How was iodine commercially recovered from seaweed ash?

To recover iodine from seaweed on a commercial scale, the seaweed is first dried and burned. The resulting ash is leached with water, and impurities such as sodium sulfate and sodium chloride are removed through crystallization.1

The remaining solution is concentrated by evaporating the water. This concentrated liquid is then treated with sulfuric acid to decompose any sulfites. Finally, manganese dioxide is added to release the iodine, which is vaporized and purified through sublimation. Alternatively, the addition of cupric sulfate can be used to produce cuprous iodide.1

Historically, the discovery of the element occurred in 1811 when French chemist Bernard Courtois observed a violet vapor while heating seaweed ashes with sulfuric acid as a by-product of saltpeter production.2