Fehling Test

Fehling's reagent, a blue colored basic solution of bistartratocuprate(II) complex, is added to three different aqueous sugar solutions immersed in beakers of warm water. A brick-red precipitate forms in the solutions containing glucose and fructose. There is no reaction in the test tube containing sucrose solution. 

Curriculum Notes 

This demo is appropriate for use in an organic chemistry or biochemistry course when the reactions of carbohydrates are being studied. The fructose reaction could also be used earlier in an organic chemistry course as an illustration of a reaction that proceeds via a pathway that relies upon keto-enol tautomerism. This demo can easily be scaled up for visibility if video projection is unavailable in the classroom.

Lead Time 
One day of lead time is required for this project.
Discussion 

Fehling's solution is used as a chemical test used to differentiate between water-soluble aldehyde and ketone functional groups, and as a test for monosaccharides. The test was developed by German chemist Hermann von Fehling in 1849.[1]

Fehling's solution is always prepared fresh in the laboratory. It is made initially as two separate solutions, known as Fehling's A and Fehling's B. Fehling's A is a blue aqueous solution of copper(II) sulfate pentahydrate crystals, while Fehling's B is a clear solution of aqueous potassium sodium tartrate (also known as Rochelle salt) and a strong alkali (commonly sodium hydroxide).

Equal volumes of the two mixtures are mixed together to get the final Fehling's solution, which is a deep blue colour. In this final mixture, aqueous tartrate ions from the dissolved Rochelle salt chelate to Cu2+ (aq) ions from the dissolved copper sulfate crystals, as bidentate ligands giving the bistartratocuprate(II)complex as shown in the accompanying illustration.

Fehling's can be used to determine whether a carbonyl-containing compound is an aldehyde or a ketone. The bistartratocuprate(II) complex in Fehling's solution is anoxidizing agent and the active reagent in the test. The compound to be tested is added to the Fehling's solution and the mixture is heated. Aldehydes are oxidized, giving a positive result, but ketones do not react, unless they are alpha-hydroxy-ketones. The bistartratocuprate(II) complex oxidizes the aldehyde to a carboxylateanion, and in the process the copper(II) ions of the complex are reduced to copper(I) ions. Red copper(I) oxide then precipitates out of the reaction mixture, which indicates a positive result i.e. that redox has taken place (this is the same positive result as with Benedict's solution. A negative result is the absence of the red precipitate; it is important to note that Fehling's will not work with aromaticaldehydes; in this case Tollens' reagent should be used.

Fehling's test can be used as a generic test for monosaccharides. It will give a positive result for aldose monosaccharides (due to the oxidisable aldehyde group) but also for ketosemonosaccharides, as they are converted to aldoses by the base in the reagent, and then give a positive result.[2] For this reason, Fehling's reagent is sometimes referred to as a general test for monosaccharides.

Fehling's can be used to screen for glucose in urine, thus detecting diabetes. Another use is in conversion / breakdown of starch to glucose syrup andmaltodextrins, to measure the amount of reducing sugars and calculating thedextrose equivalent (DE) of the starch sugar.

(Wikipedia, "Fehling's Solution." 6/3/11.)

  1. H. Fehling (1849). "Die quantitative Bestimmung von Zucker und Stärkmehl mittelst Kupfervitriol". Annalen der Chemie und Pharmacie 72 (1): 106-113.doi:10.1002/jlac.18490720112.
  2. P. Keusch, Demonstration Experiments on Video, "Fehling's Test." University of Regensburg, 6/3/11. http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Di...
Materials 
  • 3 ea. labeled 16x125 mm test tubes containing 10 mL each of 1% glucose, fructose, and sucrose solutions (see prep notes).
  • 3 ea. 250 mL beakers about 2/3 full of warm water (~60o C).
  • A small bottle of Fehling's solution (see prep notes).
  • A 1 mL Beral pipette.
Procedure 
  • Place each test tube in a beaker of warm water
  • Add 1 mL of Fehling's solution to each of the test tubes.
  • Within 90 s a brick-red precipitate begins to form in the test tubes containing glucose and fructose solutions.
Safety Precautions 

Fehling's solution is corrosive and toxic. Wear appropriate personal protective devices such as gloves and goggles when preparing the solution and when performing the demonstration.

Prep. Notes 
  • Fehling I consists of 7 g of hydrated copper(II) sulfate dissolved in 100 mL of dist. water.
  • Fehling II is made by dissolving 35 g of potassium sodium tartrate and 10 g of sodium hydroxide in 100 mL of dist. water.
  • Fehling's reagent: Equal volumes of Fehling I and Fehling II are mixed to form a deep blue solution.

(P. Keusch, Demonstration Experiments on Video, "Fehling's Test." University of Regensburg, 6/3/11. http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Di...)

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group