Boron and compounds

Description

Boron is an extremely valuable mineral and it is used in many products from cookware and medicine to nuclear waste storage and space exploration. Boron compounds are mainly used in borosilicate glass products, but are also used in agriculture, in fire retardants, and in soaps and detergents.

Boron is used in special-purpose alloys, in cementation of iron, as oxygen scavenger for copper and other metals, as fibres and filaments in composites with metals or ceramics, as semiconductor, for nuclear reactors, as a shield for nuclear radiation and in instruments used for detecting neutrons. Boron is used in pyrotechnic flares (distinctive green colour), for rockets (as an igniter), in boron-coated tungsten wires and in high temperature brazing alloys.

Applications for specific boron compounds follow.

Borates are used mostly to produce glass. They are also used in fire retardants, leather tanning industries, cosmetics, photographic materials, soaps and cleaners, adhesives and for high-energy fuel. Some pesticides used for cockroach control and some wood preservatives also contain borates.

Borax is used in soldering metals, as a cleansing flux in welding, in the manufacture of glazes and enamels (e.g. for covering steel of refrigerators and washing machines), in tanning, in cleaning compounds, to artificially age wood, as a preservative against wood fungus (either alone or with other antiseptics), and in fireproofing fabrics. It is also used for curing and preserving skins, in cockroach control and as a water softener in washing powders.

Boric acid is used for weatherproofing and fireproofing fabrics, as a preservative, in the manufacture of cements, crockery, porcelain, enamels, glass, borates, leather, carpets, hats, soaps, and artificial gems, and in nickel-plating baths. It is also used in the manufacture of cosmetics, in ointments and eye washes, as a mild antiseptic, in printing and dyeing, in photography, for impregnating wicks, for hardening steel, in welding flux, copper brazing, as an insecticide for cockroaches and carpet beetles, and in fungus control for citrus fruits.

Boron oxide is used in metallurgy, in the analysis of silicon dioxide in silicates, in blowpipe analysis, for the production of boron, in heat-resistant glassware, as a fire-resistant additive for paints, in electronics and as an herbicide.

Boron carbide is used as an abrasive, in the manufacture of hard and chemical-resistant ceramics or wear-resistant tools, in the refractory industry, in light weight cermets, in armour tiles, in radiation protection and shielding, in the nuclear industry in control rods in nuclear reactors (high capture cross-section to absorb thermal neutrons), as raw material for producing other boron containing materials (e.g. titanium boride), and in solid fuel (propellant for ducted rockets).

Boron nitride is used as a refractory material, laboratory reagent, and abrasive. Boron trichloride is used in the manufacture and purification of metal alloys, in bonding of iron and steel, in soldering fluxes, and in the manufacture of electrical resistors. It is also used to extinguish magnesium fires in heat resisting furnaces.

Boron trifluoride is widely used to promote various organic reactions. Boron filaments are high-strength, lightweight materials that are used in fibre optics research and for advanced aerospace structures.

Substance details

Substance name: Boron & compounds

CASR number: 7440-42-8

Molecular formula: B

Synonyms: No synonyms for elemental boron.

Boron compounds include borax or sodium tetraborate decahydrate (CASR# 1303-96-4), boric acid (CASR# 10043-35-3), boron oxide (CASR# 1303-86-2), boron carbide (CASR# 12069-32-8), boron nitride (CASR# 10043-11-5), boron tribromide (CASR# 10294-33-4), boron trichloride (CASR# 10294-34-5) and boron trifluoride (CASR# 7637-07-2).

Physical properties

Boron is a metalloid (an element which has both metallic and non-metallic properties) that has an odourless, black, hard solid appearance. It can also appear as a brown amorphous powder. It is a semiconductor. It is extremely difficult to prepare pure boron because of its high melting point and the corrosiveness of the liquid.

Atomic Number: 5

Atomic Mass: 10.8

Melting Point: 2167°C

Boiling Point: 3658°C

Specific Gravity: 2.4

The properties of boron compounds vary greatly. Properties of selected boron compounds follow.

Borax comes in hard, odourless crystals, granules or powder. It has a melting point of 75°C and boils at 320°C. It loses water of crystallisation when heated. Its specific gravity is 1.73.

Boric acid comes as colourless, odourless, transparent crystals, or white granules or powder. It has a specific gravity of 1.44-1.51 and decomposes at 169°C. Boric acid loses chemically combined water upon heating, forming boron oxide at higher temperatures.

Boron oxide comes in white rhombic crystals or colourless, semi-transparent vitreous granules or flakes, or hygroscopic lumps or powder. It usually forms a glass. The crystals have a specific gravity of 1.84-2.46 and melt at 450°C.

Boron carbide appears as black, shiny rhombohedra or octahedra. Its hardness is very high, being the third hardest material next to industrial diamond and cubic boron nitride. It is harder than silicon carbide. It is less brittle than most ceramics and has a specific gravity of 2.51. Its melting range is 2350-2455°C and its boiling point is >3500°C. It can be moulded from a pulverised powder at high temperature and pressure.

Boron nitride is a white, odourless powder. It has a specific gravity of 2.29 and sublimes at 3000°C. It exists in three different forms (hexagonal, cubic and pyrolytic). The cubic form is the second hardest material after diamond. Boron nitride behaves like an electrical insulator but conducts heat like a metal. It has lubricating properties similar to graphite.

Boron tribromide is a fuming liquid that boils at 91°C. Its melting point is -46°C.

Boron trichloride is a colourless fuming liquid at low temperature. It boils at 12.5°C. Its melting point is -107°C.

Boron trifluoride is a pungent, colourless gas. It boils at -100°C and its melting point is -127°C.

Chemical properties

Pure boron can be obtained by pyrolysis of boron hydrides and halides, or reduction of boron chloride or bromide with hydrogen. Boron is inert in its crystalline form, unaffected by boiling hydrochloric acid or hydrofluoric acid. Finely powdered boron is slowly oxidised by hot concentrated nitric acid. Many other hot concentrated oxidising agents do not or do only very slowly attack boron.

Properties of selected boron compounds follow.

Borax is soluble in water and glycerol, very slightly soluble in alcohol and insoluble in acids.

Boric acid is a weak acid. It is soluble in water, glycerol, ether, alcohol, methanol, and liquid ammonia, and slightly soluble in acetone. Boric acid can be obtained from borax, or by hydrolysing boron halides or hydrides.

Boron oxide crystals are slightly soluble in cold water and soluble in hot water. Finely ground boron oxide reacts vigorously with water to form boric acid. Boron oxide in its amorphous form is also soluble in alcohol, glycerol and acids. Boron oxide can be obtained by fusing boric acid.

Boron carbide does not mix with water and it is highly resistant to most chemical action including attack by hot hydrofluoric, nitric or chromic acids. It is decomposed by molten alkalis at red heat. It does not burn in oxygen flame.

Boron nitride is insoluble in water.

Boron trichloride is decomposed by water or alcohol to produce hydrochloric acid and oily liquids with powerful irritant and corrosive action.

Boron tribromide is decomposed by water or alcohol to produce hydrobromic acid.

Boron trifluoride reacts with water, ethers, alcohols and amines.

Further information

The National Pollutant Inventory (NPI) holds data for all sources of boron and compounds emissions in Australia.

Description

The severity of health effects will depend on how much boron a person has been exposed to, for how long, and current state of health. Once different borates are dissolved in the acid of gastric juices, they cannot be distinguished from each other on chemical or toxicological grounds. Both boric acid and borax may enter the body by absorption from the gastrointestinal tract or through mucous membranes. Although absorption can occur through undamaged skin, it is slow and toxic effects are less likely. However, absorption through damaged skin can be rapid and complete.

Ingestion of large borate quantities is unlikely, but may be harmful if it occurs. Ingestion or absorption through the skin can cause nausea, abdominal pain, diarrhoea and persistent vomiting (vomitus and faeces may sometimes contain blood), which may be accompanied by headache and weakness, lethargy, restlessness, tremors, intermittent convulsions, and characteristic erythematous (abnormally red) lesions on the skin. In severe cases, shock with fall in arterial pressure, tachycardia (increase in heart rate) and cyanosis (blue skin colour) may occur. Central nervous system stimulation followed by depression, gastrointestinal disturbance (haemorrhagic gastroenteritis) and erythematous skin eruptions (giving rise to a boiled lobster appearance) may be present. The kidneys (producing oliguria (small volume of urine), albuminuria (presence of albumin in the urine), anuria (absence of or defective excretion of urine)) and, rarely, the liver (jaundice) may also be involved. Excretion occurs mainly through the kidneys with about half excreted in the first 12 hours and the remainder over 5-12 days. Toxic symptoms may be delayed for several hours. The mean lethal dose of sodium borate or boric acid probably exceeds 30 grams in adults and death occurs due to vascular collapse in the early stages or to central nervous system depression in later stages. Children are thought to be more susceptible to the effects of borate intoxication.

Breathing moderate levels of boron dust or fume can result in irritation of the nose, throat, and eyes.

There is little information on the health effects of long-term exposure to boron. Most of the studies are on short-term exposures. Chronic intoxication with boric acid may give rise to anorexia, loss of strength, confusion and loss of hair (alopecia). Reproductive effects, such as low sperm count, were seen in men exposed to boron over the long-term.

Entering the body

Boron can be ingested or inhaled. Absorption through intact skin is unlikely but it can be readily absorbed through areas of damaged, abraded or burned skin as well as areas of active dermatitis.

Exposure

Low levels of boron can be found naturally in air, water and some food. Hence, drinking water and eating food can contribute to some normal boron intake. Certain consumer products such as cosmetics and laundry products are also likely sources of exposure. Boron is unlikely to leach from borosilicate glasses. Occupational exposure to boron and boron compounds can occur in industries that produce special glass, washing powder, soap and cosmetics, leather, cement etc. The nuclear industry (e.g. ANSTO) also uses boron.

Health guidelines

Australian Drinking Water Guidelines (NHMRC and ARMCANZ, 2004): 
Maximum of 4 mg/L (i.e. 0.004 g/L)

Worksafe Australia defines boron oxide as hazardous and has set its exposure standard to 10 milligram/m3 (TWA). Boron halides are also classified as hazardous with an exposure standard of 1 ppm (TWA). The exposure standard for the non-hazardous borax is 5 milligram/m3 (TWA).

Description

Boron is only essential to plants. It accumulates in plants and is found in foods, mainly fruits and vegetables. Boron does not appear to accumulate in fish or other organisms in water.

Entering the environment

Boron can be transported as particles released into the atmosphere or as dissolved compounds in natural waters.

Where it ends up

No information is available on how long boron remains in air, water or soil.

Environmental guidelines

No national guidelines.

Industry sources

Boron can be released from industries that use boron and boron compounds, e.g. leather tanning, cement works, and glass works.

Diffuse sources, and industry sources included in diffuse emissions data

Boron can be released from household use of consumer products containing borates such as cosmetics, washing powders, or pesticides, and sub-threshold facilities.

Natural sources

Boron is not an abundant element. The element boron does not occur in nature and boron is mostly found combined with oxygen in compounds called borates. Common borate compounds include boric acid, salts of borates and boron oxide. Natural deposits of borates are borax and kernite. Tourmaline contains about 10 % boron.

Boron is released to the environment from natural sources such as volcanoes and geothermal steam. Traces are present in rocks, soil, water and some food.

Transport sources

Mobile sources are normally not associated with the emission of boron.

Consumer products

Heat-resistant household glassware (Pyrex), laboratory glassware, some soap, some pesticides, some cosmetics and laundry products, some leather products and some cement products.

Sources used in preparing this information

  • Agency for Toxic Substances and Disease Registry (ATSDR), ToxFAQs (September 1995), Boron (accessed, May, 1999)
  • Australian and New Zealand Environment and Conservation Council (ANZECC) (1992), Australian Water Quality Guidelines for Fresh and Marine Waters.
  • ChemFinder WebServer Project (1995), Borax (accessed, May, 1999)
  • ChemFinder WebServer Project (1995), Boric acid (accessed, May, 1999)
  • ChemFinder WebServer Project (1995), Boron (accessed, May, 1999)
  • ChemFinder WebServer Project (1995), Boron carbide (accessed, May, 1999)
  • ChemFinder WebServer Project (1995), Boron nitride (accessed, May, 1999)
  • ChemFinder WebServer Project (1995), Boron oxide (accessed, May, 1999)
  • ChemFinder WebServer Project (1995), Boron tribromide (accessed, May, 1999)
  • ChemFinder WebServer Project (1995), Boron trichloride (accessed, May, 1999)
  • ChemFinder WebServer Project (1995), Boron trifluoride (accessed, May, 1999)
  • National Health and Medical Research Council (NHMRC) and Agriculture and Resource Management Council of Australia and New Zealand (ARMCANZ) (1996) Australian Drinking Water Guidelines
  • Integrated Risk Information System (IRIS, October 1, 1989), Boron (Boron and Borates only) (accessed, May, 1999)
  • National Environment Protection Council (1998), National Environment Protection Measure for the National Pollutant Inventory. (accessed, March, 1999)
  • Technical Advisory Panel (1999) Final Report to the National Environment Protection Council
  • WebElements - The periodic table on the WWW (May 1999), Bor (accessed, May, 1999)
  • Worksafe Australia (1996), Exposure Standards Database, Borax (accessed, May, 1999)
  • Worksafe Australia (1996), Exposure Standards Database, Boron oxide (accessed, May, 1999)
  • Worksafe Australia (1996), Exposure Standards Database, Boron tribromide (accessed, May, 1999)
  • Worksafe Australia (1996), Exposure Standards Database, Boron trifluoride (accessed, May, 1999)
  • Worksafe Australia (1996), Hazardous Substances Database, Boron oxide (accessed, May, 1999)
  • Worksafe Australia (1996), Hazardous Substances Database, Boron tribromide (accessed, May, 1999)
  • Worksafe Australia (1996), Hazardous Substances Database, Boron trichloride (accessed, May, 1999)
  • Worksafe Australia (1996), Hazardous Substances Database, Boron trifluoride (accessed, May, 1999)