, 2022 06:07 PM IST

What is Fehling solution test

For estimating or identifying the amount of reducing sugar present in foods, the Fehling test reaction is commonly used. A chemist in Germany named H.C. When comparing functional groups with water-soluble carbohydrates in ketones, Von Fehling is widely used. Fehling test reaction is given by formaldehyde.

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This Story also Contains

What is Fehling solution test
The Fehling solution test
Methodology
Fehling’s Test: Reactions
Tests commonly used by Fehling’s
The principle of the Fehling test reaction

The purpose of Fehling’s test

Analyse a solution for carbohydrate content.
A sugar reducer can be distinguished from a non-reducer

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The Fehling solution test

Fehling test reactions are normally performed with a solution that is freshly prepared in laboratories. A copper (II) sulphate-containing solution, Fehling solution A, is the original form of the solution. Fehling solution B is a solid solution that is formed from the reaction between Fehlings solution A and Fehling solution B. An alkali containing sodium hydroxide. Normally Fehling solution B is a clear liquid containing potassium sodium tartrate (Rochelle salt). After combining the two Fehling solution A and B in equal amounts, the final Fehling solution test is deep blue. Deep blue contains the Cu₂ complex. This solution contains tartrate tetra-anions which act as chelators.

The Fehling solution test

Methodology

Following are the steps to be followed;

Sterile test tubes should be used to hold the specimen.
Another tube should be containing purified water as a control.
Tubes will be filled with Fehling’s solution.
Water baths are ideal for holding tubes.
Inspect the precipitates for reddish colour and report it.

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A positive result is obtained if the reddish-brown precipitate is produced, whereas a negative result is obtained if there is no indication of the precipitate.

Be careful when using Fehling’s solution since it is typically corrosive. Therefore, goggles and gloves, as well as protective gear, are always advisable.

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Fehling’s Test: Reactions

According to Fehling’s Solution, the reaction between copper (II) ions and aldehydes is:

RCHO + 2 Cu²⁺ + 5 OH^- → RCOO + Cu₂O + 3 H₂O

Tartrate is added once the solution is prepared

RCHO + 2 Cu(C₄H₄O₆)₂²^- + 5 OH^- → RCOO^-+ Cu₂O + 4 C₄H₄O₆^2- + 3 H₂O

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Tests commonly used by Fehling’s

Fehling’s test is used in a few common situations. Carbonyl groups in aldehydes or ketones are characterized with this test. The oxidation of aldehydes results in a positive outcome. Alpha-hydroxy ketones are the only ketones that react. Aldose monosaccharides and ketose monosaccharides pass the Fehling test reaction, which is also used for general monosaccharide tests. Also used in medical science, the Fehling test reaction measures the presence of glucose in the urine. Diabetics should be informed if they are at risk.

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The principle of the Fehling test reaction

Chemical reagents and Fehling’s reagents are used to distinguish between aldehyde and ketone groups in a solution so the sugar reduction activity in the sample can be identified. Fehling solution A solution is an aqueous solution of the copper mixture of copper sulphate, while Fehling solution B solution consists of alkaline sodium. The chelation activity of the reagent is carried out by solution B in the reagent when both solutions are combined equally and heated.

If the test solution contains aldehydes or sugar molecules, they form chelated compounds with Fehling’s reagent. A reddish-brown precipitate forms when sugars are reduced or when an aldehyde group is present. Sugar reduction in a solution is detected by the Fehling test reaction. A precaution necessary for the efficacy of this test is the preparation of new reagents. Positive results are obtained with Fehling reagents such as glucose, fructose, and lactose.

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Frequently Asked Questions (FAQs)

1. 1. What are the uses of Fehling’s Test?

Aldehydes and ketones are separated by the use of the Fehling test reaction because ketone sugars other than ketones do not react with this test. A Fehling test reaction is conducted in medical facilities to detect the presence of glucose in urine. Whether a patient has diabetes can be determined by this test.

2. 2. Is Fehling’s Test Limited in any way?

This test does not detect aromatic aldehydes.
Alkaline environments are required for this reaction.

3. 3. Define Fehling’s test.

This test distinguishes between non-reducing and reducing sugars by using a chemical reaction. As well as distinguishing between liquid carbohydrates and carbohydrates, this test is commonly used to assess ketones. Fresh solutions are still being formulated in Fehling’s lab. A solution of copper (II) sulphate was initially created as Fehling solution A (a blue-coloured aqueous solution), and Fehling solution B (a transparent solution of potassium sodium tartrate and sodium hydroxide.)

4. 4. Aldehydes are much more reactive to nucleophilic addition than ketones?

Aldehyde exhibits a greater nucleophilic addition response than ketone due to its stereochemistry and electronic properties. It is easy for a nucleophile to attack aldehyde because it has only one hydrogen in its functional group, unlike ketone.

5. 5. Why are Fehling’s solutions A and B kept separate?

Separating Fehling’s, A and B solutions is necessary because the bitartrate to cuprite (II) complex formed in the presence of each will easily degrade in the presence of the other.