[bawr-aks, -uhks, bohr-]
borax or sodium tetraborate decahydrate, chemical compound, Na2B4O7·10H2O; sp. gr. 1.73; slightly soluble in cold water; very soluble in hot water; insoluble in acids. Borax is a colorless, monoclinic crystalline salt; it also occurs as a white powder. It readily effloresces, especially on heating. It loses all water of hydration when heated above 320°C; and fuses when heated above 740°C;; a "borax bead" so formed is used in chemical analysis (see bead test). Borax is widely and diversely used, e.g., as a mild antiseptic, a cleansing agent, a water softener, a corrosion inhibitor for antifreeze, a flux for silver soldering, and in the manufacture of enamels, shellacs, heat-resistant glass (e.g., Pyrex), fertilizers, pharmaceuticals, and other chemicals. It is sometimes used as a preservative but is toxic if consumed in large doses. Naturally occurring borax (sometimes called tincal) is found in large deposits in the W United States (Borax Lake in Death Valley, Calif.; Nevada; and Oregon) and in the Tibet region of China. Borax can also be obtained from borate minerals such as kernite, colemanite, or ulexite. California is the chief source of borate minerals in the United States.

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.



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.


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 non-enzymatically glycosylated hemoglobin (chiefly HbA1c), which is a model for long term damage done by 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.


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.

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.


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.

Other uses

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.


Borax, sodium tetraborate decahydrate, is not acutely toxic. Its LD50 (median lethal dose) score is tested at 2.66 g/kg in rats. This does not mean that it is safe, merely that a significant dose of the chemical is needed to cause severe symptoms or death. 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."

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). 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.


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 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


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