Fehling's solution

Fehling's solution is a solution used to differentiate between water soluble aldehyde and ketone functional groups. The substance to be tested is heated together with Fehling's solution; a red precipitate indicates the presence of an aldehyde. Ketones (except alpha hydroxy ketones) do not react. An example for its use is to screen for glucose in urine, thus detecting diabetes. It was developed by German chemist Hermann von Fehling.

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However formic acid (methanoic acid) also gives a positive Fehling's test result, as it does with Tollens' and many more aldehyde tests.

Fehling's is also used to test for aldoses.

Fehling's solution is always made just prior to the test. It is comprised of equal parts of the following solutions:

• 69.28 grams copper(II) sulfate pentahydrate dissolved in 1 litre of distilled water (Fehling's solution A)
• 346 grams Rochelle salt (potassium sodium tartrate tetrahydrate) and 120 grams sodium hydroxide in 1 litre of distilled water (Fehling's Solution B)

Fehling test

An aldehyde is first added to the Fehling solution and the mixture is then heated. Aldehyde oxidizes to acid and red precipitates of cuprous oxide are formed The cupric ion is complexed with the tartrate ion. Contact with an aldehyde group reduces it to a cuprous ion, which then precipitates as red Cu₂O (copper(I) oxide). Ketones (except alpha hydroxy ketones) and aromatic aldehydes do not respond to the Fehling test.

Fehling's solution is nowadays often replaced by Benedict's solution.

Note: Fehling's solution can only be used to test for aliphatic aldehydes, whereas tollen's reagent can be used to test for both aliphatic and aromatic aldehydes

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