Lab colour reactions of carbohydrates.pptx

colour reactions of carbohydrates

1. Colour reactions of Carbohydrates Dhanya Hegde(200971) & Kratika Rathi 1 st MSc MBT Manipal School of Life Sciences

2. CARBOHYDRATES Carbohydrates are Optically active polyhydroxy aldehydes or ketones or the compounds which produce units of such type on hydrolysis. 2) Cx (H2O)y –General Formula 3)Classification of Carbohydrates based on number of sugar units.

3. TESTS FOR THE QUALITATIVE ANALYSIS OF CARBOHYDRATES MOLISCH’S TEST IODINE TEST BENEDICT’S TEST BARFOED’S TEST SELWINOFF’S TEST

4. MOLISCH’S TEST USE: To test the presence of carbohydrate in the given sample. PRINCIPLE: The carbohydrate undergoes dehydration upon the introduction of concentrated hydrochloric acid or sulphuric acid, resulting in the formation of an aldehyde. This aldehyde undergo condensation with α- napthol present in the reagent, resulting in the formation of a purple-coloured complex.

5. PROCEDURE: To 1ml of test sample add 2-3 drops of α- napthol . Then add 2ml of conc. Sulphuric acid along the sides of the test tube. RESULT: The formation of a purple ring at the junction of two layers indicate a positive test. All carbohydrate give a positive result for this test (except trioses and tetroses )

6. IODINE TEST USE: To test the presence of Starch in the given test sample. PRINCIPLE: The iodine molecules slip into the helical structure of Amylose unit of starch and forms a blue-black solution.

7. PROCEDURE: To 1ml of test sample add 3-4 drops of iodine solution and mix well. Heat the solution to observe the colour disappear and re-appear on cooling. RESULT: The appearance of blue-black solution indicates a positive test. The presence of starch gives a positive result for this test.

8. BENEDICT’S TEST USE: To test the presence of reducing sugars. PRINCIPLE: In the alkaline medium, reducing sugars reduce the cupric ions to cuprous ions. These participates as cuprous oxide changing the colour from blue to red. The sugar solution and Benedict’s reagent in the ratio 1:10(V/V) gives a rough assessment of the concentration ( green-0.5%, yellow-1%, orange-1.5%, brick red ->2%)

9. PROCEDURE: Add 8 drops of sugar solution to 5ml Benedict’s reagent, mix, boil gently for 2 minutes over flame. RESULT: The appearance of Brick-red precipitate indicate a positive result for Benedict’s test. All Reducing sugars( Monosaccharide like Glucose, Fructose and Disaccharides( except Sucrose)) gives a positive result for this test.

10. BARFOED’S TEST USE- To detect the presence of monosaccharides in the given sample. PRINCIPLE- Barfoed reagent, containing copper acetate in dilute acetic acid, reacts faster with monosaccharides due to their strong reducing properties, forming a red precipitate in about 1-2 minutes. Disaccharides take longer (7-8 minutes) as they first need to hydrolyze in the acidic solution before reacting. The distinction in precipitation time helps differentiate between monosaccharides and disaccharides.

11. PROCEDURE- Take 1ml of test sample and add 2ml of barfoed’s reagent.Mix and keep it in water bath. RESULT- Positive Barfoed’s test-development of brick red colour precipitate within 3-5 minutes. Negative Barfoed’s test- absence of red color .

12. Selwinoff’s test USE- To detect the presence of ketohexoses in the given sample. PRINCIPLE- The test reagent dehydrates ketohexoses to form 5-hydroxymethylfurfural. 5-hydroxymethylfurfural further reacts with resorcinol present in the test reagent to produce a red product within two minutes. Aldohexoses react to form the same product, but do so more slowly.

13. PROCEDURE- TO 3ml of selwinoff reagent add 1ml of sample ,mix and boil for 30 seconds. RESULT- Positive Selwinoff’s test: formation of cherry red-colored complex indicates presence of ketohexoses in the sample. Negative Selwinoff’s result: the abense of cherry red colour indicates that the sampe doesn’t have ketoses.