Carbohydrate Experiments تجارب الكربوهيدرات
Qualitative Analysis Of Carbohydrates
Carbohydrate Experiments
This reaction is a general for given by all carbohydrates (sugar).Carbohydrates
when treated with concentrated H2SO4 undergo Dehydrates
to give furfural derivatives .this compound condense with α-naphthol (in alcoholic solution) gives red- violet ring.
Procedure:-
1 ml of sugar solution + 3 drops of α-naphthol (Molisch's reagent) And mix +2ml of
concentrated H2SO4 → violet
ring that spreads with shaking
Benedict's Test
Reducing sugars as Monosaccharides and disaccharides –Exception of sucrose
–to gives in medium alkaline intermediate compound (enedioles).This indicates
the sugars of strong reducing agents reduce cupric (Cu2+)ions (Blue
colour) to cuprous (Cu+)ions(Red colour) by heating and oxidize them
to sugar acids
Procedure
Solution of sugar +5 ml of Benedict's Reagent –shake well and heat on
Boiling water both for 2-3min → Red or yellow precipitate. Indicates the presence of reducing sugars
Fehling Test
This is another reduction test to detect the presence of reducing sugar
.principle: as Benedict's test .
Procedure
1 ml of sugar Solution + 2 ml of Fehling Solution (FA+FB)
Shake well +bath water for 3-5min → reddish –brown ppt
Indicates the presence of reducing sugar
Barfoed's test
This is used to distinguish reducing monsaccharides from a reducing
disaccharides by controlling PH and time of heating monosaccharides react very
test whereas reaction with disaccharides is slow Procedure:-
1 ml of sugar solution +2ml of Barfoed's reagent –shake well + water
bath for 5 min + cool under running water →Red ppt (Cu2O) .
Indicates the presence of monosaccharides .
Seliwanoff's test :-
This test is positive for Ketohexose only fructose Ketohexoses on
treatment with hydrochloric acid form 5-hydroxyl methyl furfural which on
condensation with resorcinol gives a cherry red coloured complex .
And used in distinguish between Glucose and Fructose where gives
positive result with Fructose only.
Procedure:-
0.5ml of sugar solution +2 ml of Seliwanoff's solution + water bath for
10 min → cherry –Red
colour .
Iodine test :-
This is used to identify on polysaccharides "Starch"
Procedure:-
1ml of Starch Solution in hot
water +1-2 drops of Iodine
Solution →blue Colour
Osazone test
Reaction of phenyl hydrazine with reducing derivatives of sugars
It is a useful means of preparing crystalline derivatives of sugars
Procedure
Dry 0.5 g of mixture from (( phenyl hydrazine chloride +sodium
Acetate )) + 4 ml of sugar Solution then
shake the Solution bath water 0.5 hour +cold the Solution crystals are formed
.A coverslip preparation is mode on a clean slide and seen under the microscope
The shape of osazone Crystals are
Note
The sugar which different in the structural formula on first and second atoms
(as Glucose ,fructose ,Mannose ) give the some Osazone crystals at reacts with
phenyl hydrazine
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