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| Borax | |
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| IUPAC name | Sodium tetraborate
decahydrate |
| Identifiers | |
| CAS number | 1330-43-4 |
| Properties | |
| Molecular formula | Na2B4O7·10H2O or Na2[B4O5(OH)4·8H2O |
| Molar mass | 381.37 |
| Appearance | white solid |
| Density | 1.73 g/cm³ (solid) |
| Melting point |
741 °C |
| Boiling point |
1575 °C |
| Basicity (pKb) | see text |
| Hazards | |
| EU classification | not listed |
| NFPA 704 |
 0
1
0
|
| Flash point | Non-flammable |
| Related compounds | |
| Other anions | Sodium aluminate; sodium gallate |
| Other cations | Potassium tetraborate |
| Related compounds | Boric acid, sodium perborate |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references |
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Borax, also known as sodium borate, sodium tetraborate, or disodium tetraborate, is an important boron compound, a mineral, and a salt of boric acid. It is usually a white powder consisting of soft colorless crystals that dissolve easily in water.
Borax has a wide variety of uses. It is a component of many detergents, cosmetics, and enamel glazes. It is also used to make buffer solutions in biochemistry, as a fire retardant, as an anti-fungal compound for fiberglass, as an insecticide, as a flux in metallurgy, and as a precursor for other boron compounds.
The term borax is used for a number of closely related minerals or chemical compounds that differ in their crystal water content, but usually refers to the decahydrate. Commercially sold borax is usually partially dehydrated.
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Name
The origin of the name is traceable to the Medieval Latin borat, which comes from the Arabic buraq, which comes from either the Persian burah or the Middle Persian burak.12
Uses
Buffer
Sodium borate is used in biochemical and chemical laboratories to make buffers, e.g. for gel electrophoresis of DNA, such as TBE or the newer SB buffer. Borate buffers (usually at pH 8) are also used as preferential equilibration solution in DMP-based crosslinking reactions.
Co-complexing
Borax as a source of borate has been used to take advantage of the co-complexing ability of borate with other agents in water to complex various substances. Borate and a suitable polymer bed are used to chromatograph non-glycosylated hemoglobin differentially from glycosylated hemoglobin (chiefly HbA1c), which is an indicator of long term hyperglycemia in diabetes mellitus. Borate and a proprietary synthetic amino acid, Deselex (from Henkel) have been used to complex water "hardness" cations to make a non-precipitating water "softener". Borate alone does not have a high affinity for "hardness" cations, although it has been used for that purpose.
Flux
A mixture of borax and ammonium chloride is used as a flux when welding iron and steel. It lowers the melting point of the unwanted iron oxide (scale), allowing it to run off. Borax is also used mixed with water as a flux when soldering jewelry metals such as gold or silver. It allows the molten solder to flow evenly over the joint in question. Borax is also a good flux for 'pre-tinning' tungsten with zinc - making the tungsten soft-solderable.3
Small-scale mining
Borax is replacing mercury as the preferred method for extracting gold in small-scale mining facilities. The method is called the borax method and was first discovered in the Philippines. 4
Putty
When a borax-water solution is mixed with PVA glue (wood glue) a rubbery precipitate is formed, which is the result of cross-linking in the polymer.
Food additive
Borax is used as a food additive in some countries with the E number E285, but is banned in the United States. In consequence certain foods, such as caviar, produced for sale there contain higher levels of salt to assist preservation.5 In China it was found to have been added to hand-pulled noodles (Lamian).
Other uses
- Component of detergents
- Ingredient in enamel glazes
- Component of glass, pottery, and ceramics
- Fire retardant
- Anti-fungal compound for fibreglass and cellulose insulation
- Insecticide to kill ants and fleas
- Precursor for sodium perborate monohydrate that is used in detergents, as well as for boric acid and other borates
- Treatment for thrush in horses' hooves
- Used to make indelible ink for dip pens by dissolving shellac into heated borax
- Curing agent for snake skins
- Swimming pool buffering agent to control the pH
- Neutron absorber, used in nuclear reactors and spent fuel pools to control reactivity and to shut down a nuclear chain reaction
Natural sources
Borax occurs naturally in evaporite deposits produced by the repeated evaporation of seasonal lakes (see playa). The most commercially important deposits are found in Turkey and near Boron, California and other locations in the Southwestern United States, the Atacama desert in Chile, and in Tibet. Borax can also be produced synthetically from other boron compounds.
Toxicity
Borax, sodium tetraborate decahydrate, is not acutely toxic. 6 Its LD50 (median lethal dose) score is tested at 2.66 g/kg in rats. 7 This does not mean that it is safe, merely that a significant dose of the chemical is needed to cause severe symptoms or death. (Note that the median lethal dose for humans tends to differ for a given compound from that of rats.) Simple exposure can cause respiratory and skin irritation. Ingestion may cause gastrointestinal distress including nausea, persistent vomiting, abdominal pain, and diarrhea. Effects on the vascular system and brain include headaches and lethargy, but are less frequent. "In severe poisonings, a beefy red skin rash affecting palms, soles, buttocks and scrotum has been described. With severe poisoning, erythematous and exfoliative rash, unconsciousness, respiratory depression, and renal failure." 8
A reassessment of boric acid/borax by the United States Environmental Protection Agency Office of Pesticide Programs found potential developmental toxicity (especially effects on the testes).9 Boric acid solutions used as an eye wash or on abraded skin are known to be especially toxic to infants, especially after repeated use because of its slow elimination rate.10
Chemistry
The term borax is often used for a number of closely related minerals or chemical compounds that differ in their crystal water content:
- Anhydrous borax (Na2B4O7)
- Borax pentahydrate (Na2B4O7·5H2O)
- Borax decahydrate (Na2B4O7·10H2O)
Borax is generally described as Na2B4O7·10H2O. However, it is better formulated as Na2[B4O5(OH)4·8H2O, since borax contains the [B4O5(OH)42− ion. In this structure, there are two four-coordinate boron atoms (two BO4 tetrahedra) and two three-coordinate boron atoms (two BO3 triangles).
Borax is also easily converted to boric acid and other borates, which have many applications. If left exposed to dry air, it slowly loses its water of hydration and becomes the white and chalky mineral tincalconite (Na2B4O7·5H2O).
When borax is added to a flame, it produces a bright orange color. This property has been tried in amateur fireworks, but borax in this use is not popular because its waters of hydration inhibit combustion of compositions and make it an inferior source of the sodium which is responsible for most of its flame color, and which overwhelms the green contributed to the flame by boron.
However, commercially available borax can be mixed with flammables such as methanol to give the characteristic green flame of boron when ignited, which then slowly gives way to the characteristic yellow-orange flame of the sodium.
See also
- Buffer solution
- Borax bead test
- Sodium borohydride
- Ulexite
- Twenty-Mule-Team Borax
- Francis Marion Smith
- John Veatch
- Boric acid
References
- ^ "Borax, noun". Merriam-Webster Online Dictionary. Retrieved on 2008-05-01.
- ^ "Borax". Bartleby. Retrieved on 2008-05-01.
- ^ Dodd, J.G. (1966), "Soft soldering to tungsten wire", Am. J. Phys 34 (10): p. xvi, doi:
- ^ "The borax method". Borax replacing mercury in small-scale mining. The Geological Survey of Denmark and Greenland (GEUS). Retrieved on 2008-08-02.
- ^ "Caviar glossary". The Caviar Guide a gourmet review of caviars & fish roe. Hanson Ltd, Geneva, Switzerland. Retrieved on 2008-07-07.
- ^ Borax - toxicity, ecological toxicity and regulatory information
- ^ Mountain Fresh Dial Bar Soap
- ^ Borax - toxicity, ecological toxicity and regulatory information
- ^ Report of the Food Quality Protection Act (FQPA) Tolerance Reassessment Eligibility Decision (TRED) for Boric Acid/Sodium Borate Salts
- ^ Goodman and Gillman's: The Pharmacological Basis of Therapeutics, 6th edition, chapter on Antiseptics and Disinfectants, page 971
External links
- International Chemical Safety Card 0567
- International Chemical Safety Card 1229 (fused borax)
- National Pollutant Inventory - Boron and compounds
- NIOSH Pocket Guide to Chemical Hazards
- Sodium Borate in sefsc.noaa.gov
Wikipedia content modification information:
- This page was last modified on 19 November 2008, at 21:00.
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