Chemical Fact Sheet
Boron
| Chemical Abstract Number (CAS #) | 7440-42-8 |
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| Analytical Methods | 200.7 - 6010 |
| Molecular Formula | B |
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Synopsis
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Boron - (Ar. Buraq, Pers. Burah), B; at. wt. 10.81(5); at. no. 5; m.p. 2075 deg C; b.p. 4000 deg C; sp. gr. of crystals 2.34, of amorphous variety 2.37; valence 3. Boron compounds have been known for thousands of years, but the element was not discovered until 1808 by Sir Humphry Davy and by Gay-Lussac and Thenard. The element is not found free in nature, but occurs as orthoboric acid usually in certain volcanic spring waters and as borates in borac and colemanite. Ulexite, another boron mineral, is interesting as it is nature's own version of "fiber optics." Important sources of boron are the ores rasorite (kernite) and tincal (borax ore). Both of these ores are found in the Mojave Desert. Tincal is the most important source of boron from the Mojave. Extensive borax deposits are also found in Turkey. Boron exists naturally as 19.9% 10B isotope and 80.1% 11B isotope. Eleven isotopes of boron are known. High-purity crystalline boron may be prepared by the vapor phase reduction of boron trichloride or tribromide with hydrogen on electrically heated filaments. The impure, or amorphous, boron, a brownish-black powder, can be obtained by heating the trioxide with magnesium powder. Boron of 99.9999% purity has been produced and is available commercially. Elemental boron has an energy band gap of 1.50 to 1.56 eV, which is higher than that of either silicon or germanium. It has interesting optical characteristics, transmitting portions of the infrared, and is a poor conductor of electricity at room temperature, but a good conductor at high temperature. Amorphous boron is used in pyrotechnic flares to provide a distinctive green color, and in rockets as an igniter. By far the most commercially important boron compound in terms of dollar sales is Na2B4O7 - 5H2O. This pentahydrate is used in very large quantities in the manufacture of insulation fiberglass and sodium perborate bleach. Boric acid is also an important boron compound with major markets in textile fiberglass and in cellulose insulation as a flame retardant. Next in order of importance is borax (Na2B407 - 10H2O) which is used principally in laundry products. Use of borax as a mild antiseptic is minor in terms of dollars and tons. Boron compounds are also extensively used in the manufacture of borosilicate glasses. Other boron compounds show promise in treating arthritis. The isotope boron-10 is used as a control for nuclear reactors, as a shield for nuclear radiation, and in instruments used for detecting neutrons. Boron nitride has remarkable properties and can be used to make a material as hard as diamond. The nitride also behaves like an electrical insulator but conducts heat like a metal. It also has lubricating properties similar to graphite. The hydrides are easily oxidized with considerable energy liberation, and have been studied for use as rocket fuels. Demand is increasing for boron filaments, a high-strength, lightweight material chiefly employed for advanced aerospace structures. Boron is similar to carbon in that it has a capacity to form stable covalently bonded molecular networks. Carboranes, metalloboranes, phosphacarboranes, and other families comprise thousands of compounds. Crystalline boron (99%) costs about $8/g. Amorphous boron costs about $4/g. Elemental boron and the borates are not considered to be toxic, and they do not require special care in handling. However, some of the more exotic boron hydrogen compounds are definitely toxic and do require care. |
| Use | IN NUCLEAR CHEMISTRY AS NEUTRON ABSORBER; IN IGNITRON RECTIFIERS; IN ALLOYS, USUALLY TO HARDEN OTHER METALS (10)BORON: AS SHIELD FOR NUCLEAR RADIATION & IN INSTRUMENTS USED FOR DETECTING NEUTRONS DEOXIDIZER IN NONFERROUS METALLURGY; GRAIN REFINER IN ALUMINUM; IN DELAYED ACTION FUSES; IGNITER IN RADIO TUBES; COATING MATERIAL IN SOLAR BATTERIES CEMENTATION OF IRON; OXYGEN SCAVENGER FOR COPPER AND OTHER METALS; IN SEMICONDUCTORS, BORON-COATED TUNGSTEN WIRES; FOR FIBERS AND FILAMENTS IN COMPOSITES WITH METALS OR CERAMICS; IN HIGH-TEMPERATURE BRAZING ALLOYS AMORPHOUS: IN PYROTECHNIC FLARES TO PROVIDE DISTINCTIVE GREEN COLOR, IN ROCKETS AS IGNITER As a catalyst for olefin polymerization and dehydration of alcohols. As an abrasive it imparts superior durability to cut-off wheels, effective as the major component of an ultra-high-pressure gasketing composition. Used in composite structural materials, in high temperature abrasives in special purpose alloys, and in steel making. |
| Consumption Patterns | Principal uses for boron compounds consumed in the United States in 1988 were estimated to be glass products, 56%; soaps and detergents, 6%; agriculture, 4%; and other, 34% |
| Apparent Color | POLYMORPHIC: ALPHA-RHOMBOHEDRAL FORM, CLEAR RED CRYSTALS; BETA-RHOMBOHEDRAL FORM, BLACK;ALPHA-TETRAGONAL FORM, BLACK, OPAQUE CRYSTALS WITH METALLIC LUSTER; AMORPHOUS FORM, BLACK OR DARK BROWN POWDER; *OTHER CRYSTAL FORMS KNOWN BUT NOT ENTIRELY CHARACTERIZED;YELLOW MONOCLINIC OR BROWN AMORPHOUS POWDER; *FILAMENTS, POWDER, WHISKERS, SINGLE CRYSTALS |
| Boiling Point | 2550 DEG C |
| Melting Point | 2300 DEG C |
| Molecular Weight | 10.81 |
| Density | Amorphous, 2.3 g/cu cm; alpha-rhombohedral, 2.46 g/cu cm; alpha-tetragonal, 2.31 g/cu cm; beta-rhombohedral, 2.35 g/cu cm |
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Chemical and Physical Properties |
ATOMIC NUMBER 5; VALENCE 3; NATURALLY-OCCURRING ISOTOPES: 10, 11; 3 SHORT-LIVED ARTIFICIAL ISOTOPES: 8, 12, 13 ALPHA-RHOMBOHEDRAL FORM, 12 ATOMS/UNIT CELL; BETA-RHOMBOHEDRAL FORM, 105 ATOMS/UNIT CELL; ALPHA-TETRAGONAL FORM, 50 ATOMS/UNIT CELL Amorphous form, heat capacity: 2.858 cal/g-atom/deg C at 25 deg C; Beta-rhombohedral form, heat capacity: 2.650 cal/g-atom/deg C at 25 deg C FEEBLE CONDUCTOR OF ELECTRICITY @ ROOM TEMP, GOOD CONDUCTOR @ HIGH TEMP; SELF-LIMITING REACTION WITH OXYGEN DUE TO FORMATION OF BORIC OXIDE FILM; OXIDE COATING EVAPORATES ABOVE 1000 DEG C; UNAFFECTED BY AQ HYDROCHLORIC AND HYDROFLUORIC ACIDS; CRYSTALS ARE ALMOST AS HARD AS DIAMOND ENERGY BAND GAP OF 1.50-1.56 EV; TRANSMITS PORTIONS OF THE INFRARED HIGH NEUTRON ABSORPTION CAPACITY; AMPHOTERIC; MOHS HARDNESS 9.3 The alpha-rhombohedral structure degrades above 1200 deg C, and at 1500 deg C recrystallization to the beta-rhombohedral form occurs. The amorphous form converts to beta-rhombohedral above approx 1000 deg C, and any type of pure boron ultimately transforms to the beta-rhombohedral form when heated above the melting point and recrystallized. The electrical resistivity of boron, about 1x10+6 at room temperature, varies from 1x10+13 to 10 in the range -200 to 400 deg C. Crystalline boron is relatively unreactive. |
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Environmental Impact |
IN THE "NUISANCE" CATEGORY ARE THE BORON CARBIDE, NITRIDE, AND OXIDE, AND BORON DUST ITSELF. |
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Environmental Fate |
IN THE EARTH'S CRUST ABOUT 0.001% IN FORM OF ITS COMPOUNDS, NEVER AS THE ELEMENT. OCCURS AS ORTHOBORIC ACID USUALLY IN CERTAIN VOLCANIC SPRING WATERS AND AS BORATE IN BORAX AND COLEMANITE. ULEXITE, ANOTHER BORON MINERAL MOST IMPORTANT SOURCE IS THE MINERAL RASORITE, ALSO KNOWN AS KERNITE IN MOJAVE DESERT BORON EXISTS NATURALLY AS 19.78% (10)BORON ISOTOPE & 80.22% (11)BORON ISOTOPE. Concentration in rocks varies from 10 ppm in igneous rocks to 100 ppm in sandstones. The average soil concentration of boron is 10 ppm. Boron is widely distributed in the environment. Borax, kernite, and tourmaline are three of the more commonly mined boron minerals. High levels of boron are most likely to occur in soil derived from marine sediments and arid soils.Boron is found in soil at an average concetration of 10 mg/kg. Boron has been detected in contaminated shrimp by inductively coupled plasma spectroscopy. Total body burden= 20 mg boron(B), content of soft tissues 14 mg B, Kidney 0.9-2.6 mg boron/kg, Liver 0.4-2.3 mg boron/kg, Muscle 0.3-1.0 mg boron/kg, Bone 1.1-3.3 mg boron/kg, Hair 0.2-8.0 mg boron/kg, Brain 0.06 mg boron/kg, Whole blood 0.1-0.4 mg boron/kg. |
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Drinking Water Impact |
BORON IS FOUND IN FRESH WATER AT A CONCENTRATION OF 0.01 UG/G. Boron in ocean waters has concentration of 4.6 mg/l. |
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Disposal |
At the time of review, criteria for land treatment or burial (sanitary landfill) disposal practices are subject to significant revision. Prior to implementing land disposal of waste residue (including waste sludge), consult with environmental regulatory agencies for guidance on acceptable disposal practices. |
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