Borax can be dehydrated by heating into other forms with lesswater of hydration. The anhydrous form of borax can also be obtained from the decahydrate or other hydrates by heating and then grinding the resulting glasslike solid into a powder. It is a white crystalline solid that dissolves in water to make abasicsolution due to thetetraborateanion.
Borax was first discovered in dry lake beds inTibet. Native tincal from Tibet,Persia, and other parts of Asia was traded via theSilk Road to theArabian Peninsula in the 8th century AD.[9]
The English wordborax and its previousMiddle formboras is a Latinate loan fromOld Frenchboras ~bourras[12] which may have been fromMedieval Latinbaurach (another English spelling),borac(-/um/em), borax, along withSpanishborrax (>borraj) andItalianborrace, in the 9th century, and fromArabicبورقbawraq ~būraq ~bōraq which is attested inIbn Sayyar al-Warraq'sKitāb al-Ṭabīkh among many examples,[12][13][14] from Middle Persian bwlk' (bōrag), which yielded Persianبورهbure.
The wordstincal andtincar were adopted intoEnglish in the 17th century fromMalaytingkal and fromUrdu/Persian/Arabicتنکارtinkār/tankār; thus the two forms in English. These all appear to be related to theSanskritटांकणṭānkaṇa.[15][16]
From a chemical perspective, borax contains the[B4O5(OH)4]2− ion. In this structure, there are two four-coordinate boron centers and two three-coordinate boron centers.
It is aproton conductor at temperatures above 21 °C (70 °F). Conductivity is maximum along theb-axis.[17]
Borax is also easily converted toboric acid and otherborates, which have many applications. Its reaction with hydrochloric acid to form boric acid is:
Molten borax dissolves many metal oxides to form glasses. This property is important for its uses in metallurgy and for theborax bead test of qualitative chemical analysis.
Borax is soluble in a variety of solvents; however, it is notably insoluble in ethanol.[1]
The termborax properly refers to the so-called "decahydrate"Na2B4O7·10H2O, but that name is not consistent with its structure. It is actually octahydrate. Theanion is nottetraborate[B4O7]2− buttetrahydroxy tetraborate[B4O5(OH)4]2−,[17] so the more correct formula should beNa2B4O5(OH)4·8H2O. However, the term may be applied also to the related compounds. Borax "pentahydrate" has the formulaNa2B4O7·5H2O, which is actually a trihydrateNa2B4O5(OH)4·3H2O. It is a colorless solid with a density of1.880 kg/m3 that crystallizes from water solutions above 60.8 °C (141.4 °F) in therhombohedral crystal system. It occurs naturally as the mineraltinkhanite.[20] It can be obtained by heating the "decahydrate" above 61 °C (142 °F).[21] Borax "dihydrate" has the formulaNa2B4O7·2H2O, which is actually anhydrous, with the correct formulaNa2B4O5(OH)4. It can be obtained by heating the "decahydrate" or "pentahydrate" to above 116–120 °C (241–248 °F).[21] Anhydrous borax is sodium tetraborate proper, with formulaNa2B4O7. It can be obtained by heating any hydrate to 300 °C (572 °F).[21] It has one amorphous (glassy) form and three crystalline forms – α, β, and γ, with melting points of 1,015 K (742 °C; 1,367 °F), 993 K (720 °C; 1,328 °F) and 936 K (663 °C; 1,225 °F) respectively.α-Na2B4O7 is the stable form.[21]
Naturally occurring borax (known by the trade name Rasorite–46 in the United States and many other countries) is refined by a process ofrecrystallization.[23]
Borax is used in pest control solutions because it is toxic to ants and rats. Because it is slow-acting, worker ants will carry the borax to their nests and poison the rest of the colony.[24] Borax is more effective than zinc borate for termite control but a 1997 paper concluded that exposing at least 10% of the total colony population was needed for effective treatment. In Japan the practice of laying newspapers treated with o-boric acid and borax under buildings has been effective in controllingCoptotermes formosanus andReticulitermes speratus populations. Decaying wood treated with 0.25 to 0.5 percent DOT was also found to be effective for baitingHeterotermes aureus populations. The paper concluded: "Borate baits would undoubtably be helpful in the long-term, but do not appear sufficient as a sole method of structural protection."[25]
Borate ions (commonly supplied as boric acid) are used in biochemical and chemical laboratories to makebuffers, e.g. forpolyacrylamide gel electrophoresis of DNA and RNA, such asTBE buffer (borate buffered tris-hydroxymethylaminomethonium)[30][31] or the newer SB buffer orBBS buffer (borate buffered saline) in coating procedures. Borate buffers (usually at pH 8) are also used as preferential equilibration solutions indimethyl pimelimidate (DMP) based crosslinking reactions.[32]
Borax alone does not have a high affinity forhardness cations, although it has been used for water softening. A general chemical equation forwater softening is (R is the dissolved cation, usually calcium or magnesium):
R2+(aq) + Na2B4O7(aq) → RB4O7(s)↓ + 2 Na+(aq)
The sodium ions introduced do not make water "hard". This method is suitable for removing both temporary and permanent types of hardness.[citation needed]
Inartisanal gold mining, borax is sometimes used as part of a process known as theborax method (as a flux) meant to eliminate the need for toxic mercury in thegold extraction process, although it cannot directly replace mercury. Borax was reportedly used by gold miners in parts of the Philippines in the 1900s.[36][37] There is evidence that, in addition to reducing the environmental impact, this method achieves better gold recovery for suitable ores and is less expensive. This borax method is used in northernLuzon in the Philippines, but miners have been reluctant to adopt it elsewhere for reasons that are not well understood.[38] The method has also been promoted in Bolivia[39] and Tanzania.[40]
A rubbery polymer sometimes calledSlime,Flubber, "gluep" or "glurch" (or erroneously calledSilly Putty, which is based on silicone polymers), can be made bycross-linkingpolyvinyl alcohol with borax.[41] Making flubber frompolyvinyl acetate-based glues, such as Elmer's Glue, and borax is a common elementary science demonstration.[42][43]
Borax, given theE numberE285, is used as afood additive but this use is banned in some countries, such as Australia, China, Thailand and the United States.[44] As a consequence, certain foods, such ascaviar, produced for sale in the United States contain higher levels of salt to assist preservation.[45] In addition to its use as a preservative, borax imparts a firm, rubbery texture to food. In China, borax (Chinese:硼砂;pinyin:péng shā or Chinese:月石; pinyin:yuè shí) has been found in foods including wheat and rice noodles namedlamian (Chinese:拉面; pinyin:lāmiàn),shahe fen (Chinese:沙河粉; pinyin:shāhéfěn),char kway teow (Chinese:粿條; pinyin:guǒ tiáo), andchee cheong fun (Chinese:肠粉; pinyin:chángfěn)[clarification needed][46] In Indonesia, it is a common, but forbidden,[47] additive to such foods as noodles,bakso (meatballs), and steamed rice.
Pulverized for the prevention of stubborn pests (e.g.German cockroaches) in closets, pipe and cable inlets, wall panelling gaps, and inaccessible locations where ordinarypesticides are undesirable.[54]
Inparticle physics as an additive tonuclear emulsion, to extend the latent image lifetime of charged particle tracks. The first observation of thepion, which was awarded the 1950Nobel Prize, used this type of emulsion.[66][67]
According to one study, borax is not acutely toxic. ItsLD50 (median lethal dose) score is tested at 2.66 g/kg in rats, meaning that a significant dose of the chemical is needed to cause severe symptoms or death.[citation needed]
The lethal dose is not necessarily the same for humans; human studies in three borate exposure-rich comparison groups (U.S. Borax mine and production facility workers, Chinese boron workers, Turkish residents living near boron rich regions) produced no indicators of developmental toxicity in blood and semen tests. The highest estimated exposure was 5 mg B/kg/day, likely due to eating in contaminated workplaces, more than 100 times the average daily exposure.[68]
Borax is absorbed poorly through intact skin, although fatalities have been recorded in persistent treatment of rashes and open wounds with boric acid-containing ointments and bath solutions. Borax is readily absorbed orally, well above 90%, and mostly excreted through the urine. Fatal cases attributed to ingestion include small children mistakenly drinking pesticides or suicide attempts with large volumes of crystals. No genotoxicity or carcinogenicity has been recorded in studies.[69]
Borax has been in use as an insecticide in the United States with various restrictions since 1946. All restrictions were removed in February 1986 due to the low toxicity of borax, as reported in two EPA documents relating to boric acid and borax.[70][71]
EPA has determined that, because they are of low toxicity and occur naturally, boric acid and its sodium salts should be exempted from the requirement of a tolerance (maximum residue limit) for all raw agricultural commodities.[70]
Although it cited inconclusive data, a re-evaluation in 2006 by the EPA still found that "There were no signs of toxicity observed during the study and no evidence ofcytotoxicity to the target organ."[72] In the reevaluation, a study of toxicity due to overexposure was checked and the findings were that "The residential handler inhalation risks due to boric acid and its sodium salts as active ingredients are not a risk concern and do not exceed the level of concern", but that there could be some risk of irritation to children inhaling it if used as a powder for cleaning rugs.
Overexposure to borax dust can cause respiratory irritation, while no skin irritation is known to exist due to external borax exposure. Ingestion may cause gastrointestinal distress includingnausea, persistent vomiting, abdominal pain, anddiarrhea. Effects on the vascular system andhuman brain include headaches and lethargy but are less frequent. In severe cases, a "beefy" red rash affecting the palms, soles, buttocks and scrotum has occurred.[73]
Borax was added to theSubstance of Very High Concern (SVHC) candidate list on December 16, 2010. The SVHC candidate list is part of theEU Regulations on theRegistration, Evaluation, Authorisation and Restriction of Chemicals 2006 (REACH), and the addition was based on the revised classification of borax astoxic for reproduction category 1B under theCLP 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".[74] It was proposed for addition to REACH Annex XIV by theECHA on July 1, 2015.[75] If this recommendation is approved, all imports and uses of borax in the EU will have to be authorized by the ECHA.[needs update]
A review of the boron toxicity (as boric acid and borates) published in 2012 in theJournal of Toxicology and Environmental Health concluded: "It clearly appears that human B [boron] exposures, even in the highest exposed cohorts, are too low to reach the blood (and target tissue) concentrations that would be required to exert adverse effects on reproductive functions."[76] A draft risk assessment released byHealth Canada in July 2016 has found that overexposure to boric acid has the potential to cause developmental and reproductive health effects. Since people are already exposed to boric acid naturally through their diets and water, Health Canada advised that exposure from other sources should be reduced as much as possible, especially for children and pregnant women.
The concern is not with any one product, but rather multiple exposures from a variety of sources. With this in mind, the department also announced that certain pesticides that contain boric acid, which are commonly used in homes, will have their registrations cancelled and be phased out of the marketplace. As well, new, more protective label directions are being introduced for other boric acid pesticides that continue to be registered in Canada (for example, enclosed bait stations and spot treatments using gel formulations).[77]
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