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Iodine Test for Starch Procedure

Iodine test for starch procedure post we briefly summarises about: principle, reagents requirements, procedure, iodine test results, application and limitations of Iodine test.

Iodine Test for Starch Procedure

Iodine test used to detect polysaccharides. This test distinguishes polysaccharides such as amylase, dextrin, and glycogen from the rest of the carbohydrates. Starch and glycogen have favourable iodine test results. Glycogen, starch, and cellulose can all be distinguished using this method.

Principle

Iodine test is based on the fact that polyiodide ions form colored adsorption complex with helical chains of glucose residue of amylase (blue-black), dextrin (black), or glycogen (reddish-brown). Colorless monosaccharides, disaccharides, and branching polysaccharides such as cellulose. Amylopectin is orange-yellow in colour.

Lugol’s iodine, an aqueous solution of elemental iodine and potassium iodide, is the reagent used in the iodine test. Iodine is insoluble in water on its own. When potassium iodine is added, the iodine ion reversibly combines with iodine to generate a triiodide ion, which then reacts with another iodine molecule to form a pentaiodide ion.

The iodide, triiodide, and pentaiodide ions are colourless, whereas the bench iodine solution appears brown. The helix (coil or spring) structure of the glucose chain appears to be the key to this test. Furthermore, the colour produced is determined by the length of the glucose chains. The triiodide and pentaiodide ions formed are linear and slip inside the helix structure.

It is believed that the transfer of charge between the helix and the polyiodide ions results in changes in the spacing of the energy levels, which can absorb visible light, giving the complex its color. The intensity of the color decreases with the increase in temperature and the presence of water-miscible organic compounds like ethanol.

On heating, the blue color amylase-iodine complex dissociates but is formed again on cooling because the helical structure is disrupted; thereby amylose loses its iodine binding capacity and the blue color. The blue color reappears on cooling due to the recovery of iodine binding capacity due to regaining of the helical structure.

Requirements

Reagent

1. Lugol’s iodine: 5% elemental iodine is mixed with 10% potassium iodide to form the Lugol’s iodine.

2. Test sample

Materials

1. Test tubes

2. Test tube stand

Equipment

1. Water bath

Procedure

1. 1 mL of a given sample should be placed in a clean, dry test tube. In a separate tube, control 1 mL of pure water.

2. Add a few drops of iodine solution to both tubes and vortex them together. Examine how the colour appears in the test tubes.

3. In a water bath, heat the test tubes until the colour fades. Remove the test tubes from the oven to cool.

Iodine Test Results

Iodine Test for Starch Procedure

If iodine test results positive, the colour of the solution changes when you add iodine, it means the solution contains polysaccharide. The colour formed is used to determine the polysaccharide’s nature. 

The presence of amylase or starch in the solution is indicated by the appearance of a blue tint. Dextrin is present if a reddish-purple tint emerges. Glycogen is present if a reddish-brown tint emerges.

Applications

iodine test results are quite specific for polysaccharides in solution; cellulose does not pass this test, but cellulose is not found in solution.

Iodine test results is used to determine whether or not a sample contains starch. iodine test results are also used to evaluate the photosynthesis process in plants.

Limitations

When the solution is heated, the colour fades and then reappears when it is cooled. If the colour does not show after cooling, the iodine has evaporated during the heating process.

Further Readings

Reference