Fehling's solution is a solution used to differentiate between aldehyde or ketone functional groups. The substance to be tested is heated together with Fehling's solution; a red precipitate indicates the presence of an aldehydes. 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. However formic acid also gives fehlings test, not only fehlings but also tollen's and many more aldehyde reactions.

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
• 346 grams Rochelle salt (potassium sodium tartrate tetrahydrate) and 120 grams sodium hydroxide in 1 litre of distilled water

Fehling test

An aldehyde is first added to the Fehling solution & then the mixture is 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 do not respond to the Fehling test.

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

External links

• National Pollutant Inventory - Copper and compounds fact sheet

Copper compounds | Organic chemistry | Chemical tests | Coordination compounds

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