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Boron
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Boron is a trace element which influences calcium and magnesium metabolism. Although no recommended dietary allowance (RDA) has been established for boron, the average daily intake is highly variable, having been estimated at between 0.5 and 7 mg per day. Boron is found in most tissues, but is concentrated in the bone, spleen, and thyroid indicating boron’s functions in bone metabolism and suggesting a potential role for boron in hormone metabolism. Boron is found in relatively high levels in foods of plant origin, such as dried fruits, nuts, dark green leafy vegetables, applesauce, grape juice, and cooked dried beans and peas. Meat and fish are poor dietary sources of boron.
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Boron (B) is an essential trace element for plants and its interrelationship with mineral and bone metabolism and endocrine function in humans has been proposed. Relatively little is known about the occurrence of B in the food chain and hence a biomarker which reflects its intake is required. Two studies were carried out to quantify the urinary B concentration of subjects consuming their habitual diet and the effect of supplementation. In addition, the effect of supplementation on plasma lipoprotein cholesterol concentrations and susceptibility to oxidation and plasma steroid hormones were determined. Boron excretion, obtained on two different occasions from 18 healthy male subjects, was found to be in the range 0.35-3.53 mg/day, with no significant difference between the two occasions. Supplementation with 10 mg B/d for 4 wk resulted in 84% of the supplemented dose being recovered in the urine.
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Boron has two naturally-occurring and stable isotopes, 11B (80.1%) and 10B (19.9%). The mass difference results in a wide range of δ11B values in natural waters, ranging from -16 to +59. There are 13 known isotopes of boron, the shortest-lived isotope is 7B which decays through proton emission and alpha decay. It has a half-life of 3.26500x10-22 s. Isotopic fractionation of boron is controlled by the exchange reactions of the boron species B(OH)3 and B(OH)4. Boron isotopes are ... fractionated during mineral crystallization, during H2O phase changes in hydrothermal systems, and during hydrothermal alteration of rock. The latter effect species preferential removal of the 10B(OH)4 ion onto clays results in solutions enriched in 11B(OH)3 may be responsible for the large 11B enrichment in seawater relative to both oceanic crust and continental crust; this difference may act as an isotopic signature.
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Boron and boron compounds have numerous uses in many fields, although elemental boron is employed chiefly in the metal industry. Its extreme reactivity at high temperatures, particularly with oxygen and nitrogen, makes it a suitable metallurgical degasifying agent. It is used to refine the grain of aluminum castings and to facilitate the heat treatment of malleable iron. Boron considerably increases the high-temperature strength characteristics of alloy steels. Elemental boron is used in the atomic reactor and in high-temperature technologies. The physical properties that make boron attractive as a construction material in missile and rocket technology are its low density, extreme hardness, high melting point, and remarkable tensile strength in filament form.
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Boron does not react with water or hydrochloric acid and is unaffected by air at ordinary temperatures. At red heat it combines directly with nitrogen to form boron nitride (BN), and with oxygen to form boron oxide (B2O 3). With metals it forms borides, such as magnesium boride (Mg3 B2). The original sources of boron compounds were the minerals borax and boric acid. More recently, important boron ores have been, in the United States, ulexite (NaCaB5O9· 8H2O), colemanite (Ca2B6O11· 5H 2O), and kernite (Na2B4O7· 4H2 O); and in Germany, boracite (Mg7Cl2B16O 30). Boron ranks about 38th in natural abundance among the elements in the Earth's crust.
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Boron is an essential nutrient for certain organisms, notably vascular plants and diatoms. Cyanobacteria require boron for formation of nitrogen-fixing heterocysts and boron may be beneficial to animals. Boron deficiency in plants produces manifold symptoms: many functions have been postulated. Deficiency symptoms first appear at growing points, within hours in root tips and within minutes or seconds in pollen tube tips, and are characterized by cell wall abnormalities. Boron-deficient tissues are brittle or fragile, while plants grown on high boron levels may have unusually flexible or resilient tissues. Borate forms cyclic diesters with appropriate diols or polyols.
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Boron