Fehling's solution

Fehling's solution is a solution used to differentiate between water soluble aldehyde and ketone functional groups, although ketose monosaccharides (such as fructose) will also test positive, due to conversion to aldoses by the base in the reagent ^[1]. For this reason, Fehling's reagent is sometimes referred to as a general test for monosaccharides.

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. ^[2]

This article contains instructions, advice, or how-to content. The purpose of Wikipedia is to present facts, not to train. Please help improve this article either by rewriting the how-to content or by moving it to Wikiversity or Wikibooks. (October 2009)

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 (Fehling's solution A)
• 346 grams Rochelle salt (potassium sodium tartrate tetrahydrate) and 120 grams sodium hydroxide in 1 litre of distilled water (Fehling's Solution B)

Fehling test

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.

Images

The aldehyde form of glucose

[CuII(tartrate)2]4− complex

Crystal structure of the copper(I) oxide precipitate

References

1. ^ Fehling's Test for Reducing Sugars
2. ^ H. Fehling (1849). "Die quantitative Bestimmung von Zucker und Stärkmehl mittelst Kupfervitriol". Annalen der Chemie und Pharmacie 72 (1): 106–113. doi:10.1002/jlac.18490720112.