known since its formation in 1813 by gay-lussac and davy. it is most easily made by the dehydration of iodic acid (hio3), of which it is the anhydride. it will quickly oxidise carbon monoxide completely to carbon dioxide at room temperature, and is thus a useful reagent in determining carbon monoxide concentration. it also oxidises nitrogen oxide, ethylene, and hydrogen sulfide. it reacts with sulfur trioxide and peroxydisulfuryl difluoride (s2o6f2) to form salts of the iodyl cation, +, and is reduced by concentrated sulfuric acids to iodosyl salts involving +. it may be fluorinated by fluorine, bromine trifluoride, sulfur tetrafluoride, or chloryl fluoride, resulting iodine pentafluoride, which also reacts with iodine pentoxide, giving iodine(v) oxyfluoride, iof3. a few other less stable oxides are known, notably i4o9 and i2o4; their structures have not been determined, but reasonable guesses are iiii(ivo3)3 and +− respectively. standard reduction potentials for aqueous i species e°(couple)a(h+) = 1(acid)e°(couple)a(oh−) = 1(base) i2/i−+0.535i2/i−+0.535 hoi/i−+0.987io−/i−+0.48 io−3/i−+0.26 hoi/i2+1.439io−/i2+0.42 io−3/i2+1.195 io−3/hoi+1.134io−3/io−+0.15 io−4/io−3+1.653 h5io6/io−3+1.601h3io2−6/io−3+0.65 more important are the four oxoacids: hypoiodous acid (hio), iodous acid (hio2), iodic acid (hio3), and periodic acid (hio4 or h5io6). when iodine dissolves in aqueous solution, the following reactions occur: i2 + h2o⇌ hio + h+ + i−kac = 2.0 × 10−13 mol2 l−2 i2 + 2 oh−⇌ io− + h2o + i−kalk = 30 mol2 l−2 hypoiodous acid is unstable to disproportionation. the hypoiodite ions thus formed disproportionate immediately to give iodide and iodate: 3 io− ⇌ 2 i− + io−3k = 1020 iodous acid and iodite are even less stable and exist only as a fleeting intermediate in the oxidation of iodide to iodate, if at all. iodates are by far the most important of these compounds, which can be made by oxidising alkali metal iodides with oxygen at 600 °c and high pressure, or by oxidising iodine with chlorates. unlike chlorates, which disproportionate very slowly to form chloride and perchlorate, iodates are stable to disproportionation in both acidic and alkaline solutions. from these, salts of most metals can be obtained. iodic acid is most easily made by oxidation of an aqueous iodine suspension by electrolysis or fuming nitric acid. iodate has the weakest oxidising power of the halates, but reacts the quickest. many periodates are known, including not only the expected tetrahedral io−4, but also square-pyramidal io3−5, octahedral orthoperiodate io5−6, 2−, 4−, and i2o4−9. they are usually made by oxidising alkaline sodium iodate electrochemically (with lead(iv) oxide as the anode) or by chlorine gas: io−3 + 6 oh− → io5−6 + 3 h2o + 2 e− io−3 + 6 oh− + cl2 → io5−6 + 2 cl− + 3 h2o they are thermodymically and kinetically powerful oxidising agents, quickly oxidising mn2+ to mno−4, and cleaving glycols, α-diketones, α-ketols, α-aminoalcohols, and α-diamines. orthoperiodate especially stabilises high oxidation states among metals because of its very high negative charge of −5. orthoperiodic acid, h5io6, is stable, and dehydrates at 100 °c in a vacuum to metaperiodic acid, hio4. attempting to go further does not result in the nonexistent iodine heptoxide (i2o7), but rather iodine pentoxide and oxygen. periodic acid may be protonated by sulfuric acid to give the i(oh)+6 cation, isoelectronic to te(oh)6 and sb(oh)−6, and giving salts with bisulfate and sulfate. polyiodine compounds when iodine dissolves in strong acids, such as fuming sulfuric acid, a bright blue paramagnetic solution including i+2 cations is formed. a solid salt of the diiodine cation may be obtained by oxidising iodine with antimony pentafluoride: 2 i2 + 5 sbf5 so2⟶20 °c 2 i2sb2f11 + sbf3 the salt i2sb2f11 is dark blue, and the blue tantalum analogue i2ta2f11 is also known. whereas the i–i bond length in i2 is 267 pm, that in i+2 is only 256 pm as the missing electron in the latter has been removed from an antibonding orbital, making the bond stronger and hence shorter. in fluorosulfuric acid solution, deep-blue i+2 reversibly dimerises below −60 °c, forming red rectangular diamagnetic i2+4. other polyiodine cations are not as well-characterised, including bent dark-brown or black i+3 and centrosymmetric c2h green or black i+5, known in the asf−6 and alcl−4 salts among others. the only important polyiodide anion in aqueous solution is linear triiodide, i−3. its formation explains why the solubility of iodine in water may be increased by the addition of potassium iodide solution: i2 + i− ⇌ i−3 (keq = ~700 at 20 °c) many other polyiodides may be found when solutions containing iodine and iodide crystallise, such as i−5, i−9, i2−4, and i2−8, whose salts with large, weakly polarising cations such as cs+ may be isolated. organoiodine compounds structure of the oxidising agent 2-iodoxybenzoic acid organoiodine compounds have been fundamental in the development of organic synthesis, such as in the hofmann elimination of amines, the williamson ether synthesis, the wurtz coupling reaction, and in grignard reagents. the carbon–iodine bond is a common functional group that forms part of core organic chemistry; formally, these compounds may be thought of as organic derivatives of the iodide anion. the simplest organoiodine compounds, alkyl iodides, may be synthesised by the reaction of alcohols with phosphorus triiodide; these may then be used in nucleophilic substitution reactions, or for preparing grignard reagents. the c–i bond is the weakest of all the carbon–halogen bonds due to the minuscule difference in electronegativity between carbon (2.55) and iodine (2.66). as such, iodide is the best leaving group among the halogens, to such an extent that many organoiodine compounds turn yellow when stored over time due to decomposition into elemental iodine; as such, they are commonly used in organic synthesis, because of the easy formation and cleavage of the c–i bond. they are also significantly denser than the other organohalogen compounds thanks to the high atomic weight of iodine. a few organic oxidising agents like the iodanes contain iodine in a higher oxidation state than −1, such as 2-iodoxybenzoic acid, a common reagent for the oxidation of alcohols to aldehydes, and iodobenzene dichloride (phicl2), used for the selective chlorination of alkenes and alkynes. one of the more well-known uses of organoiodine compounds is the so-called iodoform test, where iodoform (chi3) is produced by the exhaustive iodination of a methyl ketone (or another compound capable of being oxidised to a methyl ketone), as follows: some drawbacks of using organoiodine compounds as compared to organochlorine or organobromine compounds is the greater expense and toxicity of the iodine derivatives, since iodine is expensive and organoiodine compounds are stronger alkylating agents. for example, iodoacetamide and iodoacetic acid denature proteins by irreversibly alkylating cysteine residues and preventing the reformation of disulfide linkages. halogen exchange to produce iodoalkanes by the finkelstein reaction is slightly complicated by the fact that iodide is a better leaving group than chloride or bromide. the difference is nevertheless small enough that the reaction can be driven to completion by exploiting the differential solubility of halide salts, or by using a large excess of the halide salt. in the classic finkelstein reaction, an alkyl chloride or an alkyl bromide is converted to an alkyl iodide by treatment with a solution of sodium iodide in acetone. sodium iodide is soluble in acetone and sodium chloride and sodium bromide are not. the reaction is driven toward products by mass action due to the precipitation of the insoluble salt. occurrence and production iodine is the least abundant of the stable halogens, comprising only 0.46 parts per million of earth's crustal rocks (compare: fluorine 544 ppm, chlorine 126 ppm, bromine 2.5 ppm). among the 84 elements which occur in significant quantities (elements 1–42, 44–60, 62–83, 90 and 92), it ranks 61st in abundance. iodide minerals are rare, and most deposits that are concentrated enough for economical extraction are iodate minerals instead. examples include lautarite, ca(io3)2, and dietzeite, 7ca(io3)2·8cacro4. these are the minerals that occur as trace impurities in the caliche, found in chile, whose main product is sodium nitrate. in total, they can contain at least 0.02% and at most 1% iodine by mass. sodium iodate is extracted from the caliche and reduced to iodide by sodium bisulfite. this solution is then reacted with freshly extracted iodate, resulting in comproportionation to iodine, which may be filtered off. the caliche was the main source of iodine in the 19th century and continues to be important today, replacing kelp (which is no longer an economically viable source), but in the late 20th century brines emerged as a comparable source. the japanese minami kanto gas field east of tokyo and the american anadarko basin gas field in northwest oklahoma are the two largest such sources. the brine is hotter than 60 °c from the depth of the source. the brine is first purified and acidified using sulfuric acid, then the iodide present is oxidised to iodine with chlorine. an iodine solution is produced, but is dilute and must be concentrated. air is blown into the solution to evaporate the iodine, which is passed into an absorbing tower, where sulfur dioxide reduces the iodine. the hydrogen iodide (hi) is reacted with chlorine to precipitate the iodine. after filtering and purification the iodine is packed. 2 hi + cl2 → i2↑ + 2 hcl i2 + 2 h2o + so2 → 2 hi + h2so4 2 hi + cl2 → i2↓ + 2 hcl these sources ensure that chile and japan are the largest producers of iodine today. alternatively, the brine may be treated with silver nitrate to precipitate out iodine as silver iodide, which is then decomposed by reaction with iron to form metallic silver and a solution of iron(ii) iodide. the iodine may then be liberated by displacement with chlorine. applications about half of all produced iodine goes into various organoiodine compounds, another 15% remains as the pure element, another 15% is used to form potassium iodide, and another 15% for other inorganic iodine compounds. among the major uses of iodine compounds are catalysts, animal feed supplements, stabilisers, dyes, colourants and pigments, pharmaceutical, sanitation (from tincture of iodine), and photography; minor uses include smog inhibition, cloud seeding, and various uses in analytical chemistry. chemical analysis testing a seed for starch with a solution of iodine the iodide and iodate anions are often used for quantitative volumetric analysis, for example in iodometry. iodine and starch form a blue complex, and this reaction is often used to test for either starch or iodine and as an indicator in iodometry. the iodine test for starch is still used to detect counterfeit banknotes printed on starch-containing paper. the iodine value is the mass of iodine in grams that is consumed by 100 grams of a chemical substance typically fats or oils. iodine numbers are often used to determine the amount of unsaturation in fatty acids. this unsaturation is in the form of double bonds, which react with iodine compounds. potassium tetraiodomercurate(ii), k2hgi4, is also known as nessler's reagent. it is often used as a sensitive spot test for ammonia. similarly, mayer's reagent (potassium tetraiodomercurate(ii) solution) is used as a precipitating reagent to test for alkaloids. aqueous alkaline iodine solution is used in the iodoform test for methyl ketones. spectroscopy the spectrum of the iodine molecule, i2, consists of (not exclusively) tens of thousands of sharp spectral lines in the wavelength range 500–700 nm. it is therefore a commonly used wavelength reference (secondary standard). by measuring with a spectroscopic doppler-free technique while focusing on one of these lines, the hyperfine structure of the iodine molecule reveals itself. a line is now resolved such that either 15 components (from even rotational quantum numbers, jeven), or 21 components (from odd rotational quantum numbers, jodd) are measurable. caesium iodide and thallium-doped sodium iodide are used in crystal scintillators for the detection of gamma rays. the efficiency is high and energy dispersive spectroscopy is possible, but the resolution is rather poor. spacecraft propulsion propulsion systems employing iodine as the propellant can be built more compactly, with less mass (and cost), and operate more efficiently than the gridded ion thrusters that were utilized to propel previous spacecraft, such as japan's hayabusa probes, esa's goce satellite, or nasa's dart mission, all of which used xenon as the reaction mass. iodine's atomic weight is only 3.3% less than that of xenon, while its first two ionisation energies average 12% less; together, these make iodine ions a promising substitute. use of iodine should allow more widespread application