Discovery of iodine

Iodine was discovered in 1811 by the French chemist Bernard Courtois, who made saltpeter for Napoleon's armies in Paris. He extracted soda from seaweed ash, and when he added too much sulfuric acid to the residue, a violet vapour rose that condensed into dark crystals. Courtois passed the substance to other chemists, and in 1813 Joseph Louis Gay-Lussac and Humphry Davy independently showed that it was a new element related to chlorine. It was named from the Greek ioeides, meaning violet-coloured. Iodine is needed by the thyroid gland, and its deficiency causes goitre. This led to the introduction of iodised table salt in the 1920s, a public health measure that was among the most successful of the twentieth century.

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