# Iodine

Iodine is a chemical element with symbol I and atomic number 53. It was discovered by French chemist Bernard Courtois in 1811 whereby Gay-Lussac subsequently suggested the name "iode", from the Greek word ιώδες (iodes) for violet, because of the color of iodine vapor. It is found mainly as the highly water-soluble iodide ion I− in oceans and brine pools. Like the other halogens, free iodine occurs mainly as a diatomic molecule I$_2$, and then only momentarily after being oxidized from iodide by an oxidant, like free oxygen. Iodine and its compounds are primarily used in nutrition, and industrially in the production of acetic acid and certain polymers. It is a part of many X-ray contrast materials used in modern medicine by attachment to organic compounds. A number of iodine radioisotopes, such as $^{131}$I, are also used in medical applications. Iodine has only one stable isotope.

## Quotes

* A French saltpetre manufacturer, Courtois, in 1811 discovered a strange substance in the soda obtained from sea plants; he told his discovery to Clément, who showed the body in question to Davy. Davy soon demonstrated its elementary nature and Gay Lussac, after a complete investigation of iodine, as he called it, and its compounds, succeeded in showing its marked likeness to chlorine.
** Francis Paul Armitage, A History of Chemistry (1906) p.74
* Laurent, in 1837, striving after the real truth of substitution, had pictured the organic molecule as a prism, the angles of which were occupied by carbon atoms, the centres of its edges by hydrogen atoms, or failing these by chlorine, bromine, or iodine atoms.
** Francis Paul Armitage, A History of Chemistry (1906) p.160
* From this time oxygen began to be conventionally spoken of as the "supporter of combustion," and substances which burn in it were called "combustibles." Yet this manner of speaking is purely conventional, and is due to the abundance of oxygen in the atmosphere. Combustion is merely a special case, in which the heat of combination is exceptionally augmented. After the discovery of chlorine, bromine, and iodine, and when bodies were found to burn in the vapours of these substances, as well as in oxygen, the term "supporter of combustion" was extended to every substance capable of forming vapour in which others could burn; but as the same substance may act at one time as a "supporter," and at another time as a "combustible," the distinction has gradually become obsolete.
** James Campbell Brown, A History of Chemistry from the Earliest Times (1920) p.292
* Gay-Lussac was one of the most brilliant of the French chemists, and spent his whole life in research. His principal subjects of investigation [included]... Iodine and its compounds. ...Gay-Lussac's Memoir on Iodine appeared in 1814. This substance had been discovered three years previously by Bernard Courtois. Davy, in respect of its analogy to chlorine, had declared it to be an elementary body. Gay Lussac, doubting this conclusion, made a careful investigation of iodine, and described its peculiar properties and those of its compounds. This substance was shortly afterwards recognised as an element.
** James Campbell Brown, A History of Chemistry from the Earliest Times (1920) pp. 324-327
* In 1812 [Humphry Davy's] Elements of Chemical Philosophy was published, and next year he travelled on the Continent. During this journey, while in Paris, he examined iodine, and declared it to be an element.
** James Campbell Brown, A History of Chemistry from the Earliest Times (1920) p.338
* Berthollet's conclusion that chlorine is oxymuriatic acid was universally accepted until Gay-Lussac and Thénard in 1809 endeavoured to decompose the gas and failed. They concluded that it contained water because it yielded water when passed over litharge. Their researches read to the Institute in 1809 led Davy to investigate muriatic acid (hydrochloric acid) gas, which in 1808 he had shown to be decomposed by potassium, with evolution of hydrogen. In 1810 he proved that chlorine is an element, and that muriatic acid gas is a compound of chlorine and hydrogen. He thus overturned the oxygen-acid theory, and demonstrated that muriates are compounds of metals with chlorine. He pointed to the fact that some acids, such as sulphuretted hydrogen, contain no oxygen, and argued that muriatic acid gas was one of these, chlorine in it taking the place of oxygen. ...The conclusions of Davy were at first doubted, but when iodine and bromine were also discovered, Gay-Lussac and his followers adopted Davy's views. The latter worked out fluorine, and proved that hydrofluoric acid (HF) contains no oxygen. Berzelius also opposed Davy until the discovery of iodine, but embraced the latter's opinion in 1820.
** James Campbell Brown, A History of Chemistry from the Earliest Times (1920) p.340-341
* Never have experiments been conducted with greater care, caution, and exactitude than those of Stas. It is said that the greatest variation between his many individual determinations of the atomic weight of the same element was from .005 to .01. ...Some illustrations may be given of his care in purifying materials. He purified iodine either (1) by dissolving it to saturation in an aqueous solution of potassium iodide, precipitating it with an excess of water, rather less than was sufficient to throw down the full amount possible, washing, distilling with steam, and drying under a bell-glass beside calcium nitrate (the only salt which was found not to affect the purity of iodine); or (2) by acting with solution of ammonia on the iodine to be treated, thus forming nitrogen iodide, or diodamine, as he considered it, decomposing the violently explosive body by heating with excess of water to 60° or 65° Centigrade, and distilling and drying as before. By this latter method he treated some 10 kilogrammes of iodine in lots of 500 grains at a time.
** James Campbell Brown, A History of Chemistry from the Earliest Times (1920) p.384
* Meyer published along with [Paul] Jacobson a "Text book of Organic Chemistry" [Lehrbuch der Organischen Chemie], which was not finished; also, either alone or in conjunction with his pupils, upwards of 300 memoirs and papers... His researches [included]...Di- and tri-bromobenzene, in work on which he gained further evidence in support of his view that in the case of the chlorine, bromine, iodine, and nitroxyl (NO$_2$) derivatives of aromatic amines, obtained by the direct substitution of an element, it is always the hydrogen contiguous to the NH$_2$ group that is replaced.
** James Campbell Brown, A History of Chemistry from the Earliest Times (1920) p.391
* Curious relations, triads and octaves, were discovered to exist among the atomic weights of substances possessing similar physical characters and chemical properties. Chlorine, iodine, and bromine formed such a triad. Adding the atomic weights of chlorine and iodine, 35 and 125, the sum is 160. The mean or half, therefore, is 80, which is the atomic weight of bromine. These three substances are strictly analogous in their modes of combination, their salts are isomorphous, and with the increase in their atomic weight there is a uniform modification of their properties—chlorine is a green gas, bromine a very volatile red liquid, and iodine a slate-coloured solid, volatilised only by heat. Chlorine is the most active chemically, and can expel the others, bromine is next, and iodine is least active. Chloride of silver is easily soluble in ammonia, the bromide soluble with difficulty, and the iodide quite insoluble.
** James Campbell Brown, A History of Chemistry from the Earliest Times (1920) p.397
* In developing his ideas of nuclei which were used as a basis of classification in Gmelin's large Handbook of Chemistry [Handbuch der Chemie], as well as in his own Chemical Method, and in his theory of types, he introduced to science the words "anhydride," "amide," "imide," "amidic acid," and others. His Theory of Nuclei was originally mentioned in his Inaugural Dissertation at the Faculté des Sciences at Paris in 1837, but it must not be forgotten that these doctrines are also associated with the name of Gerhardt, who has a share in the honour of their conception and introduction. The Nucleus Theory amounted to this:—The molecules of organic bodies, he said, are either nuclei, or compounds of nuclei, with other substances placed outside them. These nuclei are groups of carbon atoms united with other elements, and are either (1) fundamental nuclei, consisting of carbon and hydrogen only, or (2) derivative nuclei, derived from the fundamental nuclei by substitution. The bodies substituted are generally chlorine, bromine, iodine, oxygen, or nitrogen, but compound bodies, acting as radicles, may be in like manner substituted for hydrogen and enter into the nucleus... In constructing his nuclei and comparing them to prisms, he abandoned the dualistic system, and viewed the compound as a single unitary whole like a crystal. Although the Nucleus Theory has not survived, it laid the foundation of the theory of types which we still use.
** James Campbell Brown, A History of Chemistry from the Earliest Times (1920) pp.412-413
* Speaking broadly... we understand by the Atomicity of a chemical element its power of combining with other elements in equivalent proportions; by its Valency or quantivalence, the difference between that element and other elements in their respective powers of combining with hydrogen as a standard. Thus... Hydrogen, chlorine, bromine, iodine, and fluorine combine with hydrogen atom for atom and are termed Monovalent elements or Monads.
** James Campbell Brown, A History of Chemistry from the Earliest Times (1920) p.457
* Iodine was discovered by Courtois of Paris in 1812 in the mother-liquors collected in the process of manufacturing the sodium salts from kelp or burnt sea-weeds. The name is derived from a Greek word meaning "violet" in allusion to the colour of the vapour of iodine. Its elementary character was established by Gay-Lussac in 1815.
** James Campbell Brown, A History of Chemistry from the Earliest Times (1920) pp.506-507
* I need only remind you of Davy's great researches: nitrous oxide; electric conduction and decomposition—resulting, on the one hand, in the separation of potassium and sodium, the decomposition of the earths following as a necessary consequence, and on the other in the electro-chemical theory; iodine and chlorine—resulting in the extension and confirmation of the word element, the discovery of the so-called hydrogen acids, and the important modification of the French theory of the constitution of acids; the investigation of gaseous explosion and of flame, and the invention of the safety lamp. These are the contributions to science which stand out more prominently in connection with Davy. But over and above all this is the peculiar manner of his discoveries. He was no patient plodder. He did not elaborate his work in minute detail. He dashed it off in broad masses; but just on that account there has never been anyone to follow up his investigations. Davy's mantle fell on no one, not even on Faraday.
** John Ferguson, Recent Inquiries Into the Early History of Chemistry (1879) pp.xxi-xxii
* Gay Lussac's work is of a different kind. Less broad and striking, it is of the most thorough and comprehensive kind. Though he did not discover potassium, he invented a mode of preparing it in practical quantity. He did not at once grasp the significance of the elemental characters of iodine and chlorine, and was not at once prepared to accept the hydrogen acid theory, but he did at last give in his adherence, and his monograph on iodine and its compounds is a classical research, and exhausted the subject.
** John Ferguson, Recent Inquiries Into the Early History of Chemistry (1879) pp.xxi-xxii
* Lavoisier believed that oxygen was the essential constituent in every acid and gave the element its name on that account. ... If muriatic acid contains oxygen then oxymuriatic acid (chlorine) must contain still more, but all Davy's attempts to obtain oxygen from it were equally fruitless. ...Numerous chemists who admired Lavoisier and valued a logical system more than experimental evidence, preferred for some time longer to make this assumption, and Gay-Lussac and Thénard were particularly hard to convince. Indeed, they supported the muriaticum theory with some very ingenious experiments. As fate would have it, however, they soon themselves furnished the most satisfactory evidence against the theory. In 1813 Gay-Lussac published a famous paper upon iodine, then recently discovered by Courtois, and in the following year another paper on cyanogen which is equally noteworthy. These investigations involved a study of hydriodic and hydrocyanic acids; and the important analogies which connect these with hydrochloric, together with the certainty that there is no oxygen in hydrocyanic acid soon satisfied all that there was no oxygen in any of them.
** Forris Jewett Moore, A History of Chemistry (1918) pp.95-97
* In 1812 and 1813 iodine, discovered by Courtois, a French soapmaker, and investigated by Gay Lussac, was added to the list of acidifiers.
** Francis Preston Venable, A Short History of Chemistry (1909)
* Gay-Lussac (1778 1850)... enriched chemical literature by many excellent investigations, working often in company with Thenard, Humbolt, and Liebig. His most noteworthy work was upon iodine, cyanogen (the first compound radical), the alkaline oxides, the isolation of boron, improved methods for organic analysis, and many similar studies.
** Francis Preston Venable, History of Chemistry (1922) p.67
* Dumas devised an accurate and excellent method for determining the specific gravities, or densities, of gases which could be used at high temperatures, thus enabling him to experiment upon the vapor densities of iodine, phosphorus, sulphur, mercury, etc. His results, instead of confirming, tended rather to disprove the law of volumes. The trouble lay in the complex nature of the molecules experimented upon, but of course this was unknown to Dumas. He finally declared that even in the case of the simple gases like volumes did not contain equal numbers of chemical atoms. ...The atomic weights determined by him with the greatest care were those of silver, potassium, sodium, lithium, lead, chlorine, bromine, iodine, sulphur, and nitrogen.
** Francis Preston Venable, History of Chemistry (1922) pp.75-77
* The first numerical regularities observed between the atomic weights were the triads of Döbereiner. This chemist seems to have observed first that the combining weight of strontium was the arithmetical mean of those of calcium and barium. A like regularity was noted with regard to certain physical properties of these elements and some of their compounds. This led him for a while to question the independent existence of strontium. Several similar triads were discovered among the other elements as lithium, sodium, and potassium; chlorine, bromine, and iodine; sulphur, selenium, and tellurium. He was careful not to let this grouping depend upon the atomic weights alone but insisted that only elements exhibiting decided analogies of properties should be considered together. This idea was taken up by other chemists, notably by Gmelin in his Handbook, and many analogies and groups were sought for. In 1857 [Ernst] Lennsen returned to this grouping, endeavoring to force all the elements into some twenty groups. Then Odling sought to build upon them an elaborate system of the elements which he called the Natural System. Such groupings were often forced, and failures. The science was not far enough advanced to enable one to understand the real meaning of these regularities.
** Francis Preston Venable, History of Chemistry (1922) pp.83-84
* The table of Mendeleeff was changed but little for thirty years. Its anomalies, as the omission of hydrogen and the rejection of the atomic weight as the deciding factor in such cases as cobalt and nickel, tellurium and iodine, etc., were recognized; but greater knowledge was needed before these could be explained or the underlying law grasped.
** Francis Preston Venable, History of Chemistry (1922) p.85-86