Borax

Borax, also known as sodium borate, sodium tetraborate, or disodium tetraborate, is a compound with formula Na
2H
4B
4O
9•nH
2O or, more precisely, [Na•(H
2O)+
m]
2 [B
4O
5(OH)2−
4].[9][10]

For other uses, see Borax (disambiguation).

Borax
Names
IUPAC name
Sodium tetraborate decahydrate
Other names
Borax octahydrate
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
EC Number
  • 215-540-4
E number E285 (preservatives)
KEGG
UNII
Properties
Na2B4O7·10H2O or Na2[B4O5(OH)4]·8H2O
Molar mass 201.22 (anhydrous)
381.38 (decahydrate)
Appearance white solid
Density 2.4 g/cm3 (anhydrous, solid)[1]
1.73 g/cm3 (decahydrate, solid)[1]
Melting point 743 °C (1,369 °F; 1,016 K) (anhydrous)[1]
75 °C (decahydrate, decomposes)[1]
Boiling point 1,575 °C (2,867 °F; 1,848 K) (anhydrous)[1]
31.7 g/L (both)[1]
−85.0·10−6 cm3/mol (anhydrous)[2]
n1=1.447, n2=1.469, n3=1.472 (decahydrate)[3]
Structure[4]
Monoclinic, mS92, No. 15
C2/c
2/m
a = 1.1885 nm, b = 1.0654 nm, c = 1.2206 nm
α = 90°, β = 106.623°, γ = 90°
1.4810 nm3
4
Pharmacology
S01AX07 (WHO)
Hazards
GHS pictograms
H360
P201, P308+313
NFPA 704 (fire diamond)
NIOSH (US health exposure limits):
PEL (Permissible)
 none[6]
REL (Recommended)
 TWA 1 mg/m3 (anhydrous and pentahydrate)[6][7]
TWA 5 mg/m3 (decahydrate)[8]
IDLH (Immediate danger)
 N.D.[6]
Related compounds
Other anions
 Sodium aluminate
Other cations
 Lithium tetraborate
Related compounds
 Boric acid, sodium perborate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

The formula is often improperly written as Na
2B
4O
7•(n+2)H
2O, reflecting an older incorrect understanding of the anion's molecular structure. The name may refer to any of a number of closely related boron-containing mineral or chemical compounds that differ in their water of crystallization content. The most commonly encountered one is the octahydrate Na
2H
4B
4O
9•8H
2O or [Na(H
2O)+
4]
2 [B
4O
5(OH)2−
4] (or Na
2B
4O
7•10H
2O, the "decahydrate", in the older notation).[9] It is a colorless crystalline solid that dissolves in water.

Borax is a component of many detergents, cosmetics, and enamel glazes. It is used to make buffer solutions in biochemistry, as a fire retardant, as an anti-fungal compound, in the manufacture of fiberglass, as a flux in metallurgy, neutron-capture shields for radioactive sources, a texturing agent in cooking, as a cross-linking agent in slime, as an alkali in photographic developers, as a precursor for other boron compounds, and is useful as an insecticide (similarly to boric acid).

In artisanal gold mining, borax is sometimes used as part of a process (as a flux) meant to eliminate the need for toxic mercury in the gold extraction process, although it cannot directly replace mercury. Borax was reportedly used by gold miners in parts of the Philippines in the 1900s.[11]

Borax was first discovered in dry lake beds in Tibet and was imported via the Silk Road to the Arabian Peninsula in the 8th century AD.[12] 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 20 Mule Team Borax trademark, named for the method by which borax was originally hauled out of the California and Nevada deserts.[13][14]

Chemistry
The structure of borax according to X-ray crystallography.

Structure

The term borax is often used for a number of closely related minerals or chemical compounds that differ in their crystal water content:

  • anhydrous sodium tetraborate, Na2B4O7
  • sodium tetraborate pentahydrate, Na2B4O7·5H2O
  • sodium tetraborate decahydrate, Na2B4O7·10H2O or equivalently the octahydrate, Na2B4O5(OH)4·8H2O

From the 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.

Reactions

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:

Na2B4O7·10H2O + 2 HCl → 4 B(OH)3 + 2 NaCl + 5H2O

The "decahydrate" is sufficiently stable to find use as a primary standard for acid base titrimetry.[15]

When borax is added to a flame, it produces a yellow green color.[16] Borax is not used for this purpose in fireworks due to the overwhelming yellow color of sodium. Boric acid is used to color methanol flames a transparent green.

Borax is very soluble in ethylene glycol, moderately soluble in diethylene glycol and methanol, slightly soluble in acetone.[17] It is poorly soluble in cold water, but its solubility increases significantly with temperature.[17][18]

Etymology

The English word borax is Latinized: the Middle English form was boras, from Old French boras, bourras. That may have been from medieval Latin baurach (another English spelling), borac(-/um/em), borax, along with Spanish borrax (> borraj) and Italian borrace, in the 9th century. Another name for borax is tincal, from Sanskrit.[12]

The word tincal /ˈtɪŋkəl/ "tinkle", or tincar /ˈtɪŋkər/ "tinker", refers to crude borax, before it is purified, as mined from lake deposits in Tibet, Persia, and other parts of Asia. The word was adopted in the 17th century from Malay tingkal and from Urdu/Persian/Arabic تنکار‎ tinkār/tankār; thus the two forms in English. These all appear to be related to the Sanskrit टांकण ṭānkaṇa.[19][20]

Natural sources
Borax "cottonball"

Borax occurs naturally in evaporite deposits produced by the repeated evaporation of seasonal lakes. Fibrous nodules of ulexite and proberite (sodium and calcium borate minerals) known as "cottonball" were found in salt deposits on the valley floor of California's Death Valley in the 1880s and taken to market on 20-mule team wagons. By the 1890s railroads were transporting purer forms of mined borax such as colemanite, kernite, priceite, and ulexite.[21]

The most commercially important deposits are found in: Turkey; Boron, California; and Searles Lake, California. Borax has been found at many other locations in the Southwestern United States, the Atacama desert in Chile, and in Bolivia, Tibet, and Romania.

Naturally occurring borax is refined by a process of recrystallization.[22]

Borax can be produced synthetically from other boron compounds.

Uses
Borax-based laundry detergent

Household products

Borax is used in various household laundry and cleaning products,[23] including the "20 Mule Team Borax" laundry booster, "Boraxo" powdered hand soap, and some tooth bleaching formulas.[24]

pH buffer

Borate ions (commonly supplied as boric acid) are used in biochemical and chemical laboratories to make buffers, e.g. for polyacrylamide gel electrophoresis of DNA and RNA, such as TBE buffer (borate buffered tris-hydroxymethylaminomethonium)[25][26] or the newer SB buffer or BBS buffer (borate buffered saline) in coating procedures. Borate buffers (usually at pH 8) are also used as preferential equilibration solution in dimethyl pimelimidate (DMP) based crosslinking reactions.

Co-complexing agent

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.

Water-softening agent

Borax alone does not have a high affinity for the hardness cations, although it has been used for water-softening. Its chemical equation for water-softening is given below:

Ca2+ (aq) + Na2B4O7 (aq) → Ca B4O7 (s)↓ + 2 Na+ (aq)
Mg2+ (aq) + Na2B4O7 (aq) → Mg B4O7 (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.

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, where it allows the molten solder to wet the metal and flow evenly into the joint. Borax is also a good flux for "pre-tinning" tungsten with zinc — making the tungsten soft-solderable.[27] Borax is often used as a flux for forge welding.

Small-scale gold mining
Traction engine hauling borax, Death Valley, 1904

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.[28]

Flubber
Main article: Flubber (material)

A rubbery polymer sometimes called Slime, Flubber, 'gluep' or 'glurch' (or erroneously called Silly Putty, which is based on silicone polymers), can be made by cross-linking polyvinyl alcohol with borax. Making flubber from polyvinyl acetate-based glues, such as Elmer's Glue, and borax is a common elementary-science demonstration.[29][30]

Food additive

Borax, given the E number E285, is used as a food additive, but is banned in some countries, including the United States, China,[31] and Thailand. As a consequence, certain foods, such as caviar, produced for sale in the United States contain higher levels of salt to assist preservation.[32] 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 named; lamian, shahe fen, kway teow, and chee cheong fun.[33] 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 five to ten years.[34] Borax was one of the chemicals used in 19th century industrialised food production that was investigated by Dr. Harvey W. Wiley with his famous 'Poison Squad' as part of the US Department of Agriculture. This led up to the 1906 Pure Food and Drug Act, a landmark event in the early history of food regulation in the United States.

Other uses
Rio Tinto borax mine pit, Boron, California

Medicinal

Other

Toxicity

Borax, sodium tetraborate decahydrate, according to one study, is not acutely toxic. Its LD50 (median lethal dose) score is tested at 4.55-6.08 g/kg in rats as determined by US EPA (1969),[51] later in 1972 found to be 4.5 g/kg,[52] meaning that a significant dose of the chemical is needed to cause severe symptoms or death. The lethal dose is not necessarily the same for humans. On pesticide information websites it is listed as a non-lethal compound and of no hazardous concerns.

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.[53][54]

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.[53]

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 of cytotoxicity to the target organ."[55] 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.

Sodium tetraborate decahydrate has no known hazard issues.[56]

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 including nausea, persistent vomiting, abdominal pain, and diarrhea. Effects on the vascular system and human 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.[57]

Due to concerns about the toxicity of borax which was withdrawn as a cleaning and laundry product, sodium sesquicarbonate is sold in the European Union (EU) as "Borax substitute". It is also known as one of the E number food additives E500.

During the 1951 Indianapolis 500, race winner Lee Wallard coated his racing suit with a fire retardant mixture of borax crystals and water. He suffered serious chafing, and required treatment at the infield hospital after the victory lane celebration.

Risk to fertility and pregnancy

Borax was added to the Substance of Very High Concern (SVHC) candidate list on December 16, 2010. The SVHC candidate 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".[58] It was proposed for addition to REACH Annex XIV by the ECHA on July 1, 2015.[59] If this recommendation is approved, all imports and uses of borax in the EU will have to be authorized by the ECHA.

Review of the boron toxicity (as boric acid and borates) published 2012 in Journal 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."[60]

A draft risk assessment released by Health 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 registrations for 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).[61]

See also

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Cited sources
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