Properties
The triiodide ion is the simplest polyiodide; several higher polyiodides exist. 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 react with starch. Iodide does not react with starch; nor do solutions of iodine in nonpolar solvents. Lugol's iodine contains potassium iodide as well, so that significant amounts of triiodide ion can exist in solution.
Tincture of iodine, although nominally a solution of elemental iodine in ethanol, also contains significant amounts of triiodide, due to its content of both iodide and water.
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