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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Fehling's solution is the name given to a chemical reagent, primarily used to differentiate between water-soluble carbohydrate and ketone functional groups. It can also be used as a test for reducing sugars and non reducing sugars. This reagent is prepared from two solutions, a basic solution of potassium sodium tartrate and a solution of copper(II) sulfate.
Fehling's solution oxidizes aldehydes, but not ketones, into their corresponding carboxylic acids.
- 2Cu2+ + R-CHO + 5 OH- → Cu2O + R-COO- + 3 H2O
If the test is positive, a brick-red precipitate of Copper(I) oxide indicates the redox reaction. The solution usually turns various shades of green prior to this.
Fehling's "A" is a light blue solution that when combined with "B" turns a deep blue due to the complex formation of the copper with the tartrate.
Copper(II) sulfate pentahydrate is a blue crystalline solid and potassium sodium tartrate a colorless or white crystalline solid.
Reagent "A" and Reagent "B" are available from Sigma Aldrich.
- 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 of Copper(I) oxide.
- Test for reducing sugars
- Make Copper(I) oxide
Copper salts are moderately toxic, especially to aquatic life.
Solution A and B should be stored separately in tightly stoppered bottles. They are stable under normal temperatures and pressures.
Do not pour it down the drain. Either reduce the copper to metal by using metals like zinc or iron or deal accordingly with the aqueous copper waste.
Preparation from UWI at Mona, Jamaica: []