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From Greek "Iodes" = violet
resublimed iodine dissolved in alcohol
Minerals; Inorganic; Column Seven
Hahnemann: "Chronic Diseases," 2nd Edition, vol.3, p. 376, 1837, by the provers Hahnemann, Gerdorff, Hartlaub and Trinks, Gross and Schreter,
Description of the substance
Iodine is a nonmetallic, nearly black crystalline solid that is used in medicine, in the synthesis of some organic chemicals, in the manufacture of dyes, in analytical chemistry, and in photography. At room temperature, solid iodine sublimes to a deep violet vapour that is irritating to the eyes, nose, and throat. Iodine dissolves in alcohol and slightly in water to give brown solutions and in carbon tetrachloride and carbon disulfide to give violet solutions. Iodine is never found in nature uncombined. It is sparingly present in seawater as iodide ion, I-, to the extent of approximately 50 milligrams of iodine per ton of seawater. Appreciable quantities are found in a number of natural brines.
Iodine occurs in animals and plants, generally in very small amounts, but very abundantly in seaweeds. Dispersed as a trace element in rocks, iodine is not sufficiently concentrated to form independent minerals. A formerly important source of iodine for commercial preparation is the saltpetre or nitrate deposits of Chile, in which iodine is present as solid iodates, especially calcium iodate, Ca(IO3)2. A French manufacturer of saltpetre from seaweeds, Bernard Courtois, discovered (1811) iodine in water that was used to extract the soluble material from seaweed ash. Since that time, in France and Great Britain and, more recently, in Japan, iodine has been recovered from seaweeds.
Iodine is an essential microconstituent in the human body, which contains an average of 14 mg (0.00049 ounce) of the element, concentrated mostly in the thyroid gland. The thyroid gland secretes iodine-bearing hormones, especially thyroxine, which are essential for maintaining normal metabolism in all the body's cells. Where insufficient iodine is present in the food supplies, as in North America's Great Lakes region, The Netherlands, and most mountainous areas of the world, the main source of iodine is table salt to which potassium iodide or other iodizing chemicals have been added. Iodine deficiency leads to endemic goitre and myxedema. Iodine at high concentration is poisonous and may cause serious damage to skin and tissues. In dilute alcoholic solution (tincture of iodine) or aqueous solution, it has limited use as a topical antiseptic.
In chemical compounds the common oxidation states are -1 (iodides), +5 (iodates), and +7 (periodates). Iodine combines readily with most metals and some nonmetals to form iodides. The iodide ion is a strong reducing agent; that is, it readily gives up one electron. Although the iodide ion is colourless, iodide solutions may acquire a brownish tint as a result of oxidation of iodide to free iodine by atmospheric oxygen. Molecules of elemental iodine, consisting of two atoms (I2), combine with iodides to form polyiodides (typically I2 + I- ® I-3), accounting for the high solubility of iodine in solutions containing soluble iodide. The aqueous solution of hydrogen iodide (HI), known as hydriodic acid, is a strong acid that is used to prepare iodides by reaction with metals or their oxides, hydroxides, and carbonates. Iodine exhibits a +5 oxidation state in the moderately strong iodic acid (HIO3), which can be readily dehydrated to yield the white solid iodine pentoxide (I2O5). Periodates may take a form represented by potassium metaperiodate (KIO4) or silver paraperiodate (Ag5IO6), for example, because the large size of the central iodine atom allows a relatively large number of oxygen atoms to get close enough to form bonds. The only naturally occurring isotope of iodine is stable iodine-127
. An exceptionally useful radioactive isotope is iodine-131, which has a half-life of eight days. It is employed in medicine to monitor thyroid gland functioning, to treat goitre and thyroid cancer, and to locate tumours of the brain and of the liver. It is also used in investigations to trace the course of compounds in metabolism. Several iodine compounds are used as contrast mediums in diagnostic radiology. In aqueous solution even minute amounts of iodine in the presence of starch produce a blue-black colour.
Occurrence and distribution
Because of its violet-coloured vapours, the element was given the name iodine from the Greek word ioeides, "violet coloured." Iodine occurs to a small extent in seawater and is formed in seaweeds, oysters, and cod livers. Sodium iodate (NaIO3) is contained in crude Chile saltpetre (sodium nitrate, NaNO3). The human body contains iodine in the compound thyroxine, which is produced in the thyroid gland. It occurs in small quantities in much animal and vegetable matter.Production and use Iodine is produced commercially either from Chile saltpetre or from iodine-containing brines.
In the former process, the salt is dissolved in hot water and the saltpetre allowed to crystallize on cooling. The mother liquor is used for further extractions until the extracts contain up to nine grams of iodine per litre. Sodium hydrogen sulfite is then added in order to reduce all iodate to iodide, and the solution is nearly neutralized with sodium carbonate. Fresh mother liquor is then added until all iodide is oxidized by the iodate to free iodine, according to the equation: The solid, containing up to 80 percent iodine, is collected, washed with water, and pressed into cheeselike blocks. These are heated to distill off both iodine and water. Natural brines, or brines extracted from oil wells containing up to 150 milligrams of iodine per litre, are found in Java, California, and northern Italy. Impurities, such as clay, sand, and oil, are removed by filtration, and the solution is passed through a stream of sulfur dioxide and then through a number of containers containing bundles of copper wire. The copper(I) iodide that forms is removed by filtration, washed with water, dried, and finely ground. The product is heated with potassium carbonate to give potassium iodide, which is then oxidized to the free element with dichromate and sulfuric acid: In an alternate process, chlorine is used as the oxidizing agent: For a long time, iodine has been recovered on a commercial scale from seaweed. This is dried and burnt; the ash is leached with water; sodium sulfate and sodium chloride are removed by crystallization; and the remaining solution is concentrated by evaporation of water.
The final solution, which contains 30 to 100 grams of iodine per litre, is treated with sulfuric acid in order to decompose any sulfite, and sulfide and manganese dioxide are added to release iodine, which is vapourized and purified by sublimation. Alternatively, addition of cupric sulfate gives cuprous iodide. Iodine is widely used as a disinfectant and antiseptic, frequently in a solution of alcohol and water containing potassium iodide. Several compounds of iodine, such as iodoform (CHI3), also serve as antiseptics.
Because iodine is converted to thyroxine in the thyroid gland, a small amount of iodine is essential for the body. In many places drinking water contains sufficient iodine for this purpose. In the absence of iodine in the water supply, however, goitre is prevalent, and a small quantity of iodine is frequently added to table salt in order to ensure against iodine deficiency. Iodine and its compounds are used extensively in analytical chemistry, many analytical procedures being based on the release or uptake of iodine and its subsequent titration with sodium thiosulfate (iodometry). Unsaturation of fats (that is, the number of double bonds between carbon atoms) is determined by addition of free iodine (iodine number).
Iodine compounds are also employed as catalysts in certain classes of organic reactions. Iodine, silver iodide, and potassium iodide are used in photography. Silver iodide is also used to seed clouds to induce rain. Iodine has been introduced into metallurgical processes for the production of certain transition metals in a high state of purity, among them titanium, zirconium, thorium, chromium, and cobalt. Electronic equipment, such as scintillation counters or neutron detectors, contain single-crystal prisms consisting of alkali metal iodides.