A test for the presence of starch in which the sample turns blue-black in color when a few drops of potassium iodide solution is placed on the sample.
The reaction is due to the formation of polyiodide chains from the reaction of starch and iodine. The amylose in starch forms helices where iodine molecules assemble, forming a dark blue or black color. When starch is broken down or hydrolyzed into smaller carbohydrate units, the blue-black color is not produced. Therefore, this test can also indicate completion of hydrolysis when a color change does not occur.
Also called as: Starch-iodine test.
See also: starch, hydrolysis.