Fehling’s Test Procedure

A chemical reagent, called Fehling’s solution, is used to distinguish between the water-soluble carbohydrates and ketonic functional groups. It can also be used as a test for the reducing sugars and the non-reducing sugars, which is supplementary to Tollen’s one. The Fehling’s test was advanced by Hermann von Fehling, a German chemist, in the year 1849. 

How Fehling’s solution is prepared

Fehling’s solution is made by the combination of two different solutions:

• Fehling’s A 
• Fehling’s B 

Fehling’s A is the aqueous solution of copper(II) sulfate (Cupric sulfate or Blue vitriol or Bluestone). It is deep blue in color. 

Fehling’s B is the aqueous solution of potassium sodium tartrate (KNaC4H4O6·4H2O) and is also known as Rochelle salt. It is made strongly of alkali by NaOH (Sodium Hydroxide). Fehling’s B is colorless. 

Both Fehling’s solutions (A and B) are stable when they are in separate conditions but during the test, when they are combined, it becomes unstable. This is because the copper (II) from Fehling’s A solution, forms complexes during combination which is unstable. It starts decomposing slowly into copper hydroxide because of alkaline conditions. The tartrate complex of Cu²+ is the active reagent. It serves as the oxidizing agent. The role of the ligand is served by the tartrate. 

General Reaction

2Cu(OH)2 + reducing sugar —› 2Cu2O + Aldonic acid

Principle of Fehling’s test

Fehling’s solution test is one of the sensitive tests used for the detection of reducing sugars. It consists of two solutions, Fehling’s solution A and Fehling’s solution B. Fehling’s solution A is the aqueous copper sulfate solution and Fehling’s solution B is an alkaline solution of sodium potassium tartrate (KNaC4H4O6·4H2O). Rochelle salts in this reagent act as a chelating agent in the following test. Before the performance of the test, these two solutions are mixed into a test tube in equal amounts. On heating an aldehyde or reducing sugars with this Fehling’s solution, it gives a reddish-brown precipitate. The formation of this red precipitate of cuprous oxide indicates the presence of reducing sugars. 

Fehling’s Test Procedure

In a clean and dry test tube, take 1 ml of the given sample. Test sample concentration should be 5% (weight/volume). 

• Take control of 1 ml of distilled water in a different test tube 
• Add 2-3 drops of Fehling’s reagent into both the test tubes and mix them. 
• Give a water bath to both the test tubes for about 1-2 minutes. 
• Observe the color change in the test tubes. 
• Note down the color seen in the test tubes.

 A reddish-brown precipitate appearance indicates a positive result and that the reducing sugars are present. 

The absence of a reddish precipitate or an appearance of a deep blue color indicates a negative result and that reducing sugars are not present. 

Uses of the Fehling’s Reagent

Fehling’s solution is used to differentiate between the aldehyde and ketonic functional groups in a given organic compound. Here are some Fehling’s test procedure examples to elucidate.

To perform this test, the compound that is to be tested is added into the Fehling’s solution and the mixture obtained is heated. Positive results are given by aldehydes as they are oxidized while ketones generally do not react until they are α-hydroxy ketones. 

Aldehydes are oxidized to carboxylate ions by the action of the bistartratocuprate (II) complex. During this process, the copper(II) ions of bistartratocuprate (II) are reduced into copper (I) ions. The indication of a positive test is given when the red copper(I) oxide precipitates out of the mixture during the reaction. 

This test can also be used to test the monosaccharides or the other reducing sugars, for example – Maltose.  A positive test is given by aldose monosaccharides because of the presence of an oxidisable aldehyde group. In ketose monosaccharides, the base in the reagent converts them into aldoses due to which, they also give a positive result. 

Diabetes can also be determined in a person by testing his urine with Fehling’s solution as the glucose can be screened in the urine. In revealing the DE (dextrose equivalent) of starch sugar, it breaks down starch and converts it into glucose and maltodextrins. Formic acid gives a positive Fehling’s test also. 

Note- Fehling’s solution cannot distinguish between Acetone and Benzaldehyde. 

Net Reaction 

The overall reaction between the copper (II) ions and the aldehyde in Fehling’s solution is as follows –

RCHO + 2 Cu²+ + 5 OH− —› RCOO− + Cu2O + 3 H2O

or with the tartrate included

RCHO + 2 Cu(C4H4O6)22− + 5 OH− —› RCOO− + Cu2O + 4 C4H4O6²− + 3 H2O

Limitations of Fehling’s Test

This test is unable to detect aromatic aldehydes due to the resonance. 

The reaction requires an alkaline environment to take place. If the environment is acidic, the copper ions will be stable and do not get oxidized and fail the reaction. 

Conclusion

Fehling’s solution is used to test reducing and non-reducing sugars and also to distinguish ketone functional group carbohydrates and water-soluble carbohydrates. It consists of two solutions which are Fehling’s A and Fehling’s B. 

The compound to be tested is mixed into the solutions and is allowed to heat. The formation of a red precipitate indicates the positive Fehling’s test. It is very necessary for Fehling’s solution to be freshly prepared as it decomposes with time.