Tests for Carbohydrates

tests for carbohydrates for medical students

1. Tests for Carbohydrates Dr.S.Sethupathy

2. Conc.Sulfuric acid dehydrates carbohydrates into furfural derivatives. Pentoses yield furfurals Hexoses yield 5-Hydroxy methyl furfurals Furfurals condense with α – naphthol to form colored products

3. Molisch test

4. Importance General test for carbohydrates Nucleic acids , Glycoproteins and glycolipids give a positive test due to their carbohydrate content.

5. Benedict’s test Reducing sugars give positive Benedict’s test due to their free aldehyde or keto group. Glucose + 2 Cu ++ + 2 H 2 O Gluconic acid + Cu 2+ O +4H + Cu 2+ O - red precipitate

6. The principle of Benedict's test When reducing sugars are heated in the presence of an alkali, they become powerful reducing compounds known as enediols . Enediols reduce the cupric ions (Cu 2+ ) present in the Benedict's reagent to cuprous ions (Cu + ) which get precipitated as insoluble red Cuprous oxide(Cu 2 O ).

7. Benedict’s reagent Copper sulfate- Cupric ions Sodium carbonate – Alkaline Medium for enediol formation Sodium citrate – prevents precipitation and keeps cupric ions in solution

8. Semiquantitative Reagent – blue color A greenish precipitate indicates about 0.5 g% concentration; yellow precipitate indicates 1 g% concentration ; orange indicates 1.5 g% red indicates 2 g% or higher concentration. Spectrum of light- VI B GYOR

9. Blue,Green,Yellow,Orange,Red

10. Bial’s rest for pentoses HCl in the Bial’s reagent dehydrates pentoses to furfural derivatives Furfural reacts with orcinol and ferric ions in the reagent to form blue color product Procedure: 2 ml solution is treated with 2 ml Bial’s reagent and gently heated Formation of bluish color indicates the presence of pentoses Hexoses give different colors such as brown, yellow , red etc.

11. Bial ’ test – blue color is positive

12. Osazones Reducing sugar solution when heated with phenyl hydrazine , characteristic crystalline yellow colored oazones are formed Needle shaped crystals by glucose, sunflower shaped crystals by maltose and powder puff shaped crystals by lactose are formed. Sugar phenyl hydrazine Osazone

13. Osazones

14. Mucic acid test for galactose The  mucic acid test is used to identify the presence of the sugar  galactose   When galactose solution is heated with concentrated nitric  acid, a dicarboxylic  acid called  mucic acid  or galactaric acid is formed as a white precipitate, which counts as a positive result .

15. Mucic acid test

16. Barfoed’s test Monosaccharides react faster with Barfoed’s reagent than disaccharides and form reddish precipitate within three minutes of heating whereas disaccharides reacts slowly Barfoed's reagent consists of a 0.3 m  solution of neutral copper acetate in 1% acetic acid solution.

17. Barfoed’s test Reagent Positive

18. Iodine test for starch Addition of a few drops of iodine solution to starch, gives a dark blue color .

19. Iodine test – dark blue - positive

20. Thank you