How does iodine stain starch?

Answer 1

Starch is composed of polymers of glucose. Long linear chains are called amylose. Amylopectin is similar but contains a branch point about every 25th glucose or so.

Amylose coils into a helical secondary structure resembling a tube with a hollow core. Certain molecules including fatty acids and iodine can lodge inside the core. The complex of iodine lodged inside the amylose tube produces a characteristic blue-black color. The starch itself is not altered, and the staining can be reversed.

This is a very sensitive method, used industrially in the starch processing industry, to detect starch and starch fragments. Levels down to 10 ppm can be quantitated using a spectrophotometer to measure A620.

However, Iodine is a "metachromatic" stain, meaning it is a single stain that can stain different things different colors. For example: Although Amylose in corn starch stains bluish (actually a sort of blue with a reddish-purple tent), Amylopectin type starches stain a completely different color, sort of a reddish-brown. Tapioca starches stain a more true blue color. Additionally, high amylose type starches stain a shade of pink (when viewed using a polarized microscope, with cross-polarizers in place). Therefore, it is a mistake to think that "all starches" stain a bluish color.

Additionally, varying the concentration of the stain, relative to a given amount of starch, can vary the intensity of the staining result. Less stain produces a lighter shade of the expected color, and vice-versa. And the best Iodine stain is actually a blend of Iodine crystals and Potassium Iodide in water.

CheapIodine.com is a great source for Iodine.

Answer 2

When starch is reacted with iodine in aqueous phase, a colored complex of starch and iodine is formed. Most probable mechanism is believed that iodine present in I5- form interacts with the coils of starch molecules. The starch which is aqueous soluble is Beta amylose that entraps the iodine atoms in central groove of amylose coil and arranges them into a linear arrangement by mutual charge transfer between starch and iodine atoms. This atomic entrapment and charge sharing leads to changes in orientation and spacing in electrons present in the atomic orbitals, thus the resulting complex that absorbs visible light and imparts purple color.

Answer 3

This is something to do. get iodine, and starch and a plastic ziploc bag and a cup. just put starch in the cup and iodine in the ziploc bag. put the bag of iodine in the cup ZIPPED!!now wait like a few minutes and the starch should change color or some thing like that. that's how to find out. not that ive actually done it, my teacher showed us it a few years ago... :)

Answer 4

Iodine moves into starch and turns the starch from its white color to the iodine's brownish color.

Answer 5

yes it can. you do the same thing as if it was iodine.

i used it for one of my experiments, and it works!