A borate is any of a range of

boron Boron is a chemical element; it has 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 three ...

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 ...

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 equal and opposite to the charge of a proton, which is considered to be positive by conven ...

s containing

and

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

, such as

orthoborate In inorganic chemistry, an orthoborate is a Polyatomic ion, polyatomic anion with formula or a salt (chemistry), salt containing the anion; such as trisodium orthoborate . It is one of several boron oxyanions, or borates. The name is also used i ...

metaborate A metaborate is a borate anion consisting of boron and oxygen, with empirical formula . Metaborate also refers to any salt (chemistry), salt or ester of such anion (e.g. salts such as sodium metaborate or calcium metaborate , and esters such as ...

, or

tetraborate In chemistry, tetraborate or pyroborate is an anion (negative ion) with formula ; or a 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 hydrated ion a ...

; or any

salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

of such anions, such as

sodium metaborate Sodium metaborate is a chemical compound of sodium, boron, and oxygen with formula . However, the metaborate ion is trimeric in the anhydrous solid, therefore a more correct formula is or . The formula can be written also as to highlight the re ...

, and

borax The BORAX Experiments were a series of safety experiments on boiling water nuclear reactors conducted by Argonne National Laboratory in the 1950s and 1960s at the National Reactor Testing Station in eastern Idaho.

. The name also refers to

ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...

s of such anions, such as

trimethyl borate Trimethyl borate is the organoboron compound with the formula B(OCH3)3. It is a colourless liquid that burns with a green flame. It is an intermediate in the preparation of sodium borohydride and is a popular reagent in organic chemistry. It i ...

Natural occurrence

Borate ions occur, alone or with other anions, in many

borate A borate is any of a range of boron oxyanions, anions containing boron and oxygen, such as orthoborate , metaborate , or tetraborate ; or any salt of such anions, such as sodium metaborate, and borax . The name also refers to esters of su ...

and

borosilicate Borosilicate glass is a type of glass with silicon dioxide, silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficient of thermal expansion, coefficients of thermal expansion ( ...

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

s such as

boracite Boracite is a magnesium borate mineral with formula: Mg3 B7 O13 Cl. It occurs as blue green, colorless, gray, yellow to white crystals in the orthorhombic - pyramidal crystal system. Boracite also shows pseudo-isometric cubical and octahedral fo ...

ulexite Ulexite (), sometimes called TV rock or TV stone due to its unusual optical properties, is a hydrous borate hydroxide of sodium and calcium with the chemical formula . The mineral occurs as silky white rounded crystalline masses or in parall ...

(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, contributing to the absorption of low-frequency sound in seawater. Common borate salts include

(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. It is thought to be the Highest temperature recorded on Earth, hottest place on Earth during summer. Death Valley's Badwat ...

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 1898. They traveled from mining, mines across the Mojave Desert to the nearest Rail transport, railr ...

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

Boron Boron is a chemical element; it has 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 three ...

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

on the edge of the

Mojave Desert The Mojave Desert (; ; ) is a desert in the rain shadow of the southern Sierra Nevada mountains and Transverse Ranges in the Southwestern United States. Named for the Indigenous peoples of the Americas, indigenous Mohave people, it is located pr ...

. The

Atacama Desert The Atacama Desert () is a desert plateau located on the Pacific Ocean, Pacific coast of South America, in the north of Chile. Stretching over a strip of land west of the Andes Mountains, it covers an area of , which increases to if the barre ...

Chile Chile, officially the Republic of Chile, is a country in western South America. It is the southernmost country in the world and the closest to Antarctica, stretching along a narrow strip of land between the Andes, Andes Mountains and the Paci ...

also contains mineable borate concentrations. 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 ...

, found in

sodium tetrahydroxyborate Sodium tetrahydroxyborate is a salt (ionic compound) with chemical formula or . It is one of several sodium borates. At room temperature it is a colorless crystalline solid. The element ratio corresponds to the oxide mixture , but the structure ...

. *

, found in

trisodium orthoborate Trisodium borate is a chemical compound of sodium, boron, and oxygen, with formula , or . It is a sodium salt of the orthoboric acid . The compound is also called trisodium orthoborate, sodium orthoborate, or just sodium borate. However, "sodium ...

* , found in the calcium yttrium borosilicate oxyapatite * perborate , as in

sodium perborate Sodium perborate are chemical compounds with chemical formula (H2O)x. Commonly encountered salts are the anhydrous form (x = 0) and as a hydrate, hexahydrate (x = 6). These two species are sometimes called, respectively, "monohydrate" or PBS-1 a ...

or its cyclic trimer , found in

* diborate , found in magnesium diborate (suanite) , * triborate , found in calcium aluminium triborate (johachidolite) , *

, found in anhydrous

* tetrahydroxytetraborate , found in borax "decahydrate" * tetraborate(6-) , found in lithium tetraborate(6-) * 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 colorless crystalline solid, soluble in water. The compound is often encountered or traded ...

* 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 as ...

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, sal soda, and soda crystals) is the inorganic compound with the formula and its various hydrates. All forms are white, odourless, water-soluble salts that yield alkaline solutions in water ...

yielded on cooling two crystalline compounds with definite compositions, consistent with anhydrous borax (which can be written ) and sodium octaborate (which can be written ).

Structures

Borate anions (and 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 ...

and/or

tetrahedral In geometry, a tetrahedron (: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (geometry), vertices. The tet ...

structural units, joined together via shared oxygen atoms (corners) or atom pairs (edges) into larger clusters so as to construct various ions such as , , , , , etc. These anions may be cyclic or linear in structure, and can further polymerize into infinite chains, layers, and tridimensional frameworks. The terminal (unshared) oxygen atoms in the borate anions may be capped with hydrogen atoms () or may carry a negative charge (). The planar units may be stacked in the crystal lattice 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 In physics, acoustics, and telecommunications, a harmonic is a sinusoidal wave with a frequency that is a positive integer multiple of the ''fundamental frequency'' of a periodic signal. The fundamental frequency is also called the ''1st har ...

s generation,

birefringence Birefringence, also called double refraction, 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 described as birefrin ...

, and UV transmission. Polymeric borate anions may have linear chains of 2, 3 or 4 trigonal structural units, each sharing oxygen atoms with adjacent unit(s). as in , contain chains of trigonal structural units. Other anions contain cycles; for instance, and contain the cyclic ion, consisting of a six-membered ring of alternating boron and oxygen atoms with one extra oxygen atom attached to each boron atom. 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 orthoborate, trihydroxidoboron or boracic acid. It is usually encountered as colorless crystals or a white ...

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

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

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 ...

, 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

hydroxide ion Hydroxide is a polyatomic ion, diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually Self-ionization ...

produced by the water

autoprotolysis In chemistry, autoprotolysis is a molecular autoionization, a chemical reaction in which a proton is transferred between two identical molecules, one of which acts as a Brønsted acid, releasing a proton that is accepted by the other molecule, wh ...

: : + 2 + (p''K'' = 8.98) This reaction is very fast, with a 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 to one second, ...

. 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

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 . Like the complexed borates mentioned above, these ions are more acidic than boric acid. As a result, the pH of a concentrated polyborate solution will increase more than expected when diluted with water.

Borate salts

Several metal borates are known. They 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 carbonates, 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 terms ...

s, borate phosphates.

Complex oxyanions containing boron

More complex anions can be formed by condensing borate triangles or tetrahedra with other

s 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 borotellu ...

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.

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), ma ...

, also known as

pyrex Pyrex (trademarked as ''PYREX'' and ''pyrex'') is a brand introduced by Corning Inc. in 1915, initially for a line of clear, low-thermal-expansion borosilicate glass used for laboratory glassware and kitchenware. It was later expanded in the 1 ...

, can be viewed as a

silicate 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 also used ...

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. It forms a glass with a low

coefficient of thermal expansion Thermal expansion is the tendency of matter to increase in length, area, or volume, changing its size and density, in response to an increase in temperature (usually excluding phase transitions). Substances usually contract with decreasing temp ...

, thus resistant to cracking when heated, unlike

soda glass Soda or SODA may refer to: *Soft drink, a sweetened, carbonated, and usually flavored drink Chemistry * Some chemical compounds containing sodium ** Sodium carbonate, washing soda or soda ash ** Sodium bicarbonate, baking soda ** Sodium hyd ...

Uses

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 emis ...

and

ICP-MS Inductively coupled plasma mass spectrometry (ICP-MS) is a type of mass spectrometry that uses an inductively coupled plasma to ionize the sample. It atomizes the sample and creates atomic and small polyatomic ions, which are then detected. It ...

. Borate fusion and energy dispersive X-ray fluorescence spectrometry with polarized excitation have been used to analyse 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 as ...

tetrahydrate (commonly abbreviated DOT) is used as a

wood preservative Wood preservation refers to any method or process, or even technique, used to protect the wood and extend its service life. Most wood species are susceptible to both biological (''biotic'') and non-biological (''abiotic'') factors that cause d ...

or fungicide.

Zinc borate Zinc borate refers to a family of inorganic compounds consisting of borate of zinc. They are white solids with the formulas 4ZnO·B2O3·H2O, ZnO·B2O3·1.12H2O, ZnO·B2O3·~2H2O, 6ZnO·5B2O3·3H2O, 2ZnO·3B2O3·7H2O, 2ZnO·3B2O3·3H2O, 3ZnO·5B2O3 ...

is used as a

flame retardant Flame retardants are 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 combustion, ignition source and pr ...

. 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 Nonlinearity, 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 ...

Borate esters

Borate ester In organic chemistry, borate esters are organoboron compounds which are conveniently prepared by the stoichiometric condensation reaction of boric acid with alcohols. There are two main classes of borate esters: orthoborates, and metaborates, ...

s are

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

s, which are conveniently prepared by the

stoichiometric Stoichiometry () is the relationships between the masses of reactants and products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must equal the total m ...

condensation reaction of boric acid with

alcohols In chemistry, an alcohol (), is a type of organic compound that carries at least one hydroxyl () functional group bound to a Saturated and unsaturated compounds, saturated carbon atom. Alcohols range from the simple, like methanol and ethanol ...

(or their

chalcogen The chalcogens (ore forming) ( ) are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. Group 16 consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and the rad ...

analogs).

Thin films

Metal borate thin films have been grown by a variety of techniques, including liquid-phase

epitaxy Epitaxy (prefix ''epi-'' means "on top of”) is 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 cry ...

(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 ...

(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 wiktionary:precu ...

(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), hypothesized genus of fossil birds that was composed of fossilized parts of unre ...

composed of the tris(pyrazolyl)borate

ligand In coordination chemistry, a ligand is an ion or molecule with a 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 el ...

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 ''electr ...

to deposit CaB₂O₄, SrB₂O₄, BaB₂O₄, Mn₃(BO₃)₂, and CoB₂O₄ films.

Physiology

Borate anions are found largely as 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 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 severely abraded skin. It is distributed throughout the body, 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 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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