Iodine Test for Starch
The Iodine Test for Starch is used to determine the presence of starch in biological materials. At this level, you will be testing for the presence of this complex carbohydrate in foods, or in leaves as part of a photosynthesis experiment. If you study chemistry as well, you should pay attention principle of the test, as it helps explain how starch acts as an indicator for Redox Titrations or in the 'Iodine Clock Reaction'.
The sole reagent required for the test is bench iodine solution (0.1 M potassium triiodide solution).
N.B. The iodine test for starch cannot be performed on very dark liquids which do not permit the obervation of a colour change.
Observations and Interpretation
Recall that starch is a storage molecule found only in plants. Only plants and plant-based foods should test positive for the presence of starch.
Principle of the Iodine Test for Starch
Starch is a polysaccharide consisting of glucose units joined together by glycosidic bonds.The chains formed during the condensation reaction are either linear or highly branched molecules.
Linear - both straight and helical - molecules of starch are referred to as Amylose.
Whereas branched molecules of starch are called Amylopectin.
Natural starches - from plants - consist of a mixture of amylose (10 - 25%) and amylopectin (75-90%).The the structure of the helical amylose is key to the Iodine-starch reaction. A helix is a coil or a spring.
Iodine on its own (small non-polar molecule) is insoluble in water.Therefore Potassium triiodide solution - Iodine dissolved in potassium iodide solution - is used as a reagent in the test.
To be more specific, potassium iodide dissociates, and then the Iodide ion reacts reversibly with the Iodine to yield the the triiodide ion. A further reaction between a triiodide ion and an iodine molecule yields the pentaiodide ion.
Since molecular iodine is always present in solution, the bench iodine solution appears brown; the iodide and triiodide pentaiodide ions are colourless.
The triiodide and pentaiodide ions formed are linear and slip inside the helix of the amylose (form of starch).
The starch-iodide complex is formed as charge - recall electrons are charged particles - is transferred between the starch and iodide ion.The transfer of charge between the starch and the iodide ion changes the spacing between the energy levels/ orbitals.This change results in the starch-iodide complex absorbing light at a different wavelength - than any other species aforementioned - resulting in an intense purple colour; Biologists call this colour blue-black.
Foods which are high in amylose have more intense blue-black colour. As the Beer-Lambert Law is obeyed spectrometric analyses can quantify the quantity of amylose in starches.