Iodine

Bernard Courtois was making saltpetre for Napoleon's gunpowder from seaweed ash in 1811 when he added too much sulfuric acid and a cloud of violet vapour rose from the vat, condensing into dark crystals — iodine, named for the Greek for violet. The thyroid concentrates iodine to build thyroxine, and deficiency causes goitre and, in the developing brain, irreversible impairment; Michigan began iodising salt in 1924 in a state where goitre affected a third of schoolchildren, and the rate fell to near zero within a decade. Universal salt iodisation is one of the cheapest public health interventions ever devised. Iodine is also the reason potassium iodide tablets are stockpiled near reactors: flooding the thyroid with stable iodine blocks uptake of radioactive iodine-131. In the laboratory, triiodide with starch gives the deep blue of the iodine clock.