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Fehling's solution

 
Dictionary: Feh·ling's solution   ('lĭngz) pronunciation
n.
An aqueous solution of copper sulfate, sodium hydroxide, and potassium sodium tartrate used to test for the presence of sugars and aldehydes in a substance, such as urine.

[After Hermann von Fehling (1812-1885), German chemist.]


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Columbia Encyclopedia: Fehling's solution
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Fehling's solution ('lĭngz), deep-blue, alkaline solution used to test for the presence of aldehydes (e.g., formaldehyde, HCHO) or other compounds that contain the aldehyde functional group, -CHO. The substance to be tested is heated with Fehling's solution; formation of a brick-red precipitate indicates the presence of the aldehyde group. Simple sugars (e.g., glucose) give a positive test, so the solution has been used to test for the presence of glucose in urine, a symptom of diabetes; Benedict's solution, which gives the same test, is now more widely used. Fehling's solution is prepared just before use by mixing equal volumes of two previously prepared solutions, one containing about 70 grams cupric sulfate pentahydrate per liter of solution and the other containing about 350 grams Rochelle salt (potassium sodium tartrate tetrahydrate) and 100 grams sodium hydroxide per liter of solution. The cupric ion (complexed with tartrate ion) is reduced to cuprous ion by the aldehyde (which is oxidized) and precipitates as cuprous oxide (Cu2O); for this reason, sugars that react with Fehling's solution are called reducing sugars.


Veterinary Dictionary: Fehling's solution
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A solution used in clinical pathology as a test for glucose in solutions such as urine. (1) 34.66 g cupric sulfate in water to make 500 ml; (2) 173 g crystallized potassium and sodium tartrate and 50 g sodium hydroxide in water to make 500 ml; mix equal volumes of (1) and (2) at time of use.

Wikipedia: Fehling's solution
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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]

Fehling's solution is always made just prior to the test. It is comprised of equal parts of the following solutions:

Fehling test

Fehling test.png

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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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. 

 
 

 

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Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.  Read more
Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/ Read more
Veterinary Dictionary. Saunders Comprehensive Veterinary Dictionary 3rd Edition. Copyright © 2007 by D.C. Blood, V.P. Studdert and C.C. Gay, Elsevier. All rights reserved.  Read more
Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Fehling's solution" Read more