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. 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:
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.