| Borax | |
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Sodium tetraborate decahydrate |
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| Identifiers | |
| CAS number | 1303-96-4 (decahydrate) |
| ChemSpider | 17339255 |
| UNII | 91MBZ8H3QO |
| EC number | 215-540-4 |
| Jmol-3D images | Image 1 |
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| Properties | |
| Molecular formula | Na2B4O7·10H2O or Na2[B4O5(OH)4]·8H2O |
| Molar mass | 381.37 (decahydrate) |
| Appearance | white solid |
| Density | 1.73 g/cm³ (solid) |
| Melting point |
743 °C (anhydrous) [1] |
| Boiling point |
1575 °C |
| Related compounds | |
| Other anions | Sodium aluminate; sodium gallate |
| Other cations | Potassium tetraborate |
| Related compounds | Boric acid, sodium perborate |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
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, a texturing agent in cooking, 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 word borax:بورق is Arabic - the Arabic is said to be from the Persian burah, a word that may have meant potassium nitrate or another fluxing agent. Another name for borax is tincal, from Sanskrit.[2]
Borax was first discovered in dry lake beds in Tibet and was imported via the Silk Road to Arabia.[2] Borax first came into common use in the late 19th century when Francis Marion Smith's Pacific Coast Borax Company began to market and popularize a large variety of applications under the famous 20 Mule Team Borax trademark, named for the method by which borax was originally hauled out of the California and Nevada deserts in large enough quantities to make it cheap and commonly available.[3][4]
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Borax is used in various household laundry and cleaning products,[5] including the "20 Mule Team Borax" laundry booster and "Boraxo" powdered hand soap. However, despite its name, "Borateem" laundry bleach no longer contains borax or other boron compounds. Borax is also present in some tooth bleaching formulas.[6]
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 or BBS (borate buffered saline) in coating procedures. 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 form complex ions with 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.
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.[7]
Borax is replacing mercury as the preferred method for extracting gold in small-scale mining facilities. The method is called the borax method and is used in the Philippines.[8]
A rubbery polymer sometimes called flubber, gluep or glurch can be made by cross linking polyvinyl alcohol with a borax. Making flubber from polyvinyl acetate based glues, such as Elmer's Glue, and borax is a common elementary education experiment.[9]
Borax, given the E number E285, is used as a food additive in some countries but is banned in the United States. As a consequence, certain foods, such as caviar, produced for sale in the U.S. contain higher levels of salt to assist preservation.[10] Its use as a cooking ingredient is to add a firm rubbery texture to the food, or as a preservative. In oriental cooking it is mostly used for its texturing properties. In Asia, Borax (Chinese: 硼砂; pinyin: péng shā) or (Chinese: 月石; pinyin: yuè shí) was found to have been added to some Chinese foods like the hand-pulled noodles lamian and some rice noodles like Shahe fen, Kway Teow, and Chee Cheong Fun recipes.[11] In Indonesia it is a common, but forbidden, additive to such foods as noodles, bakso (meatballs), and steamed rice. The country's Directorate of Consumer Protection warns of the risk of liver cancer with high consumption over a period of 5–10 years.[12]
Borax occurs naturally in evaporite deposits produced by the repeated evaporation of seasonal lakes. The most commercially important deposits are found in Turkey; Boron, California; and Searles Lake, California. Also, it has been found at many other locations in the Southwestern United States, the Atacama desert in Chile, and in Tibet and Romania. Borax can also be produced synthetically from other boron compounds. Naturally occurring Borax, (known by the trade name Rasorite - 46 in USA and many other countries) is refined by a process of re-crystallization.[18]
Borax, sodium tetraborate decahydrate, is not acutely toxic.[19] Its LD50 (median lethal dose) score is tested at 2.66 g/kg in rats:[20] a significant dose of the chemical is needed to cause severe symptoms or death. The lethal dose is not necessarily the same for humans.
Sufficient exposure to borax dust 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." [21]
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).[22] Boric acid solutions used as an eye wash or on abraded skin are known to be particularly toxic to infants, especially after repeated use, because of the slow elimination rate.[23]
Borax was added to the Substance of Very High Concern (SVHC) candidate list on 16 December 2010. The SVHC candidlate list is part of the EU Regulations on the Registration, Evaluation, Authorisation and Restriction of Chemicals 2006 (REACH), and the addition was based on the revised classification of Borax as toxic for reproduction category 1B under the CLP Regulations. Substances and mixtures imported into the EU which contain Borax are now required to be labelled with the warnings "May damage fertility" and "May damage the unborn child".[24]
The term borax is often used for a number of closely related minerals or chemical compounds that differ in their crystal water content:
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. Its reaction with hydrochloric acid to form boric acid is:
The "decahydrate" is sufficiently stable to find use as a primary standard for acid base titrimetry.[25]
When borax is added to a flame, it produces a yellow green color.[26] 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 boron that is responsible for most of the green color, and that is overwhelmed by the yellow contributed to the flame by sodium.
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.