Fehling Test And Benedicch Test

Molisch Test is used to detect the presence of carbohydrate and a negative result indicates the absence of carbohydrates. In the presence of concentrated sulphuric acid the carbohydrates are dehydrated forming furfuryl derivates. Naphthol-(1), the Molisch reagent reacts with the cyclic aldehydes to form purple colored condensation products (furfuryl-diphenyl-methane-dyes) (1). A red-violet layer at the interface between the acid (bottom) and aqueous (upper) layers is a positive test for carbohydrates.

Precaution steps in this test is to always remember to use a glass Pasteur pipette to add the concentrated sulphuric acid and do not use a mechanical pipettor with concentrated acids. And always add the concentrated acid to water.

Both Fehling Test and Benedict Test are used to determine reducing sugar but Fehling Test differs from Benedict Test because it contain sodium potassium tartrate in place of sodium citrate and potassium hydroxiden as an alkali while sodium carbonate in Benedict reagent. It is not a preferred test over Benedict Test since the strong alkali present will cause the caramalzation of the sugars and cause it o be less sensitive. For Fehling Test, the presence of aldehydes is detected by reduction of the deep blue solution of copper(II) to a red precipitate of insoluble copper oxide. The test is commonly used for reducing sugars but is known to be NOT specific for aldehydes. A positive test is indicated by a green suspension and a red precipitate. Benedict