Iodine I

Discovery

In 1811 Bernard Courtois, a Paris manufacturer of saltpetre, was processing seaweed ash to make the nitrate needed for gunpowder, which was in great demand during the Napoleonic Wars.

When he added concentrated sulfuric acid to a solution of the ash, a violet vapour rose and condensed into dark crystals on cold surfaces. Courtois shared samples with other chemists, and in 1813 Joseph Louis Gay-Lussac and, independently, the English chemist Humphry Davy confirmed that it was a new element.

In the 1920s Switzerland and the United States began adding iodine to table salt, and goitre caused by iodine deficiency declined dramatically. Salt iodisation is now regarded as one of the most successful public health measures in history.

Origin of the name

The name comes from the Greek iodes, "violet", describing the colour of its vapour. The symbol is I.

Main uses

• Nutrition: iodised salt and supplements prevent thyroid disorders caused by deficiency.
• Disinfection: povidone-iodine is widely used to disinfect skin before surgery and to clean wounds.
• Medical imaging: iodine-containing contrast agents absorb X-rays strongly, making blood vessels and organs visible in CT scans and angiography.
• Displays: the polarising films in LCD screens are made from PVA film stained with iodine.
• Chemical industry: iodine compounds act as promoters in catalytic processes for making acetic acid.
• Radiation emergencies: potassium iodide tablets can block the thyroid from absorbing radioactive iodine after a nuclear accident, but they should be taken only when authorities advise.

Isotopes

Iodine has only one stable isotope, iodine-127. Radioactive iodine-131, with a half-life of about eight days, is used to treat overactive thyroid and thyroid cancer, but after nuclear accidents it is a major hazard because it concentrates in the thyroid. Iodine-123 (half-life about 13 hours) is used in diagnostic imaging, and iodine-125 (about 60 days) in brachytherapy seeds. Iodine-129 has a half-life of about 15.7 million years.

In everyday life

Most iodine comes from nitrate deposits in Chile and from natural-gas brines in Japan. Seawater iodine is concentrated by seaweeds, so kelp and similar seaweeds are extremely rich in it; in countries with high seaweed consumption, such as Korea, excess intake can be more of a concern than deficiency.

Adding iodine solution to starch produces a deep blue-black colour, a classic school experiment. Solid iodine and its vapour irritate the skin and eyes, so they should not be touched or inhaled.