[Chemistry Class Notes] Fehling Solution Pdf for Exam

Fehling’s test is used for the detection of reducing sugars or to differentiate between water soluble carbohydrates or ketone functional groups. It is a deep blue alkaline solution used to identify the existence of aldehyde or any groups that contain aldehyde functional group -CHO in addition with the Tollen’s reagent in order to differentiate between the reducing and non-reducing agents. Fehling solution also finds its use in differentiating a ketone group and water-soluble carbohydrates.  The test was developed by German Chemist Hermann von Fehling in 1849. In the test Fehling’s solution is used to get the result. In this article we will discuss the preparation of the Fehling solution, the Fehling test procedure and the reactions involved in this, the result of the Fehling test and its applications. 

 

To carry out the Fehling test, the substance that is to be tested is heated with the fehling solution. If there is the presence of any aldehyde group, it is indicated by the formation of a brick-red precipitate. 

 

Fehling’s Solution

The Fehling solution for the Fehling test is prepared by combining two separate solutions called Fehling’s Solution A and Fehling’s Solution B. Fehling solution A and Fehling solution B are mixed together in equal amounts before the test to get Fehling’s solution.  Aqueous solution of copper sulfate is called Fehling solution A which is blue in color. Fehling solution B is prepared by mixing aqueous potassium sodium tartrate (Rochelle salt) in a strong alkali (common alkali used is NaOH). It’s a colorless solution. Fehling’s solution A and B can be stored separately in the laboratory. Fehling’s solution which is prepared freshly by mixing Fehling’s solution A and B is deep blue in color due to the bis(tartrate) complex of Cu2+. The tartrate anions act as chelating agents. 

 

Procedure of Fehling’s Test 

Procedure of Fehling’s test is as follows –

• Mix Fehling solution A and B in equal quantities to prepare fresh Fehling’s solution. 

• Take freshly prepared Fehling’s solution in a washed and dried test tube. 

• Take a sample in another washed and dried test tube. 

• Take distilled water in another test tube as control. 

• Now tubes are kept in a boiling water bath. 

• Observe and record if development of red colored precipitate takes place. 

 

Reaction Of The Fehling Test

The Reaction occurring between the Copper (II) Ions and Aldehyde in the Fehling solution is as follows; 

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

 

With the addition of Tartrate, the Reaction is changed as follows;

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

 

With the completion of the redox reaction, the copper II ions are reduced to copper I Oxide. This is indicated with the formation of the red precipitate and its insoluble nature  in water. The Sodium salt of the given acid is left behind in the solution. 

 

Result 

After mixing the Fehling solution in the sample, if you observe red precipitate then it indicates the result is positive while if you don’t observe any red (or brownish red) precipitate then the result is negative. Positive results in the Fehling test indicate presence of glucose, fructose and lactose or presence of reducing sugar in the sample. The negative result of the Fehling test indicates the presence of non-reducing sugars such as sucrose, starch. 

 

Application of Fehling’s Test or Fehling’s solution 

• It is used to test Monosaccharides. Fehling test is used as a general test to determine Monosaccharides and other reducing sugars. In case of the aldose monosaccharides, the result of the test is positive. This is mainly because of the oxidizable aldehyde group. In case of ketose Monosaccharides as well a positive result is witnessed. These get converted to aldoses with the help of the base present in the reagent. 

• It is used for detection of reducing sugar like glucose in the sample.

• It can be used to distinguish aldehyde and ketone functional groups. 

• In the medical field It can be used to screen for glucose in urine in order to detect diabetes. 

• It can be used to detect diabetes. 

• It is also used in the process of the breakdown of the starch. Here it changes to glucose syrup and maltodextrins, and is used to measure and determine the amount of reducing sugar. 

 

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