May 20, 2024

Qualitative analysis of carbohydrates: Glucose (Unknown Sample)

Qualitative analysis of carbohydrates: Glucose (Unknown Sample)

1Action of heat: Heat a small amount of substance in a dry test tube.  A     black      residue     is formed Presence of carbohydrates
2Action of conc.H2SO4 Take a small amount of sample solution in a dry test tube and add 2ml of conc. H2SO4  The solution becomes black in color  Presence of carbohydrates
3Molisch’s test: To 2 ml of given sample solution and add 5 drops of Molisch’s reagent. Then slowly add 2ml of conc. H2SO4 along the sides of the test tube.  Violet ring is formed at the junction of two layers  Presence  of carbohydrates
4Benedict’s test: To 5ml of Benedict’s reagent, add 8drops of given sample. Boil over a flame for 2 minutes or place in a boiling water bath for three minutes and allow to cool.  An appearance of green, yellow, orange or brick red precipitate.  Presence of reducing sugar like glucose, fructose, lactose and maltose.
5Fehling’s test: To the given sample solution, add fehling’s A & B solution. Boil for 15 minutes on the water bath.  Brick red colour precipitate is formed  Presence     of     reducing sugars
6Tollen’s Mirror test: To 2ml of tollen’s reagent add 2ml of sugar solution, mix well and boil for 2 minutes  Silver mirror is formed  Presence     of     reducing sugars
7Barfoed’s test: To 2ml of Barfoed’s reagent, add 1ml of sample solution and heat in boiling water bath for 2 minutes (or) boil directly for 30 seconds. Cool under running tap water.Red precipitate is formed in 1 minute  Presence                        of monosaccharide
8Seliwanoff’s test: To 2.5ml of seliwanoff’s reagent add 5 drops of sugar solution and heat the mixture to boil for 30 seconds and cool.  No cherry red colour is formed.  Presence of glucose
9Foulger’s test: Take 3ml of foulger’s reagent add 5 drops of sugar solution boil and leave the test tube in the rack  Green colour is formed  Presence of glucose
10Osazone test: Take 10ml of sample solution in a dry test tube. Add one spatula full of phenyl hydrazine and two spatulas full of sodium acetate and 1ml of glacial acetic acid. Mix well and filtered in a test tube. Filtrate is kept in boiling water bath for 30 minutes. And examine the shape of the crystal under the microscope.Dark yellow broomstick or needle-like appearancePresence of glucose and fructose.


The given unknown sample was found to be Glucose

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