Editing Fehling's solution

{{Short description|Chemical test for the reducibility of a sugar}}
{{Infobox chemical analysis
| name           = Fehling's test
| image          = Fehling.JPG
| caption        = On the left, the solution in the absence of reducing sugars. On the right, copper oxide, which would appear in the bottom of the solution if reducing sugars are present.
| acronym        = 
| classification = Colorimetric method
| analytes       = Monosaccharides
| manufacturers  = 
| related        = 
| hyphenated     = 
}}

In [[organic chemistry]], '''Fehling's solution''' is a chemical [[reagent]] used to differentiate between water-soluble [[carbohydrate]] and [[ketone]] ({{chem2|>C\dO}}) [[functional group]]s, and as a test for [[reducing sugars]] and non-reducing sugars, supplementary to the [[Tollens' reagent]] test. The test 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
 | trans-title = The quantitative determination of sugar and starch by means of copper sulfate
 | author = H. Fehling
 | year = 1849 
 | url = https://zenodo.org/record/1427028
 | doi = 10.1002/jlac.18490720112}}
</ref>

==Laboratory preparation==
Fehling's solution is prepared by combining two separate solutions: Fehling's A, which is a deep blue aqueous solution of [[copper(II) sulfate]], and Fehling's B, which is a colorless solution of aqueous [[potassium sodium tartrate]] (also known as [[Rochelle salt]]) made strongly alkaline with [[sodium hydroxide]]. These two solutions, stable separately, are combined when needed for the test because the copper(II) complex formed by their combination is not stable: it slowly decomposes into copper hydroxide in the alkaline conditions. The active [[reagent]] is a tartrate complex of Cu<sup>2+</sup>, which serves as an [[oxidizing agent]]. The tartrate serves as a ligand. However, the coordination chemistry is complex and various species with different metal to ligand ratio have been determined.<ref name="Hörner">T. G. Hörner, P. Klüfers: ''The Species of Fehling's Solution.'' In: ''[[Eur. J. Inorg. Chem.]]'' 2016, S. 1798–1807, [[doi:10.1002/ejic.201600168]].</ref><ref name="Fangfang">Fangfang Jian, Pusu Zhao, Qingxiang Wang: ''Synthesis and crystal structure of a novel tartrate copper(II) two-dimensional coordination polymer: {[Cu<sub>2</sub>(C<sub>4</sub>H<sub>4</sub>O<sub>6</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]·4H<sub>2</sub>O}<sub>∞</sub>.'' In: ''J. Coord. Chem.'' 58, 2005, S. 1133–1138, [[doi:10.1080/00958970500148446]].</ref><ref name="Prout">C. K. Prout, J. R. Carruthers, F. J. C. Rossotti: ''Structure and stability of carboxylate complexes. Part VII. Crystal and molecular structures of copper(II)meso-tartrate trihydrate and copper(II)d-tartrate trihydrate.'' In: ''[[J. Chem. Soc.]]'' A, Inorg. Phys. Theo., 1971, S. 3336–3342, [[doi:10.1039/J19710003336]].</ref><ref name="Kotru">I. Quasim, A. Firdous, B. Want, S. K. Khosa, P. . Kotru: ''Single crystal growth and characterization of pure and sodium-modified copper tartrate.'' In: ''J. Cryst. Growth.'' 310, 2008, S. 5357–5363, [[doi:10.1016/j.jcrysgro.2008.09.021]].</ref><ref name="Jespersen">N. D. Jespersen: ''Novel Copper-Tartrate Coordination Compounds.'' In: ''Anal. Let.'' 5, 1972, S. 497–508.</ref>

Other methods of preparing comparable cupric-ion test-reagent solutions were developed at about the same time as Fehling's. These include the Viollette solution (eponymous for {{ill|Charles Cléophile Viollette|fr}} (1823–1894)) and the Soxhlet solution (eponymous for [[Franz Ritter von Soxhlet]] (1848–1926)), both containing tartrate, and Soldaïni's solution (eponymous for Arturo Soldaïni),<ref>Arturo Soldaïni (1876) Nuovo reattivo pel glucoso.Gazzetta della Chimica Italiana 6 322-324. available at https://catalog.hathitrust.org/Record/000679312/Home</ref> which instead contains carbonate.<ref>{{cite book |title= Handbook for Cane-sugar Manufacturers and Their Chemists |publisher= Wiley |year= 1898<!-- front-matter in the Google-books have both 1889 and 1898, and it's the third edition; late-1890s is more self-consistent; maybe 1889 is first edition? --> |first= Guilford L. |last= Spencer |pages= 62–63 |edition= third |url= https://books.google.com/books?id=_6w5AQAAIAAJ }}</ref>  [[Barfoed's test]] is also related and similar to Fehling's test (eponymous for [[Christen Thomsen Barfoed]] (1815–1889)).

==Use of the reagent==
Fehling's solution can be used to distinguish [[aldehyde]] vs [[ketone]] functional groups.  The compound to be tested is added to the Fehling's solution and the mixture is heated. Aldehydes are oxidized, giving a positive result, but ketones do not react, unless they are α-hydroxy ketones. The bistartratocuprate(II) complex oxidizes the aldehyde to a [[carboxylate]] anion, and in the process the copper(II) ions of the complex are reduced to copper(I) ions. Red copper(I) oxide then precipitates out of the reaction mixture, which indicates a positive result i.e. that [[redox]] has taken place (this is the same positive result as with [[Benedict's reagent|Benedict's solution]]).

Fehling's test can be used as a generic test for [[monosaccharides]] and other reducing sugars (e.g., maltose). It will give a positive result for [[aldose]] monosaccharides (due to the oxidisable aldehyde group) but also for [[ketose]] monosaccharides, as they are converted to [[aldoses]] by the base in the [[reagent]], and then give a positive result.<ref name="regensburg">{{Cite web |url=http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/D-Fehling-e.htm |title=Fehling's Test for Reducing Sugars<!-- Bot generated title --> |access-date=2008-01-19 |archive-date=2008-01-24 |archive-url=https://web.archive.org/web/20080124171744/http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/D-Fehling-e.htm |url-status=dead }}</ref>

Fehling's can be used to screen for [[glucose]] in [[urine]], thus detecting [[diabetes]]. Another use is in the breakdown of starch to convert it to glucose syrup and [[maltodextrin]]s in order to measure the amount of [[reducing sugar]], thus revealing the [[dextrose equivalent]] (DE) of the [[glucose syrup|starch sugar]].

[[Formic acid]] (HCO<sub>2</sub>H) also gives a positive Fehling's test result, as it does with [[Tollens' reagent|Tollens']] (eponymous for [[Bernhard Tollens|Bernhard Christian Gottfried Tollens]] (1841 – 1918)) test and Benedict's solution also. The positive tests are consistent with it being readily oxidizable to [[carbon dioxide]].

The solution cannot differentiate between benzaldehyde and acetone.

==Net reaction==
The net reaction between an aldehyde and the copper(II) ions in Fehling's solution may be written as:

: <chem>RCHO + 2 Cu^2+ + 5 OH- -> RCOO- + Cu2O + 3 H2O</chem>

or with the [[tartrate]] included:

: <chem>RCHO + 2 Cu(C4H4O6)2^2- + 5 OH- -> RCOO- + Cu2O + 4 C4H4O6^2- + 3 H2O</chem>

== See also ==
*[[Barfoed's test]]

== References ==
{{reflist}}

== External links ==
* {{Cite Collier's|wstitle=Fehling's Solution |short=x}}

{{Analytical reagents}}

{{DEFAULTSORT:Fehling's Solution}}
[[Category:Biochemistry detection methods]]
[[Category:Carbohydrate methods]]
[[Category:Chemical tests]]
[[Category:Coordination complexes]]
[[Category:Copper(II) compounds]]
[[Category:Oxidizing agents]]
[[Category:Analytical reagents]]