Qualitative Analysis of Carbohydrates – Practical Notes


🔹 Introduction


Carbohydrates are one of the most important biomolecules in our body. In biochemistry lab, qualitative tests are performed to detect the presence, type, and properties of carbohydrates. These tests are simple but very important for medical, paramedical, and lab science students.


🔹 Principle of Qualitative Tests


Carbohydrates contain aldehyde (-CHO) or ketone (-C=O) groups that react with specific reagents to give color changes or precipitates, helping us to identify different types of sugars.

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🔹 Common Qualitative Tests for Carbohydrates


1. Molisch’s Test (General test for carbohydrates)
Reagent: α-naphthol + conc. H₂SO₄
Observation: Violet/purple ring at the junction
Inference: Presence of carbohydrates


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2. Benedict’s Test (Test for reducing sugars)
Reagent: Benedict’s solution (alkaline Cu²⁺ solution)
Observation: Green → Yellow → Orange → Brick-red precipitate (depending on amount of sugar)
Inference: Presence of reducing sugars like glucose, fructose, maltose


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3. Fehling’s Test (Reducing sugars)
Reagent: Fehling’s A (CuSO₄) + Fehling’s B (alkaline tartrate)
Observation: Brick-red precipitate
Inference: Reducing sugars present


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4. Iodine Test (Polysaccharides – starch)
Reagent: Iodine solution
Observation: Blue-black color with starch (disappears on heating, reappears on cooling)
Inference: Presence of starch or related polysaccharides


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5. Barfoed’s Test (Monosaccharides vs. disaccharides)
Reagent: Barfoed’s reagent (copper acetate in acid medium)
Observation: Red precipitate in 2–3 min (monosaccharides), delayed (disaccharides)
Inference: Differentiates monosaccharides from disaccharides


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6. Seliwanoff’s Test (Ketoses vs. aldoses)
Reagent: Resorcinol + conc. HCl
Observation: Cherry-red color with ketoses (fructose)
Inference: Distinguishes ketoses from aldoses



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🔹 Conclusion
These tests are fundamental in biochemistry practicals for detecting and classifying carbohydrates. They help in understanding the biochemical basis of metabolism, clinical diagnosis (like diabetes), and laboratory applications.

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