How does iodine detect starch

Plants make compounds called carbohydrates which have a wide variety of uses including foods and fuels. All carbohydrates contain the elements carbon, hydrogen and oxygen. Two of the most common carbohydrates are glucose and starch. Glucose C 6 H 12 O 6 is a simple sugar unit. From the formula, you can see that it contains twice as many hydrogen atoms as carbon atoms.

Starch amylopectin does not give the color, nor does cellulose, nor do disaccharides such as sucrose in sugar. Iodine Test: When following the changes in some inorganic oxidation reduction reactions, iodine may be used as an indicator to follow the changes of iodide ion and iodine element. Soluble starch solution is added. Only iodine element in the presence of iodide ion will give the characteristic blue black color.

Neither iodine element alone nor iodide ions alone will give the color result. The reaction between amylose even though it is often present in lesser amounts and iodine is said to account for the intense color change seen. Many details of the reaction of iodine with starch are unknown, but one explanation is that when a solution of diluted iodine is added to starch, an intensely colored starch-iodine complex forms. Amylose molecules consist of single, mostly unbranched chains of glucose molecules, shaped like a spring.

It is speculated that the iodine in the form of I5- ions gets stuck in the coils of the beta amylose molecules soluble starch. The starch forces the iodine into a linear arrangement in the middle groove of the amylose coil. There is some transfer of charge between the starch and the iodine. This changes the electron arrangements and hence the spacings between energy levels. The new spacings absorb visible light selectively and give the complex its intense blue color.

Note: The effect is only seen when both iodine as an element and iodide as an ion are present. Iodine is not very soluble in water and the addition of iodide makes it soluble. Iodine, together with the iodide ion, forms a complex which dissolves in water, unlike iodine on its own. Molecular iodine I2 reacts with iodide I- which is a negatively charged ion and creates an anion I The anion dissolves easily in water which is polar.

There is still some dispute about the exact mechanism involved in producing the unmistakable color change, but this charge transfer process is widely accepted as the most likely.

The material is crystalline, and therefore, the team was able to determine its structure using X-ray crystallography. They found nearly linear polyiodide chains in-between stacks of pyrroloperylene. It turned out that the material containing these chains absorbs light at very similar wavelengths to the amylose-iodine complex, which supports the hypothesis that similar polymeric chains form in the iodine test for starch. DOI: Please note that to comment on an article you must be registered and logged in.

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