Fehling's solution
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'''Fehling's solution''' is a [[solution]] used to differentiate between water soluble [[aldehyde]] and [[ketone]] [[functional group]]s, although [[ketose]] [[monosaccharide]]s (such as [[fructose]]) will also test positive, due to conversion to aldoses by the base in the reagent <ref>[http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/D-Fehling-e.htm Fehling's Test for Reducing Sugars<!-- Bot generated title -->]</ref>. 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' reagent|Tollens']] test, [[Benedict's reagent|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. <ref>{{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}}
</ref>
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==
[[Image:Fehling test.png|550px|center]]
An aldehyde is first added to the Fehling solution and the mixture is heated. Aldehyde oxidizes to acid and red precipitates of [[copper(I) oxide|cuprous oxide]] are formed
The cupric ion is [[chemical complex|complexed]] with the tartrate ion. Contact with an aldehyde group reduces it to a cuprous ion, which then precipitates as red Cu<sub>2</sub>O (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==
<gallery>
Image:D-glucose-chain-3D-balls.png|The aldehyde form of glucose
Image:Copper-tartrate-complex-3D-balls.png|[Cu<sup>II</sup>(tartrate)<sub>2</sub>]<sup>4−</sup> complex
Image:Copper(I)-oxide-3D-polyhedra.png|Crystal structure of the copper(I) oxide precipitate
</gallery>
== References ==
<references/>
[[Category:Chemical tests]]
[[Category:Coordination compounds]]
[[Category:Copper compounds]]
[[Category:Oxidizing agents]]
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