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

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

Team Careers360Updated on 02 Jul 2025, 04:34 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.

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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Commonly Asked Questions

Q: Who developed the Fehling's test?
A:
The Fehling's test was developed by German chemist Hermann von Fehling in 1849. It was originally designed to detect the presence of glucose in urine for diabetes diagnosis but has since found wider applications in organic chemistry.
Q: What is the significance of the Fehling's test in organic chemistry?
A:
The Fehling's test is significant in organic chemistry as a qualitative test for identifying aldehydes and reducing sugars. It helps in distinguishing between aldehydes and ketones, and between reducing and non-reducing sugars, which is crucial in structural determination and compound identification.
Q: How does the Fehling's test relate to the reducing power of sugars?
A:
The Fehling's test directly demonstrates the reducing power of sugars. Sugars that can reduce the copper(II) ions to copper(I) are called reducing sugars. This reducing power comes from their ability to form an open-chain structure with an aldehyde group, or in the case of ketoses, to tautomerize to an aldose form.
Q: Can the Fehling's test be used to detect aldehydes in pharmaceutical products?
A:
Yes, the Fehling's test can be used to detect aldehydes in pharmaceutical products, especially in quality control processes. However, due to the complexity of many pharmaceutical formulations, more specific and sensitive analytical techniques are often preferred for definitive analysis.

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

Commonly Asked Questions

Q: How does the Fehling's test compare to other tests for reducing sugars?
A:
The Fehling's test is one of several tests for reducing sugars, including Benedict's test and Barfoed's test. It's generally more sensitive than Benedict's test but less specific than Barfoed's test, which can distinguish between monosaccharides and disaccharides.
Q: Can the Fehling's test detect all types of aldehydes?
A:
The Fehling's test can detect most aldehydes, including both aliphatic and aromatic aldehydes. However, some aldehydes with very bulky groups near the aldehyde functionality might react more slowly or not at all due to steric hindrance.
Q: How does the pH of the solution affect the Fehling's test?
A:
The pH of the solution is crucial for the Fehling's test. The test is carried out in strongly alkaline conditions (pH > 11) because this enhances the reducing power of sugars and aldehydes. In acidic or neutral conditions, the test may not work effectively.
Q: Can the Fehling's test be used to detect formaldehyde?
A:
Yes, the Fehling's test can detect formaldehyde. As the simplest aldehyde, formaldehyde readily reduces the copper(II) ions in Fehling's solution, producing the characteristic red-orange precipitate of copper(I) oxide.
Q: Can the Fehling's test be used to detect reducing disaccharides?
A:
Yes, the Fehling's test can detect reducing disaccharides such as maltose and lactose. These disaccharides have a free aldehyde group on one of their monosaccharide units, allowing them to reduce the copper(II) ions in Fehling's solution.

Methodology

Following are the steps to be followed;

  1. Sterile test tubes should be used to hold the specimen.

  2. Another tube should be containing purified water as a control.

  3. Tubes will be filled with Fehling’s solution.

  4. Water baths are ideal for holding tubes.

  5. 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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Commonly Asked Questions

Q: What are the components of Fehling's solution?
A:
Fehling's solution consists of two parts: Fehling's A and Fehling's B. Fehling's A is an aqueous copper(II) sulfate solution, while Fehling's B is an alkaline solution of sodium potassium tartrate (Rochelle salt). These are mixed in equal volumes just before use.
Q: Why is Fehling's solution prepared as two separate solutions?
A:
Fehling's solution is prepared as two separate solutions to increase its shelf life. When mixed, the alkaline Fehling's B solution would cause copper(II) hydroxide to precipitate from Fehling's A over time. Keeping them separate until use ensures the reagent's effectiveness.
Q: Can the Fehling's test distinguish between different types of reducing sugars?
A:
No, the Fehling's test cannot distinguish between different types of reducing sugars. It only indicates the presence of a reducing sugar or aldehyde. To identify specific sugars, additional tests or techniques would be required.
Q: What is the difference between Fehling's test and Tollens' test?
A:
Both Fehling's test and Tollens' test are used to detect aldehydes, but they use different reagents and produce different results. Fehling's test uses copper(II) ions and produces a red-orange precipitate, while Tollens' test uses silver ions and produces a silver mirror or black precipitate.
Q: What is the role of tartrate ions in Fehling's solution?
A:
Tartrate ions in Fehling's solution act as a complexing agent. They form a complex with copper(II) ions, preventing the precipitation of copper(II) hydroxide in the alkaline solution. This keeps the copper ions in solution and available for the redox reaction.

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 C4H4O62- + 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.

NCERT Chemistry Notes:

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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Commonly Asked Questions

Q: What is the Fehling's test?
A:
The Fehling's test is a chemical analysis used to detect the presence of reducing sugars and aldehydes. It involves the reduction of copper(II) ions to copper(I) oxide, which forms a red-orange precipitate in the presence of these compounds.
Q: What is the principle behind the Fehling's test?
A:
The principle of the Fehling's test is based on the reducing properties of aldehydes and certain sugars. These compounds reduce the copper(II) ions in the alkaline Fehling's solution to copper(I) oxide, which appears as a red-orange precipitate.
Q: Can ketones be detected using the Fehling's test?
A:
Generally, ketones do not react with Fehling's solution and give a negative result. This is because most ketones lack the ability to reduce copper(II) ions. However, α-hydroxy ketones can give a positive result due to their ability to tautomerize to aldehydes.
Q: Why is the Fehling's test considered more sensitive than the Benedict's test?
A:
The Fehling's test is considered more sensitive than the Benedict's test because it can detect lower concentrations of reducing sugars. This increased sensitivity is due to the stronger alkaline conditions in Fehling's solution, which enhance the reducing power of sugars.
Q: What color change indicates a positive Fehling's test?
A:
A positive Fehling's test is indicated by a color change from deep blue to brick-red or red-orange. This color change is due to the formation of copper(I) oxide precipitate.

Frequently Asked Questions (FAQs)

Q: How does the Fehling's test relate to the concept of chirality in sugars?
A:
The Fehling's test itself doesn't directly relate to sugar chirality, as it only detects the presence of a reducing group. However, the test's ability to detect reducing sugars is indirectly related to chirality, as the spatial arrangement of hydroxyl groups in sugars
Q: How does the Fehling's test relate to the concept of functional group interconversion?
A:
The Fehling's test demonstrates functional group interconversion as it involves the oxidation of an aldehyde group to a carboxylic acid group. This showcases how one functional group (aldehyde) can be converted to another (carboxylic acid) through a chemical reaction, a key concept in organic synthesis.
Q: Why is the Fehling's test sometimes referred to as a "clock reaction"?
A:
The Fehling's test is sometimes called a "clock reaction" because, under controlled conditions, the time taken for the color change can be related to the concentration of the reducing sugar. This allows for a rough quantitative analysis, where the reaction time serves as a "clock" to measure concentration.
Q: How does the presence of a metal catalyst affect the Fehling's test?
A:
The presence of a metal catalyst generally doesn't significantly affect the Fehling's test, as the test already involves a redox reaction catalyzed by the alkaline conditions. However, some metal catalysts might interfere with the test by reacting with the Fehling's solution or the analyte, potentially leading to false results.
Q: Can the Fehling's test be used to detect aldehydes in polymers?
A:
The Fehling's test can potentially detect aldehydes in polymers, but its effectiveness depends on the polymer's structure. If the polymer has accessible aldehyde end groups or side chains, these could react with Fehling's solution. However, for many polymers, the aldehyde groups may be too sterically hindered or too few to give a noticeable result.
Q: What role does the copper(II) ion play in the Fehling's test?
A:
The copper(II) ion in Fehling's solution acts as the oxidizing agent. It accepts electrons from the aldehyde or reducing sugar, becoming reduced to copper(I) in the process. This reduction is what causes the formation of the characteristic red-orange copper(I) oxide precipitate.
Q: How does the reactivity of aliphatic aldehydes compare to aromatic aldehydes in the Fehling's test?
A:
Generally, aliphatic aldehydes are more reactive in the Fehling's test compared to aromatic aldehydes. This is because the aromatic ring in aromatic aldehydes can delocalize the electrons, making the aldehyde group less reactive. However, both types of aldehydes will eventually give a positive result.
Q: Can the Fehling's test be used to detect aldehydes in environmental samples?
A:
Yes, the Fehling's test can be used to detect aldehydes in environmental samples, such as water or air samples. However, due to potential interferences from other compounds, more specific analytical techniques like gas chromatography or high-performance liquid chromatography are often preferred for environmental analysis.
Q: How does the Fehling's test relate to the concept of oxidation numbers?
A:
In the Fehling's test, the oxidation number of copper changes from +2 to +1, while the carbon in the aldehyde group increases its oxidation number. This demonstrates the concept of oxidation numbers changing in opposite directions during a redox reaction, with one species being oxidized and the other reduced.
Q: Why is it important to use freshly prepared Fehling's solution?
A:
It's important to use freshly prepared Fehling's solution because the reagent can degrade over time, especially when exposed to light or air. This degradation can lead to false positive results or reduced sensitivity. Mixing Fehling's A and B just before use ensures the reagent's effectiveness.
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