tests for carbohydrates
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TESTS FOR CARBOHYDRATESDr.S.Sethupathy
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•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
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Molisch test
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Importance
•General test for carbohydrates•Nucleic acids , Glycoproteins and glycolipids give a positive test due to their carbohydrate content.
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Benedict’s test• Reducing sugars 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
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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 (Cu2+) present in the Benedict's reagent to cuprous ions (Cu+) which get precipitated as insoluble red Cuprous oxide(Cu2O).
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Benedict’s reagent•Copper sulfate- Cupric ions•Sodium carbonate – Alkaline Medium for enediol formation
•Sodium citrate – prevents precipitation and keeps cupric ions in solution
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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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Blue,Green,Yellow,Orange,Red
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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.
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Bial’ test – blue color is positive
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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
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Osazones
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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.
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Mucic acid test
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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.
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Barfoed’s test Reagent Positive
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Iodine test for starch• Addition of a few drops of iodine solution to starch, gives a
dark blue color.
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Iodine test – dark blue - positive
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Thank you