Qualitative Analysis of Carbohydrates

Our objective

Our objective is to study some simple tests of carbohydrates.

The Theory

What are Carbohydrates?

Carbohydrates are most abundant organic compounds found in living organisms and are composed of carbon, hydrogen and oxygen. Carbohydrates act as the primary source to provide energy for functioning of living organisms. These are called carbohydrates because they can be considered as hydrates of carbon. Most of them have the general formula Cx(H2O)y.

Generally carbohydrates are defined as polyhydroxy aldehydes or polyhydroxy ketones or the compounds which produces such products on hydrolysis. Carbohydrates are called saccharides. Some of them have sweet taste and are called sugars.

How do you classify carbohydrates based on reactivity?

Based on the reactivity with Tollen’s, Benedict’s or Fehling’s reagent, carbohydrates are classified as;

Reducing sugars

Carbohydrates that can reduce Tollen’s, Benedict’s or Fehling’s reagents are called reducing sugars (sugar with free aldehyde or ketone group). All monosaccharides and most of the disaccharides are reducing sugars. Some examples are Maltose and Lactose.

Non-reducing sugars

Carbohydrates that cannot reduce Tollen’s, Benedict’s or Fehling’s reagents are called non-reducing sugars. Sucrose is a non-reducing sugar.

Some Important Tests for the Detection of Carbohydrates

Molisch’s test

Molisch’s reagent is 10% alcoholic solution of α-naphthol. This is a common chemical test to detect the presence of carbohydrates. Carbohydrates undergo dehydration by sulphuric acid to form furfural (furfuraldehyde) that reacts with α-naphthol to form a violet coloured product.

Fehling's test

This is an important test to detect the presence of reducing sugars. Fehling’s solution A is copper sulphate solution and Fehling’s solution B is potassium sodium tartrate. On heating, carbohydrate reduces deep blue solution of copper (II) ions to red precipitate of insoluble copper oxide.

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Benedict's test

Benedict’s test distinguishes reducing sugar from non-reducing sugar. Benedict’s reagent contains blue copper (II) ions (Cu2+, cupric ions) that are reduced to copper (I) ions (Cu+, cuprous ions) by carbohydrates. These ions form precipitate as red coloured cuprous (copper (I) oxide. 

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Tollen’s test

Tollen’s reagent is ammoniacal silver nitrate solution. On reacting with carbohydrate elemental silver is precipitating out of the solution, occasionally onto the inner surface of the reaction vessel. This produces silver mirror on the inner wall of the reaction vessel.

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Iodine test

Iodine test is used to detect the presence of starch. Iodine is not much soluble in water so iodine solution is prepared by dissolving iodine in water in presence of potassium iodide. Iodine dissolved in an aqueous solution of potassium iodide reacts with starch to form a starch/iodine complex which gives characteristics blue black colour to the reaction mixture.

Learning Outcomes

  • Students understand various identification tests for carbohydrates.
  • Students acquire the skill to distinguish reducing and non-reducing sugars.
  • Students acquire skill to perform the experiment in the real lab.

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