Formation of Fehling’s Solution

Fehling’s Solution is a chemical reagent used to determine reducing and non–reducing sugars found in carbohydrates. This method is known as Fehling’s Test. Fehling’s Solution is also known as Fehling’s Reagent. Fehling’s solution is prepared by mixing two separate solutions: Fehling’s A and Fehling’s B.

(i) Fehling’s A is a deep blue aqueous solution of Copper (II) sulphate. 

(ii) Fehling’s B is a colourless solution of aqueous Potassium sodium tartrate, also known as Rochelle salt.

Formation 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 a blue aqueous solution of Copper (II) sulphate. Fehling A is prepared by dissolving Copper sulphate in distilled water, and a few drops of sulfuric acid is added to the mixture.

Fehling B is a colourless aqueous solution of Rochelle salt (the common name of Potassium sodium tartrate) in an alkaline base like Sodium hydroxide. It is prepared by mixing sodium potassium tartrate and sodium hydroxide in distilled water. 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.

Fehling’s A and Fehling’s B are stable separately. They are combined when there is a need for conducting the test. Because the copper (II) complex formed due to the mixing of both solutions is not stable, it decomposes into copper hydroxide in alkaline conditions.

Both the solutions are individually prepared and then mixed to form blue coloured Fehling’s Solution.

Introduction to Fehling’s Test

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 of the redox reaction.

The reducing sugars give positive results for Fehling’s test.

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 the reaction gives an indication of a positive result.

Process of Fehling’s Test

The process involved in Fehling’s Test is as follows:

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

Reactions of Fehling’s Test

When copper (II) ions react with the aldehyde group of the sample, copper (I) oxide is formed. This copper oxide forms a red coloured precipitate.

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

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 Solution

Fehling’s Solution is used for the following purposes:

• To differentiate between the non–reducing and reducing sugar.
• To distinguish between the type of carbohydrates present in a solution.
• It is useful in screening the glucose in the urine of patients with diabetes.
• It is useful in breaking starch to form maltodextrins and glucose syrup. 

 This process is used to measure the amount of reducing sugar.

 Limitations of Fehling’s Test

• Aromatic aldehydes cannot be detected by Fehling’s Test.
• This reaction is feasible only in an alkaline medium.

The Copper (II) ions get stabilised in an acidic medium and then it becomes really difficult for it to get oxidised. Thus, the reaction may fail.

Conclusion 

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

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

Fehling’s Solution is a chemical reagent that is prepared in the laboratory by mixing equal amounts of Fehling A and Fehling B solutions. Fehling A is a blue colored Copper sulphate solution. Fehling B is an aqueous solution of Rochelle Salt.

The Fehling’s test is done by mixing the sample compound with Fehling’s solution and then heating them in a water – bath. The formation of red precipitate at the end of the reaction, confirms the presence of reducing sugars.