What happens when you add iodine to starch?
color in the presence of iodine. The triiodide ion ion slips into the coil of the starch causing an intense blue-black color. Starch Test: Add Iodine-KI reagent to a solution or directly on a potato or other materials such as bread, crackers, or flour. A blue-black color results if starch is present.
Starch consists of two types of molecules, amylose - a soluble starch and amylopectin. When starch is mixed with iodine in water, an intensely blue colored starch/iodine complex is formed. Many of the details of the reaction are still unknown. Apparently, the Iodine gets stuck in the coils of beta amylose molecules.
- When iodine and starch are dissolved together in water, gooey starch molecules instantly trap iodine ions close together to create a dark, blueish-black color. (Dripping liquid iodine onto a potato will turn it the same color, due to all of the spud's starch.)
- Iodine is strongly reactive, even though it is less extreme for iodine than for other halogens. Iodine cannot be found as an element, but rather as I2 molecules, as I- ions, or as iodate (a salt of iodinic acid with IO3- anion). I2 molecules and water molecules react to substances such as hypoiodite (OI-).
- In solution, it appears yellow in low concentration, and brown at higher concentration. The triiodide ion is responsible for the well-known blue-black color which arises when iodine solutions interact with starch. Iodide does not react with starch; nor do solutions of iodine in nonpolar solvents.
Lugol's Iodine (IKI) test for the presence of starch: Iodine dissolved in an aqueous solution of potassium iodide - reacts with starch producing a deep blue-black color. This reaction is the result of the formation of polyiodide chains from the reaction of starch and iodine.
- Iodine is a known indicator for starch. An indicator is a substance that chances color in the presence of the substance it indicates.
- The Dialysis tubing provides a semi-permeable membrane. Only allowing smaller molecules to pass through it. Iodine molecules are small enough to pass freely through the membrane, however starch molecules are complex and too large to pass through the membrane.
- Lugol's iodine. Lugol's iodine, also known as aqueous iodine and strong iodine solution, is a solution of potassium iodide with iodine in water. It is a medication and disinfectant used for a number of purposes. Lugol's iodine was first made in 1829 by the French physician Jean Lugol.
Updated: 2nd October 2019