Fehling’s Test-Definition, Principle, Procedure, Result, Uses

Fehling’s Test Definition

A chemical test called Fehling’s test is used to discriminate among reducing and non-reducing sugars. This test may potentially be utilised to distinguish between water-soluble carbohydrates and carbohydrate functional groups, including ketones.

Objectives of Fehling’s Test

  • to determine if a solution has carbohydrates.
  • to distinguish reducing carbohydrates from non-reducing sugars.

Principle of Fehling’s Test

  • Aldehyde or ketone-containing carbohydrates that include free or potentially free carbonyl groups may function as reducing sugars.
  • The Fehling’s solution is a dark blue liquid containing copper sulphate, potassium sodium tartrate, as well as a strong base, which is commonly sodium hydroxide.
  • When the sample containing the bistartarocuprate (II) complex from Fehling’s solution is warmed, the aldoses oxidise into the corresponding aldonic acids.
  • Copper (II) ions are transformed into cuprous (I) oxide (Cu2O) ions or an insoluble yellow or red precipitate throughout the process.
  • In contrast, the oxidation of ketones yields shorter chains of acids.
  • By generating a bistartarocuprate (II) complex, the tartrate ions stop the reaction between the solution’s copper sulphate and sodium hydroxide from producing the insoluble Cu(OH)2.
  • This combination slows down the release of cupric ions for reduction, inhibiting the development of dark cupric oxide.
  • Fehling’s solution generates a black cupric oxide precipitate in the absence of reducing sugars when heated.


2Cu(OH)2 + reducing sugar → 2Cu2O + Aldonic acid


  1. Reagent
  • Fehling’s solution A: Mix 100 ml of water with 7 g of CuSO4.7H2O.
  • Fehlings solution B: Mix 100 ml of water with 24 g of KOH and 34.6 g of potassium sodium tartrate.
  • Fehling’s solution: Just before using, combine equal quantities of each solution.
  • Sample (5% Glucose, 5% Sucrose, 5% Fructose, 5% Starch, 5% Lactose) 
  1. Materials Required
  • Pipettes
  • a test tube
  • Test tube stand
  1. Equipment
  • Water bath

Procedure of Fehling’s Test

  1. Put 1 ml of the sample that is provided in a dry, clean test tube. The test samples’ concentration should be 5 percent (w/v).
  2. In another tube, take control of 1 cc of distilled water.
  3. To each tube, add a few drops of Fehling’s reagent and stir them together in a vortex.
  4. For one to two minutes, leave the test tubes in the water bath.
  5. Watch how the test tubes’ appearance changes colour.
  6. Make a note of the colour appearance in the test tubes.

Result and Interpretation of Fehling’s Test

A reddish-brown precipitate’s appearance indicates a successful outcome and the presence of reducing sugars.

A bad outcome and a lack of reducing sugars are indicated by the absence of the reddish precipitate or the presence of a deep blue hue.

Uses of Fehling’s Test

  • Using Fehling’s test, which does not respond to any ketone sugars other than alpha-hydroxy-ketone, it is possible to discriminate between the presence of aldehydes and ketones in carbohydrates.
  • Medical institutions use Fehling’s test to identify the presence of glucose in urine. This aids in determining the patient’s diabetes status.

Limitations of Fehling’s Test

  • This test cannot identify aromatic aldehydes.
  • It is only in an alkaline environment that this reaction occurs. The reaction would not succeed in an acidic environment because the copper (II) ions would be stable and difficult to oxidise.

References and Sources

  • Tiwari A. (2015). Practical Biochemistry. LAP Lambert Academic Publishing.
  • 3% – https://www.researchgate.net/publication/313745155_Practical_Biochemistry_A_Student_Companion
  • 3% – https://www.answers.com/Q/What_is_the_role_of_sodium_potassium_tartrate_in_Fehling%27s_solution
  • 2% – https://en.wikipedia.org/wiki/Fehling%27s_solution
  • 2% – https://byjus.com/jee/fehlings-solution/
  • 2% – https://byjus.com/chemistry/fehling-test/
  • 2% – http://www.bioline.org.br/request?tc09039
  • 1% – https://www.medicalnewstoday.com/articles/323378
  • 1% – https://encyclopedia2.thefreedictionary.com/carbohydrate
  • 1% – https://chemdemos.uoregon.edu/demos/Fehling-Test
  • 1% – https://byjus.com/chemistry/tests-for-aldehydes-and-ketones/
  • 1% – http://wwwchem.uwimona.edu.jm/courses/Fehling.html
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