Fehling Solution

Fehling’s test is used for the detection of reducing sugars or differentiate between water soluble carbohydrates or ketone functional groups. 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 preparation of Fehling solution, Fehling test procedure and the reactions involved in this, result of Fehling test and its applications. 

Fehling’s Solution

Fehling solution for 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 fresh 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. 

Result 

After mixing 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 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. 

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

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

  • It can be used to screen for glucose in urine. 

  • It can be used to detect diabetes. 

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