The triiodide ion, represented as I3^-, is a polyatomic ion composed of three iodine atoms. It is formed when iodine interacts with iodide ions, resulting in a stable ion that has a negative charge. The triiodide ion is commonly found in solutions, particularly in the presence of potassium iodide, where it appears as a deep brown color. Triiodide is often used in various applications, including chemical analysis and medical imaging. Its unique properties make it useful as a reagent in laboratories and as a contrast agent in radiology to enhance the visibility of certain tissues during imaging procedures.