Fehling's solution

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Fehling's solution is a solution used to deferentiate 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.

Fehling's solution is always made just prior to the test. Two equal volumes of solutions are mixed:

1. 69.28 grams copper(II) sulfate pentahydrate and fill to 1 litre of solution with distilled water
2. 346 grams Rochelle salt (potassium sodium tartrate tetrahydrate) and 120 grams sodium hydroxide and fill to 1 litre of solution with distilled water

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).

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

External links

• National Pollutant Inventory - Copper and compounds fact sheetca:Fehling

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