Fehling's Test

In this test the presence of aldehydes but not ketones is detected by reduction of the deep blue solution of copper(II) to a red precipitate of insoluble copper oxide. The test is commonly used for reducing sugars but is known to be NOT specific for aldehydes. For example, fructose gives a positive test with Fehling's solution as does acetoin.

Two solutions are required:

Fehling's "A" uses 7 g CuSO4.5H2O dissolved in distilled water containing 2 drops of dilute sulfuric acid.
Fehling's "B" uses 35g of potassium tartrate and 12g of NaOH in 100 ml of distilled water.

These two solutions should be stoppered and stored until needed.

For the test:
Mix 15 ml of solution-"A" with 15 ml of solution-"B"
Add 2 ml of this mixture to an empty test tube.
Add 3 drops of the compound to be tested to the tube.
Place the tube in a water-bath at 60° C.

A positive test is indicated by a green suspension and a red precipitate.

The test is sensitive enough that even 1 mg of glucose will produce the characteristic red colour of the compound.

The structure of copper(II) D-tartrate has been determined to be:
C.K. Prout, J.R. Carruthers and F.J.C. Rossotti J. Chem. Soc. A, 3336, 1971.

Hermann Christian von Fehling
9th June 1812 - 1st July 1885
born Lubeck, Germany

Benedict's solution

A variation of this is called Benedict's solution.
This makes use of a single solution of copper(II) citrate which does not deteriorate as quickly on standing.

It is prepared by taking 8.7 g of crystallised sodium citrate and 5 g anhydrous sodium carbonate in about 35 cm3 of water. It may be necessary to filter.
To this is added 0.87 g of crystallised copper(II) sulfate in 5 cm3 of water and the mixture made up to 50 cm3 of water. The resulting solution should be clear, otherwise filter it.

The test again is to observe the reduction of the blue copper(II) solution to the red cuprous oxide. Use 5 cm3 of Benedict's solution and 0.4 cm3 of a 2 per cent solution of the carbohydrate. Boil for 2 minutes and allow to cool slowly.

A polymeric structure for copper(II) citrate has been determined.
D.Mastropaolo, D.A. Powers, J.A. Potenza and H.J. Schugar, Inorg. Chem., 15, 1444, 1976.

Stanley Rossiter Benedict
(17th March 1884 - 21st December 1936) American chemist

Barfoed's Reagent

Another variant is Barfoed's reagent;
a mixture of ethanoic (acetic) acid and copper(II) acetate, is added to the test solution and boiled. If any reducing sugars are present a red precipitate of copper(I) oxide is formed withing three minutes. The reaction will be negative in the presence of reducing disaccharide sugars as they are weaker reducing agents and react at a slower rate.
Barfoed's Reagent
This is prepared from 4.5 g crystalized, neutral cupric acetate in 100 cm3 of water, and adding 0.12 cm3 of 50% acetic acid.

The structure of the copper(II) acetate dimer has been determined.
G.M. Brown and R. Chidambaram, Acta Crystallogr., Sect.B, 29, 2393, 1973

Unlike the analogous Cr(II) complex, the copper dimer does not appear to have a bond between the metal ions although there is significant magnetic interaction. For the Cr(II) complex the quadruple bond is 247 pm, in the Cu(II) complex the separation is 261 pm.

Christen Thomsen Barfoed
(16th June 1815 - 30th April 1899) Danish chemist

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