[Chemistry Class Notes] Fehling Test Pdf for Exam

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 and common uses of the Fehling Test are available for a free download at

A substance or a compound that causes a chemical reaction, or is added to a test if a chemical occurs, that substance or a compound is called reagent, and the reagent which used to differentiate between the ketone functional groups, carbohydrates, and water-soluble is called Fehling’s solution. And this Fehling Test was developed by German Chemist Hermann von Fehling in the year 1849.

For the estimation of the identification of non-reducing sugar and reducing sugar one of the most common tests which is in use is The Fehling test.

Objectives of the Fehling Test

There are many objectives of the Fehling test, two of the most common uses are:

• To detect the presence of, or to differentiate between the carbohydrates in a solution.

• To differentiate between the non-reducing sugar and reducing sugar.

 

The Fehling Solution

Two separate solutions are used in order to prepare the Fehling Solution. The first one is the copper Sulphate, which is a deep blue aqueous and is known as Fehling A solution, and another solution is Rochelle salt made heavily with sodium hydroxide, it is also a colourless solution, it is referred to as Fehling B. Both of these A and B solutions, that is to say, Rochelle salt and copper Sulphate, are mixed together to make the Fehling Solution. Also, both the solution, which is A and B are stored during the evaluation and both of them are prepared separately.

Tartrate complex is the active reagent here, and it serves the purpose of an oxidising agent.

Process of Fehling Test

Below given is a brief overview of the procedure of the Fehling test.

• For preparing the fresh Fehling’s solution, you must first mix in equal quantities both the solutions A and B.

• Transformed this newly prepared Fehling’s solution in a test tube, which has been washed and is now completely dry.

• Put a sample in another tube, which is washed and dried.

• For control, take the distilled water in yet another test tube.

• Now keep all the tubes in the bath of boiling water, and lastly observe the development and record the same.

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 Cu^{2+} + 5 OH^{-} rightarrow RCOO^{-} + Cu_{2}O+3H_{2}O]

 

Once tartrate has been added,

 

[RCHO + 2Cu(C_{4}H_{4}O_{6})_{2}^{2-} + 5 OH^{-} rightarrow RCOO^{-}+Cu_{2}O+4 C_{4}H_{4}O_{6} ^{2-}+3H_{2}O]

 

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 oxidised 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. 

 

Uses of Fehling Test

There are many uses of the Fehling tests some of which are as under:

• It helps in differentiating between the Aldehyde vs ketone functional groups.

• Fehling test is also useful as a generic test for the monosaccharides, which are the simplest form of sugar.

• In the process of screening the glucose in the urine, for patients with diabetes, Fehling solution is helpful and is often used.

• In order to measure the amount of reducing sugar, the starch is converted to glucose syrup and maltodextrins, by breaking down the starch, and for this breakdown of the starch, the Fehling solution is useful.

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 solution is an aqueous solution of copper Sulphate, 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.