Fehling's solution

Fehling's solution is a solution used to detect aldehyde functional groups. The substance to be tested is heated together with Fehling's solution; a red precipitate indicates the presence of aldehydes or aldehyde groups. An example for its use is to screen for glucose in urine, thus detecting diabetes.

Preparation

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

1. 70 grams cupric sulfate pentahydrate
2. 350 grams Rochelle salt (potassium sodium tartrate tetrahydrate) and 100 grams sodium hydroxide

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.

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