Fehling Test

Fehling Test

Fehling solution is a generic test for Monosaccharides especially those with their function c₁ hydroxyl intact. The method was developed by Hermann Von Fehling. It is a test method that can distinguish between carbonyl functional group in aldehyde and ketone. Fehling solution is positive for aldehydes but negative for ketones.

Carbohydrates

The major carbohydrates in plants and animals are starch, cellulose, sucrose, lactose, fructose, and glucose. These sugars occur in different proportions and percentages in plants and animals. As plant store there excess carbohydrate as starch, these made starch the most common sugar in plants.

The polysaccharide starch is the storage form of carbohydrates in plants. Starch is made up of glucose as the monomeric units joined together through α-1,4-D-glycosidic bonds.

Other sugars such as Maltose (malt sugar), Sucrose (table sugar) and lactose (milk sugar) are disaccharides, while fructose and glucose are monosaccharides.

Digestion converts the larger carbohydrates to monosaccharide, which can be absorbed into the bloodstream. Glucose, a monosaccharide, is the predominant sugar in human blood. Oxidation of carbohydrates to CO₂ and H₂O in the body produces approximately 4kcal/g. In other words, every gram of carbohydrate we eat yields approximately 4 kcal of energy.

Note that carbohydrate molecules contain a significant amount of oxygen and are already partially oxidized before they enter our bodies. Sugars in food samples exist in three forms which are Monosaccharide (Glucose, Fructose, Galactose, Mannose, etc.), Oligosaccharide (Sucrose, Lactose, Maltose, etc.) and Polysaccharide (Starch, Cellulose, etc.), but here our focuses are on Reducing Sugars.

 

Principle: Fehling solution is a generic test for Monosaccharides. The method was developed by Hermann Von Fehling. Fehling’s solution can be used to determine whether a carbonyl –containing compound is an aldehyde or a ketone. Addition of equal volume of the Fehling I and II forms the Bidentate ligands giving the Bistartratocuprate(II) complex ion [(C₄H₄O₆)2]^4- (equation below). The bistartratocuprate(II) complex is an oxidizing agent and active reagent in the test.

Aldehyde are oxidized to give positive result on heating but ketones do not react except alpha-hydroxyl ketones. The bistartratocuprate(II) complex oxidizes the aldehyde to carboxylate anion and in the process copper(II) ion of the complex are reduced to copper(I)oxide. Red copper(I)oxide precipitate out of the reaction mixture to indicate a positive result.

 

The Fehling reaction equation

 

The concentration of the resulting sugar in the solution can be determined using the formula

 

Concentration (mg/g):

 

Equivalent weight × vol. of volumetric flask × dilution factor×100

Titre value × weight taken×1000