Fehling's solution

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 [ [http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/D-Fehling-e.htm Fehling's Test for Reducing Sugars ] ] . 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. [cite journal
journal = Annalen der Chemie und Pharmacie
volume = 72
issue = 1
pages = 106–113
title = Die quantitative Bestimmung von Zucker und Stärkmehl mittelst Kupfervitriol
author = H. Fehling
year = 1849
doi = 10.1002/jlac.18490720112]

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.


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  • Fehling's solution — Solution So*lu tion (s[ o]*l[=u] sh[u^]n), n. [OE. solucion, OF. solucion, F. solution, fr. L. solutio, fr. solvere, solutum, to loosen, dissolve. See {Solve}.] 1. The act of separating the parts of any body, or the condition of undergoing a… …   The Collaborative International Dictionary of English

  • Fehling's solution — [fā′liŋz] n. a blue solution of copper sulfate, Rochelle salt, and sodium hydroxide, used to test for the presence of a sugar, aldehyde, etc.: also called Fehling s reagent * * * …   Universalium

  • Fehling's solution — [fā′liŋz] n. a blue solution of copper sulfate, Rochelle salt, and sodium hydroxide, used to test for the presence of a sugar, aldehyde, etc.: also called Fehling s reagent …   English World dictionary

  • Fehling's solution — Feh·ling s solution fā liŋ(z) or Feh·ling solution liŋ n a blue solution of Rochelle salt and copper sulfate used as an oxidizing agent in a test for sugars and aldehydes in which the precipitation of a red oxide of copper indicates a positive… …   Medical dictionary

  • Fehling's solution — noun Etymology: Hermann Fehling died 1885 German chemist Date: 1873 a blue solution of Rochelle salt and copper sulfate used as an oxidizing agent in a test for sugars and aldehydes …   New Collegiate Dictionary

  • Fehling's solution — n. aqueous alkaline solution used to test for the presence of sugars and aldehydes in a substance (named after its developer: Hermann von Fehling) …   English contemporary dictionary

  • Fehling's solution — /ˌfeɪlɪŋz səˈluʃən / (say .faylingz suh loohshuhn) noun a solution of copper sulphate and Rochelle salt in alkali, which is used for the detection and quantitative estimation of sugars and other reducing agents. {named after Hermann Fehling,… …  

  • Fehling's solution — noun a liquid solution of copper sulfate and potassium tartrate and sodium hydroxide that is used to test for sugar in the urine; solution turns reddish when sugar is present • Hypernyms: ↑solution …   Useful english dictionary

  • Fehling's solution — noun an alkaline solution of copper sulphate which changes from blue to red in the presence of an aldehyde or a reducing sugar …   Wiktionary

  • Fehling — Feh ling, n. (Chem.) See {Fehling s solution}, under {Solution}. [1913 Webster] …   The Collaborative International Dictionary of English

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