Fehling Test

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Fehling Solution

The Fehling test is one of the most common tests used for the estimation or identification of reducing sugar and non-reducing sugars. This test by the German chemist H.C. Von Fehling is often used to distinguish between functional groups of ketones and carbohydrates that are water-soluble.


Objectives of Fehling’s Test

  • To detect the presence of carbohydrates in a solution. 

  • To distinguish between sugar reducers and non-reducers.


Fehling’s Solution

The Fehling test consists of a solution that is normally freshly prepared in laboratories. The solution initially occurs in the form of two different solutions known as Fehling's A and Fehling's B. Fehling's A is a blue copper(II) sulfate containing solution. Fehling's B is a clear liquid consisting of potassium sodium tartrate (Rochelle salt) and a powerful alkali, normally sodium hydroxide. Solutions A and B are separately prepared and stored during the evaluation.

In order to get the final Fehling solution that is deep blue, the two solutions are later combined in equal amounts. Cu2 complex is the deep blue ingredient. The tartrate tetra-anions in the solution act as a chelating agent.

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Procedure

The process can be carried out as follows; 

  • Add the specimen to a sterile test tube.

  • As a control, purified water should be retained in another tube. 

  • Fehling’s solution to be added to the tubes.

  • It is best to hold the tubes in a water bath.

  • Make observations and report whether any red precipitate growth occurs.

Notably, the result is positive if the reddish-brown precipitate is produced, whereas the result is negative if there is no sign of such a transition.

Precaution-  Fehling's solution is often corrosive in nature. Therefore, wearing protective gear including goggles and gloves is always better.


Reactions of Fehling’s Test

The response between copper(II) ions and an aldehyde is expressed in Fehling's solution as:

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

Once tartrate has been added,

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


Common Uses of Fehling’s Test

There are some common uses of Fehling's test. It is used to decide if an aldehyde or a ketone is a carbonyl group. Aldehydes tend to become oxidized and create a positive outcome. Ketones do not react, apart from alpha-hydroxy-ketones.

The Fehling test is also used as a general test for monosaccharides, where aldose monosaccharides and ketose monosaccharides have a positive outcome. Apart from this, the Fehling test is used to assess the presence of glucose in the urine in the medical field. It helps to know whether or not the person is diabetic. 


Principle/Aim of the Fehling Test

To differentiate between aldehyde and ketone groups in a solution, a chemical reagent and Fehling's reagent is used to allow the identification of sugar reduction in the test sample. Fehling's reagent is composed of two solutions, solution A and solution B. Fehling's A solution is an aqueous solution of copper sulfate, whereas Fehling's solution B is formed by alkaline sodium potassium tartrate. 

When both solutions are combined in equal amounts and heated, the solution B present in the reagent carries out the chelation activity. If an aldehyde or sugar molecule is the test solution, they form a reddish-brown chelated compound with Fehling's reagent. The formation of this reddish-brown precipitate is indicative of the presence of sugar reduction or an aldehyde group. The aim of performing the Fehling test is to detect sugar reduction in a solution. The new preparation of reagents is one precaution required for the efficacy of this test. For the Fehling reagent test, sugars such as glucose, fructose, and lactose yield positive results.

FAQ (Frequently Asked Questions)

1. Explain the uses and limitations of Fehling’s Test

Uses of Fehling’s Test

  • The Fehling test is used to differentiate between the presence of aldehydes and ketones in carbohydrates since, in this test, ketone sugars other than alpha-hydroxy-ketone do not react. 

  • In medical facilities, Fehling's test is conducted to detect the presence of glucose in urine. This helps to define whether or not the patient has diabetes.


Limitations of Fehling’s Test

  • This test does not detect aromatic aldehydes. 

  • This reaction takes place only in an alkaline atmosphere. The copper(II) ions would be stabilized and not easily oxidized in an acidic environment so that the reaction would fail.

2. Define Fehling’s test.

Fehling’s test is a chemical test used to differentiate between reducing and non-reducing sugars. This test can also be used to differentiate between carbohydrates and liquid carbohydrates in the ketone functional community.


Fehling's solution is still freshly formulated in the lab. Initially, it was created as two separate solutions, known as Fehling's A and Fehling's B. Fehling's A is a blue aqueous solution of pentahydrate crystals of copper(II) sulfate, while Fehling's B is a transparent solution of tartrate of aqueous potassium sodium (also known as Rochelle salt) and a solid alkali (commonly sodium hydroxide).

To get the final Fehling's solution, which is a deep blue colour, equivalent amounts of the two mixtures are blended together. Aqueous tartrate ions from the dissolved Rochelle salt chelate to Cu2+ (aq) ions from the dissolved copper sulfate crystals in this final mixture, as bidentate ligands, as shown in the accompanying diagram, give the bistartratocuprate(II) complex.

3. Answer the following:

A. Does benzaldehyde give Fehling test?
B. Does formaldehyde give a Fehling test?
C. Why are Fehling’s solutions A and B kept separate?
D. Which sugar does not reduce Fehling’s solution?
E. Why are aldehydes more reactive to the nucleophilic addition reaction than ketones?

Ans

A. No. For aromatic alcohol, Fehling's test cannot be used.
B. Yes. For formaldehyde, Fehling's test may be used.
C. Fehling's A and B solutions are kept separate because the bistartratocuprate (II) complex that is formed will easily degrade if they are combined.
D. As it does not have a free aldehyde or ketone group, Sucrose does not reduce Fehling's solution.
E. Aldehyde, due to its stereochemistry and electronic properties, is more reactive to nucleophilic addition response than ketone. Aldehyde, unlike ketone, has single hydrogen on one side of the functional group of carbonyl, making it simpler for a nucleophile to strike.