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Borax
Borax crystals
Ball-and-stick model of the unit cell of borax decahydrate
IUPAC name
Identifiers
CAS number (anhydrous) 1330-43-4 (anhydrous)
Identifiers
CAS number (decahydrate) 1303-96-4 (decahydrate)
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

743 °C[1]

Boiling point

1575 °C

Hazards
EU classification not listed
NFPA 704
NFPA 704.svg
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
Old steam tractor and Borax wagons, Death Valley

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 is from Persian and originates in the Middle-Persian būrak.

Contents

Uses

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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 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.

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.[2]

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 Sweden and used in the Philippines. [3]

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.[4]

Food additive

Borax, given the E number E285, is used as a food additive in some countries but is banned in the United States. In consequence certain foods, such as caviar, produced for sale in the U.S. contain higher levels of salt to assist preservation.[5] Its use in cooking 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 China it was found to have been added to hand-pulled noodles (lamian) and some Cheung Fun recipes. In Indonesia it is a common, but forbidden, additive to such foods as noodles, 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.[6]

Vaccine

Sodium borate is an ingredient in the vaccine Gardasil, manufactured by Merck.

Other uses

Natural sources

Borax "cottonball"

Borax occurs naturally in evaporite deposits produced by the repeated evaporation of seasonal lakes. The most commercially important deposits are found in Turkey and Boron, California and 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.

Toxicity

Borax, sodium tetraborate decahydrate, is not acutely toxic. [9] Its LD50 (median lethal dose) score is tested at 2.66 g/kg in rats. [10] This does not mean that it is safe, merely that a significant dose of the chemical is needed to cause severe symptoms or death. 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." [11]

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).[12] 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.[13]

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 added to a flame, it produces a yellow green color.[14] 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 which is responsible for most of the green color, and which 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.

See also

References

  1. ^ CRC Handbook of Chemistry and Physics 86th edition (2005-2006) section 4 page 88
  2. ^ Dodd, J.G. (1966), "Soft soldering to tungsten wire", Am. J. Phys 34 (10): p. xvi, doi:10.1119/1.1972398  
  3. ^ "The borax method". Borax replacing mercury in small-scale mining. The Geological Survey of Denmark and Greenland (GEUS). http://www.geus.dk/program-areas/common/int_ssm_fact_sheet_07.pdf. Retrieved 2008-08-02.  
  4. ^ Parratore, Phil. Wacky Science: A Cookbook for Elementary Teachers. Dubuque, IA: Kendall Hunt. p. 26. ISBN 0787227412.  
  5. ^ "Caviar glossary". The Caviar Guide a gourmet review of caviars & fish roe. Hanson Ltd, Geneva, Switzerland. http://www.hanscon.ch/caviar_website/glossary.htm. Retrieved 2008-07-07.  
  6. ^ Staff writer (2006). "Watch Out For The Food We Consume" (in English). Directorate of Consumer Protection, Jakarta, Indonesia. http://pkditjenpdn.depdag.go.id/English/index.php?page=infodtl&InfoID=8&dtl=1. Retrieved 2009-02-10.  
  7. ^ Borax at UC Berkeley
  8. ^ http://chemistry.about.com/cs/howtos/a/aa052703a.htm
  9. ^ Borax - toxicity, ecological toxicity and regulatory information
  10. ^ Mountain Fresh Dial Bar Soap
  11. ^ Borax - toxicity, ecological toxicity and regulatory information
  12. ^ Report of the Food Quality Protection Act (FQPA) Tolerance Reassessment Eligibility Decision (TRED) for Boric Acid/Sodium Borate Salts
  13. ^ Goodman and Gillman's: The Pharmacological Basis of Therapeutics, 6th edition, chapter on Antiseptics and Disinfectants, page 971
  14. ^ Staff. "Creating Flame Colors". The Science Company. http://www.sciencecompany.com/sci-exper/flamecolors.htm. Retrieved November 30 2008.  

External links


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

BORAX (sodium pyroborate or sodium biborate), Na2B407, a substance which appears in commerce under two forms, namely "common" or prismatic borax, Na 2 B 4 O 7.10H 2 O, and "jewellers'" or octahedral borax, Na 2 B 4 O 7.5H 2 O. It is to be noted that the term "borax" was used by the alchemists in a very vague manner, and is therefore not to be taken as meaning the substance now specifically known by the name. Prismatic borax is found widely distributed as a natural product (see below, Mineralogy) in Tibet, and in Canada, Peru and Transylvania, while the bed of Borax Lake, near Clear Lake in California, is occupied by a large mass of crystallized borax, which is fit for use by the assayer without undergoing any preliminary purification. The supply of borax is, however, mainly derived from the boric acid of Tuscany, which is fused in a reverberatory furnace with half its weight of sodium carbonate, and the mass after cooling is extracted with warm water. An alternative method is to dissolve sodium carbonate in lead-lined steamheated pans, and add the boric acid gradually; the solution then being concentrated until the borax crystallizes. Borax is also prepared from the naturally occurring calcium borate, which is mixed in a finely divided condition with the requisite quantity of soda ash; the mixture is fused, extracted with water and concentrated until the solution commences to crystallize.

From a supersaturated aqueous solution of borax, the pentahydrate, Na2B407.5H20, is deposited when evaporation takes place at somewhat high temperatures. The same hydrate can be prepared by dissolving borax in water until the solution has a specific gravity of 1.246 and then allowing the solution to cool. The pentahydrate is deposited between 79° C. and 56° C.; below this temperature the decahydrate or'ordinary borax,Na2B407.10H20, is deposited. Crystals of ordinary borax swell up to a very great extent on heating, losing their water of crystallization and melting to a clear white glass. The crystals of octahedral borax fuse more easily than those of the prismatic form and are less liable to split when heated, so that they are preferable for soldering or fluxing. Fused borax dissolves many metallic oxides, forming complex borates which in many cases show characteristic colours. Its use in soldering depends on the fact that solder only adheres to the surface of an untarnished metal, and consequently a little borax is placed on the surface of the metal and heated by the soldering iron in order to remove any superficial film of oxide. It is also used for glazing pottery, in glass-making and the glazing of linen.

Boric acid (q.v.) being only a weak acid, its salts readily undergo hydrolytic dissociation in aqueous solution, and this property can be readily shown with a concentrated aqueous solution of borax, for by adding litmus and then just sufficient acetic acid to turn the litmus red, the addition of a large volume of water to the solution changes the colour back to blue again. The boric acid being scarcely ionized gives only a very small quantity of hydrogen ions, whilst the base (sodium hydroxide) produced by the hydrolysis occasioned by the dilution of the solution, being a "strong base," is highly ionized and gives a comparatively large amount of hydroxyl ions. In the solution, therefore, there is now an excess of hydroxyl ions; consequently it has an alkaline reaction and the litmus turns blue.

Mineralogy

The Tibetan mineral deposits have been known since very early times, and formerly the crude material was exported to Europe, under the name of tincal, for the preparation of pure borax and other boron salts. The most westerly of the Tibetan deposits are in the lake-plain of Pugha on the Rulangchu, a tributary of the Indus, at an elevation of 15,000 ft.: here the impure borax (sohaga) occurs over an area of about 2 sq. m., and is covered by a saline efflorescence; successive crops are obtained by the action of rain and snow and subsequent evaporation. Deposits of purer material (chic 'sale or water borax) occur at the lakes of Rudok, situated to the east of the Pugha district; also still farther to the east at the great lakes Tengri Nor, north of Lhasa, and several other places. More recently, the extensive deposits of borates (chiefly, however, of calcium; see Colemanite) in the Mohave desert on the borders of California and Nevada, and in the Atacama desert in South America, have been the chief commercial sources of boron compounds. The boron contained in solution in the salt lakes has very probably been supplied by hot springs and solfataras of volcanic origin, such as those which at the present day charge the waters of the lagoons in Tuscany with boric acid. The deposits formed by evaporation from these lakes and marshes or salines, are mixtures of borates, various alkaline salts (sodium carbonate, sulphate, chloride), gypsum, &c. In the mud of the lakes and in the surrounding marshy soil fine isolated crystals of borax are frequently found. For example, crystals up to 7 in. in length and weighing a pound each have been found in large numbers at Borax Lake in Lake county, and at Borax Lake in San Bernardino county, both in California.

In 1927 a very large deposit of relatively pure Borax was discovered at what is now Boron, CA, in the Mojave Desert. The deposit was worked as an underground mine until 1957, when it became an open pit mine. Virtually all other Borax mining operations stopped when this mine opened, including the main source of Borax at the time in Death Valley. This mine and a major mine in Turkey, which rose to prominence in the 1990s, now supply nearly all the world's borax.

Borax crystallizes with ten molecules of water, the composition of the crystals being Na2B407+10H20. The crystals belong to the monoclinic system, and it is a curious fact that in habit and angles they closely resemble pyroxene (a silicate of calcium, magnesium and iron). There is a perfect cleavage parallel to the orthopinacoid and less perfect cleavages parallel to the faces of the prism. The mineral is transparent to opaque and white, sometimes greyish, bluish or greenish in colour. Hardness 2-21; sp. gr. 1.69-1.72.

The optical characters are interesting, because of the striking crossed dispersion of the optic axes, of which phenomenon borax affords the best example. The optic figure seen in convergent polarized light through a section cut parallel to the plane of symmetry of a borax crystal is symmetrical only with respect to the central point. The plane of the optic axes for red light is inclined at 2° to that for blue light, and the angle between the optic axes themselves is 3° greater for red than for blue light.


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