A borate is any of several

boron Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...

oxyanions,

negative ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s 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 wel ...

, such as

orthoborate In inorganic chemistry, an orthoborate is a polyatomic anion with formula or a salt containing the anion; such as trisodium orthoborate . It is one of several boron oxoanions, or borates. The name is also used in organic chemistry for the trival ...

, metaborate , 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 quantitie ...

with such anions, such as sodium metaborate, and

disodium tetraborate Borax is a salt (ionic compound), a hydrated borate of sodium, with chemical formula often written . It is a colorless crystalline solid, that dissolves in water to make a basic solution. It is commonly available in powder or granular form, ...

. 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 rest ...

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 bioch ...

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 ar ...

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), ma ...

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 (ionic compound), a hydrated borate of sodium, with chemical formula often written . It is a colorless crystalline solid, that dissolves in water to make a basic solution. It is commonly available in powder or granular form, ...

, 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 in 18 ...

. 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 Tetrahydroxyborate is an inorganic anion with the chemical formula or . It contributes no colour to tetrahydroxyborate salts. It is found in the mineral hexahydroborite, , originally formulated . It is one of the boron oxoanions, and acts as a ...

, found in sodium tetrahydroxyborate . *

, found in trisodium orthoborate * perborate , as in sodium perborate * metaborate 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 Suanite is a magnesium borate mineral with formula Mg2B2O5. It was first described in 1953 by Japanese scientist Takeo Watanabe from the University of Tokyo. His first contact with the mineral was during analysis of gold- and copper- bearing skar ...

), * triborate , found in

calcium aluminum triborate Calcium is a chemical element with the Symbol (chemistry), symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties ...

( johachidolite), * tetraborate , found in anhydrous

* tetrahydroxy tetraborate , found in borax "decahydrate" *

tetraborate(6-) In chemistry, tetraborate or pyroborate is an anion (negative ion) with formula ; or a salt (chemistry), salt containing that anion, such as sodium tetraborate, . It is one of the boron oxoacids, that is, a borate. The name is also applied to the ...

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 Boron trioxide or diboron trioxide is the oxide of boron with the formula . It is a colorless transparent solid, almost always glassy (amorphous), which can be crystallized only with great difficulty. It is also called boric oxide or boria. It h ...

and

sodium carbonate Sodium carbonate, , (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield moderately alkaline solutions ...

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 In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of an equilateral triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands a ...

or tetrahedral 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 finding ...

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'', the ...

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 birefring ...

, 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 can act as a weak Brønsted acid, that is, a

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

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 In chemistry, an electron pair or Lewis pair consists of two electrons that occupy the same molecular orbital but have opposite spins. Gilbert N. Lewis introduced the concepts of both the electron pair and the covalent bond in a landmark paper ...

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 ...

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 ...

borate nitrate Borate nitrates are mixed anion compounds containing separate borate and nitrate anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equ ...

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 ...

borate phosphate Borate phosphates are mixed anion compounds containing separate borate and phosphate anions. They are distinct from the borophosphates where the borate is linked to a phosphate via a common oxygen atom. The borate phosphates have a higher ratio of ...

Complex oxyanions containing boron

More complex anions can be formed by condensing borate triangles or tetrahedra with other oxyanions to yield materials such as

borosulfates The borosulfates are heteropoly anion compounds which have sulfate groups attached to boron atoms. Other possible terms are sulfatoborates or boron-sulfur oxides. The ratio of sulfate to borate reflects the degree of condensation. With (SO4)4sup>5 ...

boroselenates The boroselenates are chemical compounds containing interlinked borate and selenate groups sharing oxygen atoms. Both selenate and borate groups are tetrahedral in shape. They have similar structures to borosulfates and borophosphates The borophosp ...

borotellurates The borotellurates are heteropoly anion compounds which have tellurate groups attached to boron atoms. The ratio of tellurate to borate reflects the degree of condensation. In eO4(BO3)2sup>8- the anions are linked into a chain. In eO2(BO3)4sup>10� ...

, boroantimonates,

borophosphates The borophosphates are mixed anion compounds containing borate and phosphate anions, which may be joined together by a common oxygen atom. Compounds that contain water or hydroxy groups can also be included in the class of compounds. Borophosphates ...

, or

boroselenites The boroselenites are heteropoly anion chemical compounds containing selenite and borate groups linked by common oxygen atoms. They are not to be confused with the boroselenates with have a higher oxidation state for selenium, and extra oxygen ...

. Borosilicate glass, also known as pyrex, can be viewed as a

silicate In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is al ...

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, 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 Highest temperature recorded on Earth, hottest place on Earth. Death Valley's Badwater Basin is the ...

and shipped with

twenty-mule team Twenty-mule teams were teams of eighteen mules and two horses attached to large wagons that transported borax out of Death Valley from 1883 to 1889. They traveled from mining, mines across the Mojave Desert to the nearest Rail transport, railroad ...

s from 1883 to 1889. In 1925, deposits were found at

Boron Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

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 in ...

. The Atacama Desert 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 east a ...

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 Inductively coupled plasma atomic emission spectroscopy (ICP-AES), also referred to as inductively coupled plasma optical emission spectroscopy (ICP-OES), is an analytical technique used for the detection of chemical elements. It is a type of emiss ...

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 Zinc borate is an inorganic compound, a borate of zinc. It is a white crystalline or amorphous powder insoluble in water. Its toxicity is low. Its melting point is 980 °C. Variants Several variants of zinc borate exist, differing by the zinc/bo ...

is used as a flame retardant. Some borates with large anions and multiple cations, like and have been considered for applications in nonlinear optics.

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. The ...

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 (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 (also ...

(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 electr ...

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 " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxid ...

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.

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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