Borax

For other uses, see Borax (disambiguation).

Borax
General
Systematic name	Sodium tetraborate decahydrate
Molecular formula	Na2B4O7·10H2O
Molar mass	381.37 g/mol
Appearance	white solid
CAS number	1303-96-4
Properties
Density and phase	1.73 g/cm³, solid
Solubility in water	5.1 g/100 ml (20 °C)
Melting point	75 °C
Boiling point	320 °C
Basicity (pKb)	see text
Structure
Coordination geometry	?
Crystal structure	Monoclinic
Thermodynamic data
Std enthalpy of formation ΔfHo298	-3276.75 kJ/mol
Standard molar entropy So298	189.53 J·K−1·mol−1
Hazards
MSDS	External MSDS
EU classification	not listed
NFPA 704	0 1 0
Flash point	non-flammable
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
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)

Borax, also called sodium borate, or 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 fibreglass, 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.

Name

The origin of the name is traceable to the Medieval Latin borax, which comes from the Arabic buraq, which comes from either the Persian burah [1] or the Middle Persian burak[2].

Uses

Buffer

Sodium borate is used in biochemical and chemical laboratories to make buffer solutions, e.g. for gel electrophoresis of DNA. It has a lower conductivity, produces sharper bands, and can be run at higher speeds than can gels made from TBE Buffer or TAE Buffer (5 - 35 V/cm as compared to 5 - 10 V/cm). At a given voltage, the heat generation and thus the gel temperature is much lower than with TBE or TAE buffers, therefore the voltage can be increased to speed up electrophoresis so that a gel run takes only a fraction of the usual time. Downstream applications, such as isolation of DNA from a gel slice or Southern blot analysis, work as expected with sodium borate gels. Borate buffers (usually at pH 8) are also used as preferential equilibration solution in DMP-based crosslinking reactions.

Lithium borate is similar to sodium borate and has all of its advantages, but permits use of even higher voltages due to the lower conductivity of lithium ions as compared to sodium ions. However, lithium borate is much more expensive.

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.

Food additive

Borax is used as a food additive in some countries with the E number E285, but is banned in the United States. Its use is similar to salt, and it appears in French and Iranian caviar.

Other uses

• component of detergents
• component of cosmetics
• ingredient in enamel glazes
• component of glass, pottery, and ceramics
• fire retardant
• anti-fungal compound for fibreglass and cellulose insulation
• component of Slime
• 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 horse's hoofs
• used to make indelible ink for dip pens by dissolving shellac into heated borax

Natural sources

Borax "cottonball"

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

Boric acid, sodium borate, and sodium perborate are estimated to have a fatal dose from 0.1 to 0.5g/kg. These substances are toxic to all cells, and have a slow excretion rate through the kidneys. Kidney toxicity is the greatest, with liver fatty degeneration, cerebral edema, and gastroenteritis. Boric acid solutions used as an eye wash or on abraded skin are known to be especially toxic to infants, especially after repeated use due to its slow elimination rate.

Chemistry

The structure of the anion [B₄O₅(OH)₄]²⁻ in borax

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)₄]²⁻ 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 burned, it produces a bright orange-colored flame. Because of this, it is sometimes used for homemade pyrotechnics.

See also

• Boron
• Buffer solution
• Mineral
• Borax bead test
• Sodium borohydride
• Ulexite

References

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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

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