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IUPAC name Sodium tetraborate


CAS number 1303-96-49008755454154
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
EU classification not listed
NFPA 704

NFPA 704.svg

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 disclaimer and references
File:Borax wagons.jpg
Old steam tractor and Borax wagons, Death Valley

Borax (from Persian burah), 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.


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.[1][2]



Sodium borate is used in biochemical and chemical laboratories to make SB buffers, 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.[3] However, lithium borate is much more expensive.


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.[4]


When a borax-water solution is mixed with P.V.A glue it will harden forming a putty.

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 common table salt, and it appears in French and Iranian caviar.

Other uses

Natural sources

File:Borax ball.jpg
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.


Boric acid, sodium borate, and sodium perborate are estimated to have a lethal dose (LD50) from 5 to 20 grams in humans [5]. The estimated lethal dose (ingested) for adults is 15-20 grams; less than 5 grams can kill a child or pet. 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. 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).[6] 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.[7]


The structure of the anion [B4O5(OH)4]2−</sub> 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−</sub> 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


  1. "Borax". Bartleby. Retrieved 2008-05-01. 
  2. Brody, Jonathan R. (2004), "History and principles of conductive media for standard DNA electrophoresis", Analytical Biochemistry, 333: pp. 1–13, doi:10.1016/j.ab.2004.05.054 
  3. Dodd, J.G. (1966), "Soft soldering to tungsten wire", Am. J. Phys, 34 (10): p. xvi, doi:10.1119/1.1972398 
  4. Handbook of Poisoning, Robert H. Dreisback, eighth edition, p.314
  5. Report of the Food Quality Protection Act (FQPA) Tolerance Reassessment Eligibility Decision (TRED) for Boric Acid/Sodium Borate Salts
  6. Goodman and Gillman's: The Pharmacological Basis of Therapeutics, 6th edition, chapter on Antiseptics and Disinfectants, page 971

External links

bn:বোরাক্স bs:Boraks ca:Bòrax cs:Tetraboritan sodný de:Boraxeo:Boraksogl:Bórax id:Bleng it:Borace hu:Bórax nl:Boraxsr:Боракс sv:Boraxuk:Бура