strach green turn blue after staining in iodine
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Explanation:
Why Does Iodine Turn Starch Blue?
Iodine Test
Using iodine to test for the presence of starch is a common experiment. A solution of iodine (I2) and potassium iodide (KI) in water has a light orange-brown color. If it is added to a sample that contains starch, such as the bread pictured above, the color changes to a deep blue. But how does this color change work?
Starch is a carbohydrate found in plants. It consists of two different types of polysaccharides that are made up of glucose units which are connected in two different ways. One is the linear amylose and the other is the branched amylopectin (pictured below).
Components of starch
Amylose is the compound that is responsible for the blue color. Its chain forms a helix shape, and iodine can be bound inside this helix (pictured below).
Amylose helix
Charge-Transfer Complexes
The colors are caused by so-called charge transfer (CT) complexes. Molecular iodine (I2) is not easily soluble in water, which is why potassium iodide is added. Together, they form polyiodide ions of the type In–, for example, I3–, I5–, or I7–. The negatively charged iodide in these compounds acts as charge donor, the neutral iodine as a charge acceptor. Electrons in such charge-transfer complexes are easy to excite to a higher energy level by light. The light is absorbed in the process and its complementary color is observed by the human eye.
In the case of the aqueous solution of polyiodides, the absorptions of the different species lead to an overall brownish color. Once amylose is added, it forms another CT complex, Here, the amylose acts as a charge donor and the polyiodide as an acceptor. This complex absorbs light of a different wavelength than polyiodide, and the color turns dark blue.
Polyiodide Chains
The exact structure of the polyiodides inside the amyloid helix is not clear. The amylose-iodine complex is amorphous (i.e., it does not form ordered crystals), which has made it difficult to determine its structure. It has been proposed that the species inside the helix are repeated I3– or I5– units.