Qualitative tests for carbohydrates involve detecting the presence of specific functional groups or characteristics indicative of different types of carbohydrates. Here are some common qualitative tests for carbohydrates:
1. Benedict’s Test (Reducing Sugars)
Principle: Detects reducing sugars that can reduce copper ions (Cu²⁺) to form a colored precipitate.
Procedure: Mix the sample with Benedict’s reagent and heat it. The presence of reducing sugars is indicated by a color change from blue to green, yellow, orange, or red.
2. Iodine Test (Starch)
Principle: Iodine interacts with the helical structure of starch, forming a blue-black complex.
Procedure: Iodine solution is added to the sample. A color change to blue-black indicates the presence of starch.
3. Fehling’s Test (Reducing Sugars)
Principle: Like Benedict’s test, Fehling’s reagent detects reducing sugars by reducing copper ions.
Procedure: A sample is mixed with Fehling’s reagent and heated. The formation of a red precipitate indicates a positive result.
4. Molisch’s Test (General Carbohydrates)
Principle: In concentrated sulfuric acid, carbohydrates undergo dehydration, forming a complex with α-naphthol.
Procedure: A sample is mixed with α-naphthol and then treated with sulfuric acid. The appearance of a violet ring at the junction of the two liquids indicates the presence of carbohydrates.
5. Barfoed’s Test (Monosaccharides)
Principle: Differentiates between monosaccharides and disaccharides based on the rate of reduction of copper ions.
Procedure: A sample is mixed with Barfoed’s reagent and heated. Monosaccharides give a positive result faster than disaccharides, producing a brick-red precipitate.
6. Seliwanoff’s Test (Ketoses)
Principle: Differentiate between aldoses and ketoses based on the reaction rate with resorcinol in the presence of concentrated hydrochloric acid.
Procedure: A sample is mixed with resorcinol and hydrochloric acid. Ketoses give a positive result faster than aldoses, resulting in a cherry-red color.
7. Osazone Test (Glucose and Mannose)
Principle: Formation of crystalline osazones from the reaction between reducing sugars and phenylhydrazine.
Procedure: A sample is reacted with phenylhydrazine hydrochloride and sodium acetate. Crystalline osazones that form are indicative of the presence of glucose or mannose.
These tests provide qualitative information about the presence or absence of specific types of carbohydrates based on their structural and chemical characteristics.
