Group polyhalide anions

Iodine forms the most extensive group of polyhalide anions, with triiodide, I3, being the best-known example. The formation of this anion is responsible for the increased solubility of I2 in water on addition of potassium iodide (KI). Triiodide is used extensively in analytical chem-isfly, many analytical procedures being based on the release or uptake of iodine and its subsequent titration with sodium thiosulfate (iodometry). The largest polyhalide anion known, Ijg, consists of an alternate arrangement of triiodide anions and neutral iodine molecules. 

---