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Tests for Carbohydrates
This document describes several common tests used to identify different types of carbohydrates: - Molisch test is a general test for all carbohydrates that involves using concentrated sulfuric acid and α-naphthol to form colored compounds. - Benedict's test uses copper sulfate and heat to detect reducing sugars, forming a red copper (II) oxide precipitate. - Bial's test detects pentoses using orcinol and ferric ions to form a blue color. - Osazone tests form characteristic crystal shapes when reducing sugars are heated with phenylhydrazine. - Mucic acid test detects galactose by forming a white precipitate of mucic acid with nitric
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Tests for Carbohydrates
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- 3. •Conc.Sulfuric acid dehydrates carbohydratesinto furfural derivatives. •Pentoses yield furfurals •Hexoses yield 5-Hydroxy methyl furfurals •Furfurals condense with α – naphthol to form colored products
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- 6. Importance •General test forcarbohydrates •Nucleic acids , Glycoproteins and glycolipids give a positive test due to their carbohydrate content.
- 7. Benedict’s test • Reducingsugars give positive Benedict’s test due to their free aldehyde or keto group. • Glucose + 2 Cu++ + 2 H2O Gluconic acid + Cu2+O +4H+ • Cu2+O - red precipitate
- 8. The principle ofBenedict'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 (Cu2+) present in the Benedict's reagent to cuprous ions (Cu+) which get precipitated as insoluble red Cuprous oxide(Cu2O).
- 9. Benedict’s reagent •Copper sulfate-Cupric ions •Sodium carbonate – Alkaline Medium for enediol formation •Sodium citrate – prevents precipitation and keeps cupric ions in solution
- 10. 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- VIBGYOR
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- 12. Bial’s rest forpentoses • 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.
- 13. Bial’ test –blue color is positive
- 14. Osazones • Reducing sugarsolution 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
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- 16. Mucic acid testfor 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. •
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- 18. Barfoed’s test • Monosaccharidesreact 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.
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- 20. Iodine test forstarch • Addition of a few drops of iodine solution to starch, gives a dark blue color.
- 21. Iodine test –dark blue - positive
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