Iodine compounds are important in organic chemistry and very useful in medicine. Thirty isotopes are recognized. Only one stable isotope, I-127 is found in nature. Lack of iodine is the cause of goiter. Iodides, and thyroxin which contains iodine, are used internally in medicine, and as a solution of KI and iodine in alcohol is used for external wounds.
Potassium iodide finds use in photography.
The deep blue color with starch solution is characteristic of the free element. CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, Editor in Chief.