Fehling's is used to test for aldoses and ketoses, although formic acid (methanoic acid) also gives a positive Fehling's test result, as it does with Tollens' test, Benedict's test, and many more aldehyde tests.
To carry out the test 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 in 1849.
Fehling's solution is always made just prior to the test. It is comprised of equal parts of the following solutions:
An aldehyde is first added to the Fehling solution and 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 (except alpha hydroxy ketones such as are present in ketoses and other metabolites) and aromatic aldehydes do not respond to the Fehling test.
Fehling's solution in the present day is 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.