Qualitative Analysis of Carbohydrates - MeitY OLabs - YouTube

QUALITATIVE ANALYSIS OF CARBOHYDRATES

Carbohydrates are large biological molecules consisting of carbon, hydrogen and oxygen

atoms, having general formula Cm(H2O)n. They are one of the important nutrients present

in our food. During the process of photosynthesis carbohydrates are produced from carbon dioxide

and water in the presence of chlorophyll and sunlight. Glucose, lactose, sucrose and starch

are commonly known carbohydrates. Our aim here is to study some simple tests

of carbohydrates. They are:

Solubility test Molisch's test

Fehling's test Benedict's test

Tollen's test Iodine test

Solubility test:

Materials required:

Glucose, lactose, sucrose, starch, distilled water and

test tubes.

Procedure:

Take a small amount of glucose, lactose, sucrose and starch in four test tubes A, B, C and

D respectively. Now add small volume of distilled water to

glucose, lactose, sucrose and starch. Shake each test tube well.

We can see that glucose, lactose and sucrose are soluble in water whereas starch is insoluble

in water.

Molisch's test:

Aqueous solutions of glucose, lactose, sucrose, suspension of starch, Molisch's reagent, concentrated

Procedure:

Take a small quantity of aqueous solutions of glucose, lactose, sucrose and suspension

of starch in four test tubes A, B, C and D respectively.

Using a dropper add a few drops of Molisch's reagent to the test tubes A, B, C and D.

Take the test tube A and pour a small amount of concentrated sulphuric acid slowly along

the side of the test tube. Similarly pour concentrated sulphuric acid

to the other test tubes B, C and D. Concentrated sulphuric acid dehydrates carbohydrates

to form furfuraldehyde or its derivative which further reacts with a-naphthol present in

the Molisch's reagent to form a coloured product that appears as a purple ring at the interface

between the acid layer and the test layer.

Fehling's solution B, test tubes, droppers and water bath.

Procedure:

Take a small quantity of aqueous solutions of glucose, lactose, sucrose and suspension

of starch in four test tubes A, B, C and D respectively.

Using a dropper add a small quantity of Fehling's solution A into the test tubes A, B, C and

D. Now, using another dropper, add a small quantity

of Fehling's solution B into the test tubes A, B, C and D.

Heat the test tubes in a boiling water bath for sometimes.

The reducing sugars glucose and lactose reduce the Copper (II) ions in the test reagent to

form red precipitate of cuprous oxide whereas no such precipitate is formed by the non-reducing

sugars sucrose and starch.

Benedict's test:

Aqueous solutions of glucose, lactose, sucrose, suspension of starch, Benedict's reagent,

Procedure:

Take a small quantity of aqueous solutions of glucose, lactose, sucrose and suspension

of starch in four test tubes A, B, C and D respectively.

Using a dropper add a small quantity of Benedict's reagent into the test tubes A, B, C and D.

Heat the test tubes in a boiling water bath for sometimes.

The reducing sugars glucose and lactose reduce the Copper (II) ions in the test reagent to

form red precipitate of cuprous oxide whereas no such precipitate is formed by the non-reducing

sugars sucrose and starch.

Tollen's test:

Materials required:

Aqueous solutions of glucose, lactose, sucrose, suspension of starch, Tollen's reagent, test

tubes, dropper and water bath.

Procedure:

Take a small quantity of aqueous solutions of glucose, lactose, sucrose and suspension

of starch in four test tubes A, B, C and D respectively.

Using a dropper add a small quantity of Tollen's reagent into the test tubes A, B, C and D.

Heat the test tubes in a boiling water bath for sometimes.

The reducing sugars glucose and lactose reduce silver ions in the test reagent to elemental

silver, appears as silver mirror on the inner surface of the vessel whereas no such solver

mirror is produced by non-reducing sugars sucrose and starch.

Iodine test:

Materials required:

Aqueous solutions of glucose, lactose, sucrose, suspension of starch, iodine solution, test

tubes and dropper.

Procedure:

Take a small quantity of aqueous solutions of glucose, lactose, sucrose and suspension

of starch in four test tubes A, B, C and D respectively.

Using a dropper add a few drops of iodine solution into the test tubes A, B, C and D.

Iodine reacts with starch to form a blue colored starch/iodine complex whereas no such complex

is formed by the other samples.

Precautions:

Handle the chemicals with care. Heating should be done very carefully.