<Dd> Structure of the main complex in Fehling's solution . </Dd> <P> Fehling's 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 alpha - 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 solution). A negative result is the absence of the red precipitate; it is important to note that Fehling's will not work with aromatic aldehydes; in this case Tollens' reagent should be used . </P> <P> 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 . </P> <P> 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 maltodextrins in order to measure the amount of reducing sugar, thus revealing the dextrose equivalent (DE) of the starch sugar . </P>

How to test for a monosaccharide in an unknown liquid