Why does starch turn blue when iodine wine? Why does iodine turn blue when it encounters starch?

Starch is a kind of polysaccharide (C6H10O5), the current high school textbook Volume 3 points out: "starch and iodine interaction is blue", and the textbook "Organic Chemistry Experiment" of colleges and universities also points out: "starch can still give positive results of iodine test in parts per million".

Therefore, in the production and testing of the biochemical field, the mixture composed of starch and potassium iodide is widely used as an indicator for the titration of redox reactions, or iodine is used to confirm the existence of starch. However, in practice, it is common for starch liquid (including commercially available soluble starch) to appear in color and then fade immediately after encountering a certain amount of iodine. Moreover, when a large amount of iodine is added, the color is not necessarily blue, and sometimes there will be purple blue, purple, purple red, ochre blue, and even ochre.

What is the reason for this?

First: starch can be divided into amylose and amylopectin from the molecular structure. Commercially available soluble starch is generally extracted in the water-soluble part between 55 and 65.

It is well known that amylose turns blue when exposed to iodine, while amylopectin turns purple to purplish red when exposed to iodine. However, the "amylose" extracted from 55 -65 can only be said to be the part of starch that can be dissolved in water at this temperature, that is, the part of starch with smaller molecular weight.

Second: natural starch must contain oil, and different plants have different levels of oil in their starch. We know that oils and fats have an iodine value (i.e., they can react with iodine to fade), and iodine dissolves in oils, which will make the liquid appear red to orange-red.

Third: the aldehyde group contained in the monosaccharide at the end of the starch molecule can also be oxidized by iodine water to fade the iodine water. In view of the above problems, after repeated experimental exploration, the author found that the following methods can be used to treat starch, so as to reduce the experimental errors of discoloration and inconsistency with textbooks, as well as color fading.

Commercially available soluble starch (55 -65) can be used, and it is best to soak the soluble starch extracted at about 55 for half an hour with CCL4 at 40-65 after drying, filter, and wash with CCL4 for multiple times for degreasing treatment.

Second, after drying the above-mentioned extracted starch, it is soaked in 3% H2O2 for more than 20 minutes, filtered, and repeatedly washed with distilled water to remove the aldehyde group of the monosaccharide at the end of the starch molecule. The amount of iodine water or ki should be appropriate, not too concentrated or too large, so as not to cause color interference caused by excessive iodine. After the above-mentioned treated starch, the blue color produced can be more accurate during the test.

The fading of the color can not be observed, and the starch potassium iodide solution prepared with the treated starch can also be used as an indicator, which can also avoid the introduction error of the indicator and make the test results more accurate. What I would like to emphasize is that in the biochemical experiments in middle school, when iodine is used to test the starch contained in the roots, stems, leaves, and fruits of untreated plants, the phenomena obtained are purple-blue, purple-red, purple-red, ochre-blue, and even ochre, etc., which are all accurate objective phenomena.

Starch is a white powder that consists of amylose and amylopectin. Among them, amylose, which is dissolved in water, is in a curved form and is coiled into a helix shape by intramolecular hydrogen bonds. When iodine is added, the iodine molecule burrows into the void in the spiral and is linked to amylose with the help of van der Waals forces to form an inclusion complex.

Because when the iodine molecule enters the glucose molecule, the intermolecular force (valence bond force) binds the two very closely, and this structure causes the light to scatter (for physical reasons) when it passes through the molecule, turning blue.

It's not that complicated, starch is a sugar, and a substance in it reacts with iodine to form a substance with blue properties....That's all....Heh....

Starch can adsorb iodine, so that the wavelength of visible light absorbed by iodine shifts to a short wavelength, and the brown iodine solution turns blue. At the same time, iodine and starch also form packets and substances.

The color of the cladding complex produced by starch and iodine is related to the degree of polymerization or relative molecular weight of starch. In a certain range of polymerization or relative molecular mass, with the increase of polymerization degree or relative molecular weight, the color of the inclusion complex changes from colorless, orange, light red, purple to blue.

Starch will change color when it encounters iodine wine, and it is the starch that reacts with iodine wine to form a kind of inclusion complex, and the iodine molecule is encapsulated in the helical structure of the starch molecule, and this new substance changes the color of the light absorption performance.

The iodine in iodine wine has strong oxidizing properties, while vitamin C has reducing properties, and the two can undergo a redox reaction, turning iodine into a colorless iodide.

Amylose is blue when exposed to iodine, amylopectin is purple-red when exposed to iodine, and dextrin is blue-violet, purple, orange and other colors when exposed to iodine. These chromogenic reactions are highly sensitive and can be used as a quantitative and qualitative method to identify starch, as well as to analyze iodine content. It is used in the textile industry to measure the completeness of cloth desizing.

Amylose, dissolved in water, is in a curved form and is coiled into a helix shape by intramolecular hydrogen bonds. Iodine is added, in which the iodine molecule burrows into the void in the spiral and is linked to amylose with the help of van der Waals to form a complex. This complex can absorb visible light (wavelength range 400-750 sodium meters) except blue light relatively evenly, so that the starch becomes dark blue.

Starch is a plant polysaccharide that is formed by the dehydration and condensation of hundreds to thousands of glucose monomers. It is usually made up of two parts, amylose and amylopectin. Amylose is soluble in hot water and has a smaller molecular weight than amylopectin; Amylopectin is insoluble in cold water, and will form a paste when it reacts with hot water, and its molecular weight is larger than that of amylose.

The reason why starch and iodine will have a color reaction is due to the fact that iodine molecules enter the spiral circle of starch and form starch-iodine complexes. As for what color it appears, it is related to the length of the starch sugar chain. When the chain length is less than 6 glucosyls, it will not be colored; When the average length of the chain is 20 glucosyls, it will be red; When it is greater than 60 glucosyl groups, it is blue.

The principle of iodine turning blue when it meets starch is that the essence of the reaction between starch and iodine is to form a kind of inclusion complex, which makes the color blue.