Iodine

Iodine has a colorful history—literally! It was discovered in 1811 by Bernard Courtois, a French chemist, whose family produced saltpeter, a key ingredient in gunpowder. While prepping one of the ingredients necessary for saltpeter, Courtois accidentally added too much sulfuric acid, causing a cloud of violet vapor to appear! When the vapor cooled, dark crystals formed. Courtois knew he was onto something new, but he didn’t have the resources to keep investigating.

Several scientists began studying the substance, including Charles Desormes, Nicolas Clément, and Joseph Louis Gay-Lussac. Gay-Lussac confirmed that the mysterious violet substance was a new element, calling it "iodine", from the Greek word for violet, iodēs. Around the same time, British chemist Sir Humphry Davy also identified it as a new element. There was some debate over who figured it out first, but everyone agreed that Courtois had been the first to isolate iodine.

Iodine is a bluish-black, lustrous solid, volatizing at ordinary temperatures into a blue-violet gas with an irritating odor; it forms compounds with many elements, but is less active than the other halogens, which displace it from iodides. Iodine exhibits some metallic-like properties. It dissolves readily in chloroform, carbon tetrachloride, or carbon disulfide to form beautiful purple solutions. It is only slightly soluble in water.

Iodine is an essential element for life. It plays an important role in thyroid function, helping to control the body’s growth rate. To make sure people get enough iodine, it is often added to table salt, called iodized salt.

Iodine is also used in sanitation systems and as topical antiseptic to kill germs and bacteria. In its radioactive form, it can help visualize the inside of the body, helping doctors diagnose medical issues and disease. Other applications include cloud seeding, dyes, and photography.

Iodine is fairly rare but is found in both the Earth's crust and in ocean water. There is a higher concentration of iodine in the ocean than in the Earth's crust. Some ocean plants such as seaweed have a high concentration of iodine. It is also found in underground brines (concentrated salt water) near oil and natural gas reserves.

Ultrapure iodine can be obtained from the reaction of potassium iodide with copper sulfate but several other methods of isolating the element are known.