Iodine

Iodine is the heaviest of the stable halogens. At standard conditions it is a semi-lustrous, non-metallic solid that melts into a deep violet liquid. Of the stable halogens, iodine is the least volatile, yet the solid can still be seen giving off purple vapour. Iodine is the least abundant of the stable halogens, at only 0.46 parts per million of Earth's crustal rocks. Iodine vapour is very irritating to the eyes, the mucous membranes and the respiratory tract. Only one stable isotope, iodine-127, is found in nature.

Seaweed washed onto the shore was called varec in French, and the word later also referred to the ashes of such seaweed. Those ashes were used as a source of iodine and salts of sodium and potassium. Kelp stores large quantities of iodide, primarily as iodotyrosines, for unknown reasons. Natural sources of iodine include many marine organisms, such as kelp and certain seafood products.

No single chemical compound can really be pinned down as the essence of the ocean. A diverse mix of molecules contributes to the smell of the seashore. Dimethyl sulfide is one of the most common and best-understood components of seashore aroma. The brininess of wild seafood comes in large part from a class of compounds called bromophenols. In low concentrations, these chemicals are described as smelling sealike, fishlike and crablike. Some farmers have tried adding bromophenols to the diets of farm-raised seafood to re-create the flavour of ocean-faring fish, with mixed success. One challenge is striking a balance between a hint of ocean flavour and the overbearing metallic tang of iodine. One organoiodine, 1-iodopentane, was identified at 0.8% in the headspace volatiles of the alga Halopteris filicina from the Adriatic Sea.

Iodine is required for the synthesis of thyroid hormones. It also serves as a catalyst in the industrial production of acetic acid and some polymers. Tincture of iodine is an antiseptic that contains iodine, ethanol and water. It is usually 2% elemental iodine, with potassium iodide or sodium iodide, dissolved in ethanol and water. Povidone-iodine is an antiseptic used for skin disinfection before and after surgery. It is a chemical complex of povidone, hydrogen iodide and elemental iodine. It is sold under a number of brand names including Betadine.

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