Iodine

• 1. Introduction

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  Iodine, the element with atomic number 53, first announced itself as a cloud of violet vapour. A French chemist named Bernard Courtois had poured an excess of sulfuric acid onto the waste left over from processing seaweed ash. The vapour rose, met a cold surface, and settled there as dark crystals. Courtois suspected he was looking at something nobody had named.

  That purple smoke would later turn up in hospital X-ray rooms, in the thyroid gland, in rainmaking clouds, and in the desert rock of Chile. How did a puzzle about corroding copper vessels become a quarrel among the leading chemists of Europe? Why does this element behave so differently from its lighter chemical relatives? And what does the human body do with it?

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• 2. A Disputed Discovery

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  Saltpetre was in great demand in France during the Napoleonic Wars, and Courtois was born into a family of saltpetre manufacturers. Saltpetre is an essential component of gunpowder. Making it from French nitre beds required sodium carbonate, which came from seaweed gathered on the coasts of Normandy and Brittany. Workers burned the seaweed and washed the ashes with water. Courtois had been investigating why the copper vessels in this process corroded. He lacked the funding to chase the strange new substance any further.

  So in 1811 he began handing samples to friends. Charles Bernard Desormes and Nicolas Clement received some to continue the research. The chemist Joseph Louis Gay-Lussac and the physicist Andre-Marie Ampere received portions too. On the 29th of November 1813, Desormes and Clement described the substance to a meeting of the Imperial Institut de France. Courtois himself was not the one who made it public.

  On the 6th of December 1813, Gay-Lussac announced that the material was either an element or a compound of oxygen. He soon concluded it was an element. He proposed the name "iode", from the Ancient Greek word iodes, meaning violet, after the colour of its vapour. The English form became iodine.

  Ampere, meanwhile, had passed some of his sample to the British chemist Humphry Davy. Davy noticed its likeness to chlorine and also judged it an element. He sent a letter dated the 10th of December to the Royal Society, stating that he had identified a new element called iodine. Arguments erupted between Davy and Gay-Lussac over who had been first. Both men ultimately recognised Courtois as the true discoverer.

  The German name for the element, Jod, left its own mark. Early periodic tables often gave iodine the symbol J, and German texts still frequently use J in place of I.

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• 3. The Violet Halogen

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  Pure iodine melts at 114 degrees Celsius into a deep violet liquid, and it boils at 184 degrees Celsius into a violet gas. At ordinary conditions it is a semi-lustrous, non-metallic solid. It is the fourth member of group 17 of the periodic table, below fluorine, chlorine and bromine. Astatine and tennessine are radioactive, which makes iodine the heaviest stable halogen.

  The halogens darken as you move down the group. Fluorine is a very pale yellow, chlorine is greenish-yellow, and bromine is reddish-brown. Iodine finishes the sequence as violet.

  The colour changes with the solvent. In carbon tetrachloride and saturated hydrocarbons, iodine dissolves to a violet solution, the colour of its vapour. In water, alcohols and amines, it turns brown, because the solvent forms charge-transfer complexes with the iodine. Water barely dissolves it, with one gram needing 3450 millilitres at 20 degrees Celsius. Adding potassium iodide helps, because triiodide ions form.

  A classroom demonstration of iodine turning straight from solid to gas has created a misconception. Many people believe it cannot melt at atmospheric pressure. It can, and it does so at 114 degrees Celsius. The demonstration works because iodine is the least volatile stable halogen yet still gives off purple vapour.

  Electronegativity on the Pauling scale tells a similar story. Iodine scores just 2.66, against 3.98 for fluorine, 3.16 for chlorine and 2.96 for bromine. That makes it the weakest oxidising agent of the stable halogens. Its molecule, I2, also has the weakest bond of the group. About 1 percent of gaseous iodine splits into atoms at 575 degrees Celsius, while fluorine, chlorine and bromine need more than 750 degrees Celsius. The I-I bond length of 266.6 picometres is one of the longest single bonds known.

  Solid iodine has one more surprise. Its molecules interact with their next-nearest neighbours, which gives the crystal a shiny look and semiconducting behaviour. The band gap is 1.3 electronvolts. It conducts in the plane of its crystalline layers but acts as an insulator in the perpendicular direction.

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• 4. Isotopes and Fallout

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  Naturally occurring iodine consists of a single stable isotope, iodine-127. Radioactive forms are known from iodine-108 up to iodine-147. The longest-lived is iodine-129, with a half-life of 16.1 million years, and it decays to stable xenon-129. Some iodine-129 formed before the Solar System itself, but it has now decayed away completely. Scientists call it an extinct radionuclide. Its former presence shows up as an excess of xenon-129, and its mobility in the environment has let researchers date very old groundwaters.

  Most iodine-129 on Earth today comes from human nuclear activity. Its abundance rose by 3-8 orders of magnitude once such activity began. A tiny natural supply still forms when cosmic rays strike atmospheric xenon.

  Three shorter-lived isotopes serve medicine because the thyroid gland stores and concentrates iodine. Iodine-123 has a half-life of 13.223 hours and emits gamma radiation, so it suits imaging such as SPECT scans. Iodine-125, at 59.392 days, is used in biological assays, imaging and brachytherapy for prostate cancer, uveal melanomas and brain tumours. Iodine-131, at 8.0249 days, is given in radiation therapy when doctors want to destroy thyroid tissue after it takes up the iodine.

  The same iodine-131 is a hazard when it escapes. It is a common fission product and turns up in high levels in radioactive fallout. People absorb it through contaminated food, and it accumulates in the thyroid. The main risk is radiogenic thyroid cancer later in life, along with non-cancerous growths and thyroiditis.

  The defence is simple chemistry. Taking potassium iodide tablets fills the thyroid with stable iodine-127, leaving no room for the radioactive kind.

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• 5. Compounds and Reactions

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  Iodine heptafluoride is the strangest of the interhalogens, because iodine is the only halogen to form a heptafluoride. This pentagonal bipyramidal compound is an extremely powerful fluorinating agent. It attacks Pyrex glass and sets carbon monoxide on fire.

  Iodine monochloride is a volatile red-brown compound, discovered independently by Gay-Lussac and Davy in 1813-1814. It imitates bromine so closely that Justus von Liebig, after finding bromine, mistook it for iodine monochloride. Iodine monochloride and iodine monobromide are reactive enough to attack platinum and gold.

  Nitrogen triiodide has a short and fragile history. Until 1990 it was known only as an ammonia adduct. The ammonia-free compound can be isolated at minus 196 degrees Celsius, but it decomposes spontaneously at 0 degrees Celsius.

  Iodine pentoxide, a white solid, has been known since Gay-Lussac and Davy formed it in 1813. It oxidises carbon monoxide completely to carbon dioxide at room temperature, so chemists use it to measure carbon monoxide concentration. Silver iodide gives a different handle. It is extremely insoluble in water, and its formation is a common test for iodine.

  Hydrogen iodide shows the opposite extreme. One litre of water dissolves 425 litres of it. Hydroiodic acid forms an azeotrope that boils at 126.7 degrees Celsius, so evaporating water cannot concentrate it past that point.

  The carbon-iodine bond is the weakest of the carbon-halogen bonds. Many organoiodine compounds turn yellow in storage as they release a little elemental iodine. That fragility is also the appeal, since iodide is the best leaving group among the halogens. Organic chemists rely on the bond in Grignard reagents and the Williamson ether synthesis.

  The Finkelstein reaction exploits solubility instead of strength. An alkyl chloride or bromide in acetone is converted to an alkyl iodide by sodium iodide. Sodium iodide dissolves in acetone, but sodium chloride and sodium bromide do not, so they precipitate and drive the reaction forward.

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• 6. Chile, Japan and Brine

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  Iodine makes up only 0.46 parts per million of Earth's crustal rocks, which ranks it 60th in abundance. Bromine reaches 2.5 parts per million, chlorine 126, and fluorine 544. Iodide minerals are rare. Most deposits rich enough to mine are iodate minerals, such as lautarite and dietzeite.

  In Chile, these minerals occur as trace impurities in caliche, a rock whose main product is sodium nitrate. The caliche holds between 0.02 and 1 percent iodine by mass. It was the main source of iodine in the 19th century and remains important today. It replaced kelp, which is no longer economically viable. Extraction reduces sodium iodate to iodide with sodium bisulfite, then reacts that solution with fresh iodate to produce iodine, which is filtered off.

  Brines emerged as a comparable source in the late 20th century. The two largest are the Minami Kanto gas field east of Tokyo and the Anadarko Basin gas field in northwest Oklahoma. The brine rises hotter than 60 degrees Celsius. Workers acidify it with sulfuric acid, oxidise the iodide with chlorine, and blow air through the dilute solution to drive off the iodine. An absorbing tower and sulfur dioxide then capture it.

  These sources make Chile and Japan the largest producers today. The companies Sociedad Quimica y Minera and Cosayach lead iodine mining in Chile.

  Extraction sites sometimes produce a striking sight. Large purple clouds of sublimated iodine have been observed hanging over them.

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• 7. Uses Beyond the Laboratory

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  About half of all iodine produced goes into organoiodine compounds. Another 15 percent stays as the pure element, 15 percent becomes potassium iodide, and 15 percent forms other inorganic compounds.

  X-ray imaging is the top application. Iodine has a high atomic number and electron density, so it absorbs X-rays below 33.3 kiloelectronvolts efficiently. Organoiodine compounds are injected intravenously for angiography and CT scanning. Every water-soluble radiocontrast agent in use today relies on iodine-containing compounds.

  As a biocide, iodine ranks second by weight. In 1873, the French medical researcher Casimir Davaine discovered its antiseptic action. In 1908, the Istrian-born surgeon Antonio Grossich introduced tincture of iodine to rapidly sterilise the skin in the surgical field. Iodophors still work by slowly releasing I2 when applied.

  Iodine also sits inside the screen of many displays. Thin-film-transistor liquid crystal displays need polarising films, and iodine-impregnated polymer films give the highest transmission and degree of polarisation.

  In industry, iodine is a co-catalyst in the Monsanto and Cativa processes for making acetic acid. Hydroiodic acid converts methanol into methyl iodide, and hydrolysis later regenerates the acid. Most acetic acid is made this way.

  Silver iodide is a major ingredient of traditional photographic film. Thousands of kilograms of it are also used every year for cloud seeding, to induce rain. Erythrosine, an organoiodine compound, serves as a food colouring.

  Some uses are quieter. The iodine test for starch is still used to detect counterfeit banknotes printed on starch-containing paper. Because the spectrum of the I2 molecule has tens of thousands of sharp lines between 500 and 700 nanometres, it serves as a wavelength reference.

  One idea has not worked out. A three-step iodine cycle was evaluated for making hydrogen from nuclear power, with a yield of about 38 percent. It is not a competitive way to produce hydrogen.

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• 8. Iodine in the Body

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  The two main thyroid hormones are named for their iodine count: tetraiodothyronine, or T4, carries four atoms and triiodothyronine, or T3, carries three. Iodine accounts for 65 percent of the molecular weight of T4 and 59 percent of T3. These hormones regulate growth. Iodine is the heaviest element commonly needed by living organisms.

  T4 is the major form in the blood. The thyroid releases T4 and T3 in a ratio between 14 to 1 and 20 to 1. Cells convert T4 into T3, which is three to four times more potent. The enzymes that do this contain selenium, so the body needs selenium as well.

  The thyroid concentrates 15 to 20 milligrams of iodine, yet 70 percent of the body's iodine sits in other tissues. These include the mammary glands, eyes, stomach lining, thymus, salivary glands and arteries. During pregnancy, the placenta stores and accumulates iodine too. Cells take it in through the sodium-iodide symporter.

  The United States National Academy of Medicine recommends 150 micrograms a day for adults, 220 for pregnant women and 290 for women who are lactating. The thyroid itself needs 70 micrograms to make its daily hormones. The adult upper limit is 1,100 micrograms. Seafood, seaweed, kelp, dairy products and eggs are natural sources, and iodised salt is fortified with potassium iodate.

  Japan complicates the picture. Estimates once put intake from wakame and kombu between 5,280 and 13,800 micrograms a day. Newer studies suggest 1,000-3,000 micrograms. Japan revised its adult upper limit to 3,000 micrograms a day in 2015. The average person there consumes 5-14 times the usual safe limit from kombu with no evident toxicity.

  When iodine is missing, the thyroid enlarges in search of more of it, causing goitre. Deficiency also brings hypothyroidism, with extreme fatigue, mental slowing, depression and low body temperature. It affects about two billion people and is the leading preventable cause of intellectual disability. The damage occurs mainly when babies or small children lack iodine. Remote inland and mountainous regions are the most exposed. Iodised salt has largely solved the problem in wealthier areas, but deficiency remains serious in poorer ones. Iodine is on the World Health Organization's List of Essential Medicines.

  Too fast a rise can also cause trouble. After salt iodisation programmes began, some people over 40 developed iodine-induced hyperthyroidism, known as the Jod-Basedow phenomenon.

  The United States Occupational Safety and Health Administration limits workplace exposure to 0.1 parts per million over an 8-hour day, and 2 parts per million is immediately dangerous. Reactions to iodinated contrast media range from a mild rash to fatal anaphylaxis. They have created a widely held misconception, even among physicians, that some people are allergic to iodine itself. Povidone-iodine trapped against the skin has caused chemical burns in rare cases.

  One last rule comes from law enforcement. Phosphorus reduces iodine to hydroiodic acid, which can turn ephedrine and pseudoephedrine into methamphetamine. For that reason the United States Drug Enforcement Administration lists iodine as a List I precursor chemical.

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