Iodine ingested iodine and retain it for periods longer than this isotope's radiological half-life of eight days. For this reason, people at risk of exposure to environmental radioactive iodine (iodine-131) in fallout may be instructed to take non-radioactive potassium iodide tablets. The typical adult dose is one 130 mg tablet per 24 hours, supplying 100 mg (100,000 micrograms) of ionic iodine. (The typical daily dose of iodine for normal health is of order 100 micrograms; see "Dietary Intake" below.) Ingestion of this large dose of non-radioactive iodine minimises the uptake of radioactive iodine by the thyroid gland. As an element with
Iodine and iodate anions are often used for quantitative volumetric analysis, for example in iodometry and the iodine clock reaction (in which iodine also serves as a test for starch, forming a dark blue complex), and aqueous alkaline iodine solution is used in the iodoform test for methyl ketones. The iodine test for starch is still used to detect counterfeit banknotes printed on starch-containing paper. The spectra of the iodine molecule, I, consists of (not exclusively) tens of thousands of sharp spectral lines in the wavelength range 500-700 nm. It is therefore a commonly used wavelength reference (secondary standard). By measuring