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QUALITATIVE ANALYSIS OF CARBOHYDRATES
Carbohydrates, also known as saccharides, include a large group of
polyhydroxyaldehydes or polyhydroxyketones and their derivatives. They are widely distributed
in nature and make up most of the organic structures of all plants as well as being present to
some extent in all animals.
I. Below are common tests/experiments conducted to determine the presence/absence and
types of carbohydrates present in a sample. For each of the tests, it discusses (1) Its
Purpose, (2) The Principle Behind the Test, and (3) Basic Steps and Equipment Used.
1. Iodine Test for Polysaccharides
Purpose:
The purpose of this test is to determine if polysaccharides are present in biological
materials and to distinguish polysaccharide from other carbohydrates. It can also
differentiate different kinds of polysaccharide into glycogen, starch and cellulose.
Principle behind the test:
Polysaccharide is a coiled polymer and in which if Iodine is introduced then it
forms a coordinated complex between these helically coiled polysaccharide chains. It is
based on the absorptive properties of polysaccharides molecules and as absorption going
on, iodine centrally place within the helix of the chain. The color after the test indicates
whether negative if it is yellow-ish brown in color or positive. If blue appears then starch
or amylase is present, if reddish-purple then it is dextrin, and if it is reddish-brown
glycogen is present.
Basic steps and equipment used:
Equipment:
Test tubes
Test tube rack
Dropper or pipette
Solution to be tested
Iodine reagent (0.5 ml Iodine diluted in 5ml distilled water)
Steps:
Place 2 ml solution to be tested in the test tube
Drop 2-3 drops of Iodine reagent in the solution
Observe and record
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2. Seliwanoff’s Test
Purpose:
The test is done to differentiate fructose, a keto sugar, from galactose and glucose.
The test is to detect monosaccharides with a ketone as functional group.
Principle behind the test:
Keto hexoses when treated with hydrochloric acid will form 5-hydroxy methyl
furfural. These furfural derivatives formed with ketonic functional group condense with
resorcinol that will then form a chromogen and results to cherry-red color.
Basic steps and equipment used:
Equipment:
Test tube
Test tube holder
Dropper
Pipette
Stand
Gas lamp
Solution to be tested
Seliwanoff’s reagent is used that contains in 100 ml of water;
50 mg resorcinol
33 ml of concentrated HCL
Steps:
Place 3 ml of Seliwanoff’s reagent in a test tube
Add 1 ml of test solution in the test tube
Hold the test tube on flame and allow to boil for 30 seconds
Allow to cool at room temperature
Observe and record
3. Benedict’s Test
Purpose:
The test is done for reducing sugars. This determines whether or not certain
carbohydrate contains free aldehyde or ketone group.
Principle behind the test:
It is based from the ability of the reducing sugar to undergo oxidation in alkaline
solution. These reducing sugars then undergo tautomerization and form enediols. The
formed enediols then reduce the cupric ions to cuprous form and converted to sugar
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acids. The cuprous ion combines with the OH- ions to form yellow cuprous hydroxide
and then undergoes heating to convert it into red cuprous oxide. Citrate ions from the
reagent release cuprous ions slowly for reduction and it prevents Cu(HO)
2
to be formed
until oxidation-reduction process is done.
Basic steps and equipment used:
Equipment:
Test tube
Test tube holder
Dropper
Pipette
Stand
Gas lamp
Solution to be tested
Benedict’s Qualitative Reagent is used that contains;
Sodium Carbonate (to make solution alkaline)
Copper Sulfate (to provide cupric ions)
Sodium Citrate (to provide citrate ions)
Steps:
Put 5 ml of Benedict’s qualitative reagent in a test tube
Drop 8 drops of given solution in the same test tube
Mix the solutions
Hold the test tube on flame and boil for 2 minutes
Allow the solution to cool
Observe
Look for the precipitates
II. Two (2) other qualitative tests for carbohydrates, and their purpose
1. Molisch Test
Purpose:
Molisch Test is done for confirmation if a carbohydrate is present or absent in the
tested solution. It is highly sensitive but non-specific test for carbohydrates that means all
carbohydrates, monosaccharides, disaccharide, oligosaccharides and polysaccharides, are
detected.
2. Bial’s Test
Purpose:
The test is done in the purpose to determine whether or not compounds contain a
pentose sugar. The Bial’s test will give positive result when pentose sugar is present in
the compound. This is a general test for carbohydrates and is specifically sensitive only
for pentoses.

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QUALITATIVE ANALYSIS OF CARBOHYDRATES Carbohydrates, also known as saccharides, include a large group of polyhydroxyaldehydes or polyhydroxyketones and their derivatives. They are widely distributed in nature and make up most of the organic structures of all plants as well as being present to some extent in all animals. I. Below are common tests/experiments conducted to determine the presence/absence and types of carbohydrates present in a sample. For each of the tests, it discusses (1) Its Purpose, (2) The Principle Behind the Test, and (3) Basic Steps and Equipment Used. 1. Iodine Test for Polysaccharides Purpose: The purpose of this test is to determine if polysaccharides are present in biological materials and to distinguish polysaccharide from other carbohydrates. It can also differentiate different kinds of polysaccharide into glycogen, starch and cellulose. Principle behind the test: Polysaccharide is a coiled polymer and in which if Iodine is introduced then it forms a coordinated complex between these helically coiled polysaccharide chains. It is based on the absorptive properties of polysaccharides molecules and as absorption going on, iodine centrally place within the helix of the chain. The color after the test indicates whether negative if it is yellow-ish brown in color or positive. If blue appears then starch or amylase is present, if reddish-purple then it is dextrin, and if it is reddish-brown glycogen is present. Basic steps and equipment used: Equipment: Test tubes Test tube rack Dropper or pipette Solution to be tested Iodine reagent (0.5 ml Iodine diluted in 5ml distilled water) Steps: Place 2 ml solution to be tested in the test tube Drop 2-3 drops of Iodine reagent in the solution Observe and record 2. Seliwanoff’s Test Purpose: The test is done to differentiate fructose, a keto sugar, from galactose and glucose. The test is to detect monosaccharides with a ketone as functional group. Principle behind the test: Keto hexoses when treated with hydrochloric acid will form 5-hydroxy methyl furfural. These furfural derivatives formed with ketonic functional group condense with resorcinol that will then form a chromogen and results to cherry-red color. Basic steps and equipment used: Equipment: Test tube Test tube holder Dropper Pipette Stand Gas lamp Solution to be tested Seliwanoff’s reagent is used that contains in 100 ml of water; 50 mg resorcinol 33 ml of concentrated HCL Steps: Place 3 ml of Seliwanoff’s reagent in a test tube Add 1 ml of test solution in the test tube Hold the test tube on flame and allow to boil for 30 seconds Allow to cool at room temperature Observe and record 3. Benedict’s Test Purpose: The test is done for reducing sugars. This determines whether or not certain carbohydrate contains free aldehyde or ketone group. Principle behind the test: It is based from the ability of the reducing sugar to undergo oxidation in alkaline solution. These reducing sugars then undergo tautomerization and form enediols. The formed enediols then reduce the cupric ions to cuprous form and converted to sugar acids. The cuprous ion combines with the OH- ions to form yellow cuprous hydroxide and then undergoes heating to convert it into red cuprous oxide. Citrate ions from the reagent release cuprous ions slowly for reduction and it prevents Cu(HO)2 to be formed until oxidation-reduction process is done. Basic steps and equipment used: Equipment: Test tube Test tube holder Dropper Pipette Stand Gas lamp Solution to be tested Benedict’s Qualitative Reagent is used that contains; Sodium Carbonate (to make solution alkaline) Copper Sulfate (to provide cupric ions) Sodium Citrate (to provide citrate ions) Steps: Put 5 ml of Benedict’s qualitative reagent in a test tube Drop 8 drops of given solution in the same test tube Mix the solutions Hold the test tube on flame and boil for 2 minutes Allow the solution to cool Observe Look for the precipitates II. Two (2) other qualitative tests for carbohydrates, and their purpose 1. Molisch Test Purpose: Molisch Test is done for confirmation if a carbohydrate is present or absent in the tested solution. It is highly sensitive but non-specific test for carbohydrates that means all carbohydrates, monosaccharides, disaccharide, oligosaccharides and polysaccharides, are detected. 2. Bial’s Test Purpose: The test is done in the purpose to determine whether or not compounds contain a pentose sugar. The Bial’s test will give positive result when pentose sugar is present in the compound. This is a general test for carbohydrates and is specifically sensitive only for pentoses. Name: Description: ...
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