See the graphic below, which show four views in turning from a the side to an end view. Amylose in starch is responsible for the formation of a deep blue color in the presence of iodine. The iodine molecule slips inside of the amylose coil.
Iodine - KI Reagent: Iodine is not very soluble in water, therefore the iodine reagent is made by dissolving iodine in water in the presence of potassium iodide. This makes a linear triiodide ion complex with is soluble that slips into the coil of the starch causing an intense blue-black color. Introduction Amylose forms a colloidal dispersion in hot water whereas amylopectin is completely insoluble. Chemical Test for Starch or Iodine Amylose in starch is responsible for the formation of a deep blue color in the presence of iodine.
The ability to degrade starch is used as a criterion for the determination of amylase production by a microbe. Media: Starch agar is a simple nutritive medium with starch added. Composition: Peptic digest of animal tissue 5. Following incubation, flood the surface of the plates with iodine solution with a dropper for 30 seconds. Pour off the excess iodine. Examine for the clear zone around the line of bacterial growth. Expected Results Positive test: A clear zone around the line of growth after addition of iodine solution indicates that the organism has hydrolyzed starch.
Negative test: A blue, purple, or black coloration of the medium depending on the concentration of iodine. Uses It aids in the differentiation of species of genera Corynebacterium , Clostridium , Bacillus , Bacteroides, Fusobacterium, and members of Enterococcus spp. Limitations It is recommended that biochemical, immunological, molecular, or mass spectrometry testing be performed on colonies from pure culture for complete identification. 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. 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.
The colors are caused by so-called charge transfer CT complexes. Molecular iodine I 2 is not easily soluble in water, which is why potassium iodide is added. Together, they form polyiodide ions of the type I n — , for example, I 3 — , I 5 — , or I 7 —.
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.
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