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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.
It is advised to check the references for photos of reaction results. [1] Reagent testers might show the colour of the desired substance while not showing a different colour for a more dangerous additive. [2] For this reason it is essential to use multiple different tests to show all adulterants.
It is often used in place of Fehling's solution to detect the presence of reducing sugars and other reducing substances. [2] Tests that use this reagent are called Benedict's tests. A positive result of Benedict's test is indicated by a color change from clear blue to brick-red with a precipitate.
Formation of copper(I) oxide is the basis of the Fehling's test and Benedict's test for reducing sugars. These sugars reduce an alkaline solution of a copper(II) salt, giving a bright red precipitate of Cu 2 O. It forms on silver-plated copper parts exposed to moisture when the silver layer is porous or damaged.
The standard method of determining the dextrose equivalent is the Lane-Eynon titration, based on the reduction of copper(II) sulfate in an alkaline tartrate solution, [1] an application of Fehling's test. Examples: A maltodextrin with a DE of 10 would have 10% of the reducing power of dextrose which has a DE of 100.
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Another test relies on reaction of the furfural with phloroglucinol to produce a colored compound with high molar absorptivity. [4] It also gives a positive test with hydrazines, hydrazones, α-hydroxy ketones and 1,2-dicarbonyls. Both Tollens' reagent and Fehling's reagent give positive results with formic acid. [citation needed]