In 1848, a German chemist Hermann Fehling developed a method to differentiate between reducing and non – reducing sugars, this method is known as Fehling’s Test.

Initially, the Fehling Test was used to detect the presence of sugar content in the blood of diabetic patients.

Fehling’s test is an indicating reaction for reducing groups like Aldehyde functions.

The principle of this test is based on the fact that the aldehyde group of sugar is oxidised to form acids by complex Copper ions. The Red Copper (I) oxide then precipitates. The formation of precipitation is an indicator for the redox reaction.

Introduction to Fehling’s Solution:

Fehling’s Solution is also known as Fehling’s Reagent.

It is a chemical reagent which is used for determination of reducing and non – reducing sugars that are found in carbohydrates.

Fehling’s solution is not suitable for use with aromatic aldehyde.

Preparation of Fehling’s solution:

Fehling’s solution is made in the laboratory by mixing two solutions which are: Fehling A and Fehling B.

Fehling A is an aqueous solution of Copper (II) sulphate and it is blue in colour.

Fehling A is prepared by dissolving copper sulphate in distilled water and a few drops of sulfuric acid is added.

Fehling B is a colourless aqueous solution of Rochelle salt (that is the common name of Potassium sodium tartrate) in an alkaline base like Sodium hydroxide.

It is prepared by sodium potassium tartrate and sodium hydroxide in 150 ml of distilled water.

Both of these solutions are individually prepared and then they are mixed to form blue colored Fehling’s Solution.

In the final mixture, aqueous tartrate ions form bonds with Cu (II) ions as bidentate ligands and form a complex of Cu (II).

The tartrate complex formed acts as an oxidising agent.

Principle of Fehling’s Test:

Fehling’s test is given by Reducing sugars.

When aldehydes are added to Fehling’s solution,  they are easily oxidised to form acids by the bistartratocuprate (II) complex.

Also, Copper (II) ions get reduced to Copper (I) ions.

As a result, a precipitate of Copper (I) oxide is formed which is red in colour.

The occurrence of red colour at the end of reaction gives indication of Positive result.

 Process of Fehling’s Test:

The process of Fehling’s Test is as follows:

1.     The Fehling’s solution is prepared by mixing equal quantities of Fehling A and Fehling B solution.
2.     1 ml of sample is taken in a dry test tube.
3.     In another test tube, distilled water is taken as control.
4.     1 ml of Fehling’s solution is added to both of the test tubes.
5.     Both these test tubes are heated in a water bath.
6.     If red precipitate forms, then the result is positive. The sample must have glucose, fructose, etc.
7.     If there is no precipitate formation, then sample must have sucrose or starch

Reactions of Fehling’s Test:

When Copper (II) ions react with the aldehyde group of the sample, then Copper (I) oxide is formed. This Copper oxide forms red colored precipitate.

In the reaction between Fehling’s solution and the aldehyde group present in the sample compound, the aldehyde group oxidises to acid and Copper (II) ions get reduced in Cu (I) ions.

This reaction is the Redox reaction. 

Reduction reaction is as follows:

2 [Cu(C4H3O6)2]4- + 2 OH– + 2e–  → 2 CuOH + 4 (C4H3O6)3-

In this step, Copper (II) is reduced to Copper (I)

Oxidation reaction as follows:

R – CHO + 2OH– → RCOOH + H2O + 2e–

After dehydration, CuOH changes into Copper (I) oxide and forms red precipitate.

Dehydration reaction is as follows:

2 CuOH → Cu2O ↓ + H2O

Then, deprotonation of the carboxylic acid takes place:

RCOOH + 1 OH– → RCOO– + H2O

The overall reaction is as follows:

R – CHO + 2Cu2+ + 5OH– → R – COO– + Cu2O ↓ + 3H2O

The overall reaction with tartrate is as follows:

2 [Cu(C4H3O6)2]4- + 5 OH– + R – CHO → Cu2O ↓ + 3H2O + 4 (C4H3O6)3- + R – COO– 

When the redox reaction is completed, the reduced Copper (I) ion or Copper (I) oxide is formed and it is insoluble in water and has red colour.

The red colour precipitate confirms the presence of an aldehyde functional group in the sample.

Uses of Fehling’s test: 

Fehling’s test is used for the following purposes:

1. To differentiate between the non – reducing and reducing sugar.
2. To distinguish between the type of carbohydrates present in a solution.
3. It is useful in the process of screening the glucose in the urine of patients with diabetes.
4. It is useful in the breakdown of starch to form maltodextrins and glucose syrup. This process is used to measure the amount of reducing sugar.

Limitations of Fehling’s Test:

1.     Aromatic aldehydes cannot be detected by Fehling’s Test.
2.     This reaction takes place only in alkaline medium.

In an acidic environment, the Copper (II) ions would be stabilised and not easily oxidised. Thus the reaction would fail.

Conclusion 

 Fehling’s test is one of the most common tests used for the identification of reducing and non – reducing sugars.

Fehling’s test is an indicating reaction for reducing groups like Aldehyde functions.

The principle of this test is based on the fact that the aldehyde group of sugar is oxidised to form acids by complex Copper ions. The Red Copper (I) oxide then precipitates. The formation of precipitation is an indicator for the redox reaction.

Fehling’s solution is a chemical reagent which is used for determination of reducing and non – reducing sugars that are found in carbohydrates. Fehling’s solution is made in the laboratory by mixing two solutions which are: Fehling A and Fehling B.