Borates
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A borate is any of several boron
oxyanion An oxyanion, or oxoanion, is an ion with the generic formula (where A represents a chemical element and O represents an oxygen atom). Oxyanions are formed by a large majority of the chemical elements. The formulae of simple oxyanions are determine ...
s, negative ions consisting of boron and
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...
, such as orthoborate ,
metaborate A metaborate is an anion (negative ion) consisting of boron and oxygen, with empirical formula ; or any salt with such anions, such as sodium metaborate, or calcium metaborate . It is one of the boron oxoanions or borates In aqueous solut ...
, or tetraborate ; or any
salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quanti ...
with such anions, such as sodium metaborate, and disodium tetraborate . The name also refers to certain
functional group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the res ...
s in
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
s consisting of boron and oxygen, and
ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ...
s with such groups, such as triethyl orthoborate .


Natural occurrence

Borate ions occur, alone or with other anions, in many borate and
borosilicate Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ...
s such as
borax Borax is a salt (chemistry), salt (ionic compound), a hydration (chemistry), hydrated borate of sodium, with chemical formula often written . It is a colorless crystalline solid, that dissolves in water to make a base (chemistry), basic aqueo ...
, boracite, ulexite (boronatrocalcite) and
colemanite Colemanite (Ca2B6O11·5H2O) or (CaB3O4(OH)3·H2O) is a borate mineral found in evaporite deposits of alkaline lacustrine environments. Colemanite is a secondary mineral that forms by alteration of borax and ulexite. It was first described ...
. Borates also occur in seawater, where they make an important contribution to the absorption of low frequency sound in seawater. Borates also occur in plants, including almost all fruits.


Anions

The main borate anions are: * tetrahydroxyborate , found in sodium tetrahydroxyborate . * orthoborate , found in trisodium orthoborate * perborate , as in
sodium perborate Sodium perborate is chemical compound whose chemical formula may be written , , or, more properly, ·. Its name is sometimes abbreviated as PBS (not to be confused with phosphate-buffered saline). The compound is commonly encountered in anhydr ...
*
metaborate A metaborate is an anion (negative ion) consisting of boron and oxygen, with empirical formula ; or any salt with such anions, such as sodium metaborate, or calcium metaborate . It is one of the boron oxoanions or borates In aqueous solut ...
or , found in sodium metaborate * diborate , found in
magnesium diborate Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
( suanite), * triborate , found in calcium aluminum triborate ( johachidolite), * tetraborate , found in anhydrous
borax Borax is a salt (chemistry), salt (ionic compound), a hydration (chemistry), hydrated borate of sodium, with chemical formula often written . It is a colorless crystalline solid, that dissolves in water to make a base (chemistry), basic aqueo ...
* tetrahydroxy tetraborate , found in borax "decahydrate" * tetraborate(6-) found in
lithium tetraborate(6-) Lithium borate, also known as lithium tetraborate is an inorganic compound with the formula Li2B4O7. A colorless solid, lithium borate is used in making glasses and ceramics. Structure Its structure consists of a polymeric borate backbone. The Li ...
* pentaborate or , found in
sodium pentaborate Sodium pentaborate, more properly disodium decaborate, is a chemical compound of sodium, boron, and oxygen; a salt with elemental formula , , or . It is a transparent colorless crystalline solid, soluble in water. The compound is often encounte ...
* octaborate found in
disodium octaborate Disodium octaborate is a borate of sodium, a chemical compound of sodium, boron, and oxygen — a salt with elemental formula or , also written as . It is a colorless crystalline solid, soluble in water. Disodium octaborate is traded either a ...


Preparation

In 1905, Burgess and Holt observed that fusing mixtures of boric oxide and sodium carbonate yielded on cooling two crystalline compounds with definite compositions, consistent with anhydrous borax = and sodium octaborate = .


Structures

Borate anions or functional groups consist of trigonal planar or
tetrahedral In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the ...
structural units, joined together via shared oxygen atoms (corners) or atom pairs (edges) into larger clusters so as to construct various , , , , , etc. These anions may be cyclic or linear in structure, and can further polymerize into infinite chains, layers, and frameworks. The terminal (unshared) atoms in the anions may be capped with hydrogen atoms () or may carry a negative charge (). The planar units may be stacked in the crystal lattice so as to have π-conjugated
molecular orbital In chemistry, a molecular orbital is a mathematical function describing the location and wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of findi ...
s, which often results in useful optical properties such as strong
harmonic A harmonic is a wave with a frequency that is a positive integer multiple of the '' fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', ...
s generation,
birefringence Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefrin ...
, and UV transmission. Polymeric borate anions may have linear chains of 2, 3 or 4 trigonal BO3 structural units, each sharing oxygen atoms with adjacent unit(s). as in LiBO2, contain chains of trigonal BO3 structural units. Other anons contain cycles; for instance, NaBO2 and KBO2 contain the cyclic 3O6sup>2− ion. The thermal expansion of crystalline borates is dominated by the fact that and polyhedra and rigid groups consisting of these polyhedra practically do not change their configuration and size upon heating, but sometimes rotate like hinges, which results in greatly anisotropic thermal expansion including linear negative expansion.


Reactions


Aqueous solution

In aqueous solution,
boric acid Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula . It may also be called hydrogen borate or boracic acid. It is usually encountered as colorless crystals or a white powder, that dissolve ...
can act as a weak Brønsted acid, that is, a proton donor, with p''K''a ~ 9. However, it more often acts as a
Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
, accepting an electron pair from a hydroxyl ion produced by the water autoprotolysis: : + 2 + (p''K'' = 8.98) This reaction is very fast, with characteristic time less than 10
μs A microsecond is a unit of time in the International System of Units (SI) equal to one millionth (0.000001 or 10−6 or ) of a second. Its symbol is μs, sometimes simplified to us when Unicode is not available. A microsecond is equal to 1000 ...
. Polymeric boron oxoanions are formed in aqueous solution of boric acid at pH 7–10 if the boron concentration is higher than about 0.025 mol/L. The best known of these is the tetraborate ion , found in the mineral borax: :4 + 2 + 7 Other anions observed in solution are triborate(1−) and pentaborate(1−), in equilibrium with boric acid and tetrahydroxyborate according to the following overall reactions: : 2 + + 3 (fast, p''K'' = —1.92) : 4 + + 6 (slow, p''K'' = —2.05) In the pH range 6.8 to 8.0, any alkali salts of "boric oxide" anions with general formula where 3''x''+''q'' = 2''y'' + ''z'' will eventually equilibrate in solution to a mixture of , , , and . These ions, similarly to the complexed borates mentioned above, are more acidic than boric acid itself. As a result of this, the pH of a concentrated polyborate solution will increase more than expected when diluted with water.


Borate salts

A number of metal borates are known can be obtained by treating boric acid or boron oxides with metal oxides.


Mixed anion salts

Some chemicals contain another anion in addition to borate. These include
borate chloride The borate chlorides are chemical compounds that contain both borate ions and chloride ions. They are mixed anion compounds. Many of them are minerals. Those minerals that crystallise with water (hydrates) may be found in evaporite An evaporite ...
s,
borate carbonate The borate carbonates are mixed anion compounds containing both borate and carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate ...
s, borate nitrates,
borate sulfate Borate sulfates are mixed anion compounds containing separate borate and sulfate anions. They are distinct from the borosulfates The borosulfates are heteropoly anion compounds which have sulfate groups attached to boron atoms. Other possible ter ...
s, borate phosphates.


Complex oxyanions containing boron

More complex anions can be formed by condensing borate triangles or tetrahedra with other
oxyanion An oxyanion, or oxoanion, is an ion with the generic formula (where A represents a chemical element and O represents an oxygen atom). Oxyanions are formed by a large majority of the chemical elements. The formulae of simple oxyanions are determine ...
s to yield materials such as borosulfates, boroselenates, borotellurates, boroantimonates, borophosphates, or boroselenites.
Borosilicate glass Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...
, also known as pyrex, can be viewed as a silicate in which some iO4sup>4− units are replaced by O4sup>5− centers, together with additional cations to compensate for the difference in valence states of Si(IV) and B(III). Because this substitution leads to imperfections, the material is slow to crystallise and forms a glass with low
coefficient of thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kinetic ...
, thus resistant to cracking when heated, unlike
soda glass Soda or SODA may refer to: Chemistry * Some chemical compounds containing sodium ** Sodium carbonate, washing soda or soda ash ** Sodium bicarbonate, baking soda ** Sodium hydroxide, caustic soda ** Sodium oxide, an alkali metal oxide * So ...
.


Uses

Common borate salts include sodium metaborate (NaBO2) and borax. Borax is soluble in water, so mineral deposits only occur in places with very low rainfall. Extensive deposits were found in
Death Valley Death Valley is a desert valley in Eastern California, in the northern Mojave Desert, bordering the Great Basin Desert. During summer, it is the hottest place on Earth. Death Valley's Badwater Basin is the point of lowest elevation in North Am ...
and shipped with twenty-mule teams from 1883 to 1889. In 1925, deposits were found at Boron,
California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the ...
on the edge of the
Mojave Desert The Mojave Desert ( ; mov, Hayikwiir Mat'aar; es, Desierto de Mojave) is a desert in the rain shadow of the Sierra Nevada mountains in the Southwestern United States. It is named for the indigenous Mojave people. It is located primarily i ...
. The
Atacama Desert The Atacama Desert ( es, Desierto de Atacama) is a desert plateau in South America covering a 1,600 km (990 mi) strip of land on the Pacific coast, west of the Andes Mountains. The Atacama Desert is the driest nonpolar desert in the ...
in
Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the eas ...
also contains mineable borate concentrations.
Lithium metaborate Lithium metaborate is a chemical compound of lithium, boron, and oxygen with elemental formula . It is often encountered as a hydrate, , where ''n'' is usually 2 or 4. However, these formulas do not describe the actual structure of the solids. ...
, lithium tetraborate, or a mixture of both, can be used in borate fusion sample preparation of various samples for analysis by XRF, AAS, ICP-OES and ICP-MS. Borate fusion and energy dispersive X-ray fluorescence spectrometry with polarized excitation have been used in the analysis of contaminated soils.
Disodium octaborate Disodium octaborate is a borate of sodium, a chemical compound of sodium, boron, and oxygen — a salt with elemental formula or , also written as . It is a colorless crystalline solid, soluble in water. Disodium octaborate is traded either a ...
tetrahydrate (commonly abbreviated DOT) is used as a wood preservative or fungicide. Zinc borate is used as a
flame retardant The term flame retardants subsumes a diverse group of chemicals that are added to manufactured materials, such as plastics and textiles, and surface finishes and coatings. Flame retardants are activated by the presence of an ignition source a ...
. Some borates with large anions and multiple cations, like and have been considered for applications in
nonlinear optics Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in ''nonlinear media'', that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typic ...
.


Borate esters

Borate esters are
organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon- hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
s, which are conveniently prepared by the stoichiometric condensation reaction of boric acid with alcohols.


Thin films

Metal borate thin films have been grown by a variety of techniques, including liquid-phase
epitaxy Epitaxy refers to a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited crystalline film is called an epit ...
(e.g. FeBO3, β‐BaB2O4), electron-beam evaporation (e.g. CrBO3, β‐BaB2O4),
pulsed laser deposition Pulsed laser deposition (PLD) is a physical vapor deposition (PVD) technique where a high-power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. This material is vaporized from the t ...
(e.g. β‐BaB2O4,  Eu(BO2)3), and
atomic layer deposition Atomic layer deposition (ALD) is a thin-film deposition technique based on the sequential use of a gas-phase chemical process; it is a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals called precursors (a ...
(ALD). Growth by ALD was achieved using
precursors Precursor or Precursors may refer to: * Precursor (religion), a forerunner, predecessor ** The Precursor, John the Baptist Science and technology * Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of un ...
composed of the tris(pyrazolyl)borate
ligand In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ele ...
and either ozone or water as the
oxidant An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or "Electron acceptor, accepts"/"receives" an electron from a (called the , , or ). In ...
to deposit CaB2O4, SrB2O4, BaB2O4, Mn3(BO3)2, and CoB2O4 films.


Physiology

Borate anions are largely in the form of the undissociated acid in aqueous solution at physiological pH. No further metabolism occurs in either animals or plants. In animals, boric acid/borate salts are essentially completely absorbed following oral ingestion. Absorption occurs via inhalation, although quantitative data are unavailable. Limited data indicate that boric acid/salts are not absorbed through intact skin to any significant extent, although absorption occurs through skin that is severely abraded. It distributes throughout the body and is not retained in tissues, except for bone, and is rapidly excreted in the urine.


See also

* Nanochannel glass materials * Porous glass * Vycor glass * Silly Putty *
Slime (toy) Slime was a toy product manufactured by Mattel, sold in a plastic trash can and introduced in February 1976. It consisted of a non-toxic viscous, squishy and oozy green or other color material made primarily from guar gum. Different variati ...
* Tris(2,2,2-trifluoroethyl) borate


References

Robert K. Momii and Norman H. Nachtrieb (1967): "Nuclear Magnetic Resonance Study of Borate-Polyborate Equilibria in Aqueous Solution". ''Inorganic Chemistry'', volume 6, issue 6, pages 1189-1192. Miriding Mutailipu, Min Zhang, Xiaoyu Dong, Yanna Chen, and Shilie Pan (2016): "Effects of the Orientation of 5O11sup>7– Fundamental Building Blocks on Layered Structures Based on the Pentaborates". ''Inorganic Chemistry'', volume 55, issue 20, pages 10608–10616. U.S. Environmental Protection Agency (2005), "Boric Acid/Sodium Borate Salts". HED Chapter of the Tolerance Reassessment Eligibility Decision Document (TRED), EPA-HQ-OPP-2005-0062-0004, p.11 (January 2006). As cited by PubChem. Charles Hutchens Burgess and Alfred Holt (1905): "Some physical characters of the sodium borates, with a new and rapid method for the determination of melting points." ''Proceedings of the Royal Society of London, volume 74, pages 285–295. Wiberg E. and Holleman A.F. (2001) ''Inorganic Chemistry'', Elsevier Ingri N. (1962) ''Acta Chem. Scand.'', 16, 439. Rimma S. Bubnova and Stanislav K. Filatov (2008): "Strong anisotropic thermal expansion in borates". ''Basic Solid State Physics'', volume 245, issue 11, pages 2469-2476.


External links


Suanite at webmineral

Johachidolite at webmineral

Non-CCA Wood Preservatives: Guide to Selected Resources - National Pesticide Information Center
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