Borates are chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

s which contain oxoanions of boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...

 in oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

 +3. The simplest borate ion, BO33−, has a trigonal planar
Trigonal planar
In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of a triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands are identical and all bond angles are 120°. Such species belong to...

 structure. Other borates are made up of trigonal BO3 or tetrahedral BO4 structural units, sharing oxygen atoms. Boron occurs in nature mainly as borate mineral
Borate mineral
The borate minerals are minerals which contain a borate anion group. The borate units may be polymerised similar to the SiO4 unit of the silicate mineral class. This results in B2O5, B3O6, B2O4 anions as well as more complex structures which include hydroxide or halogen anions...

s and borosilicates.


When boron forms three covalent bonds, as in the borate ion, BO33-, it has a share in three pairs of electrons. VSEPR theory
VSEPR theory
Valence shell electron pair repulsion theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie–Nyholm theory after its two main developers...

 predicts a trigonal planar structure as this is the configuration that minimizes the energy of repulsion between electrons. It also predicts that in compounds where the boron atom bonds with three oxygen atoms the inter-bond angle will be 120°. In terms of valence bond theory
Valence bond theory
In chemistry, valence bond theory is one of two basic theories, along with molecular orbital theory, that were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of the dissociated atoms combine to give individual chemical bonds...

 the bonds are formed by using sp2 hybrid orbitals. The fact that there is an incomplete octet
Octet rule
The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low (The octet rule is a chemical rule of thumb that states that atoms of low (...

 means that these compounds are Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...


When a trigonal boron atom accepts a pair of electrons from a Lewis base, it adopts a tetrahedral configuration (sp3), and the octet rule is satisfied. Both trigonal and tetrahedral units can co-exist in a complex borate, such as the anion in borax (crystal structure below).

Aqueous chemistry

Boric acid, B(OH)3, while sometimes indicated as dissociate in aqueous solution,
B(OH)3 BH2O3 + H+; Ka = 5.794 x 10−10 mol/l (computed from pKa = 9.237 in citation).

it may also be represented as acidic due to its hydrolysis reaction with water molecules, forming tetrahydroxyborate
Tetrahydroxyborate, [H4BO4]− or B, is a boron oxoanion with a tetrahedral geometry. It is isoelectronic with the hypothetical compound orthocarbonic acid.B is formed by the addition of hydroxide, OH−, to boric acid, B3:...

 and releasing a proton:
B(OH)3 + H2O B(OH) + H+; Ka = 5.8x10−10 mol/l; pKa = 9.24.

In the presence of cis-diol
A diol or glycol is a chemical compound containing two hydroxyl groups A geminal diol has two hydroxyl groups bonded to the same atom...

s such as mannitol
Mannitol is a white, crystalline organic compound with the formula . This polyol is used as an osmotic diuretic agent and a weak renal vasodilator...

, glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

, sorbitol
Sorbitol, also known as glucitol, Sorbogem® and Sorbo®, is a sugar alcohol that the human body metabolizes slowly. It can be obtained by reduction of glucose, changing the aldehyde group to a hydroxyl group. Sorbitol is found in apples, pears, peaches, and prunes...

 and glycerol
Glycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...

 the pK is lowered to about 4.

Under acid conditions boric acid may undergo condensation reactions to form polymeric oxyanion
An oxyanion or oxoanion is a chemical compound with the generic formula AxOyz− . Oxoanions are formed by a large majority of the chemical elements. The formulae of simple oxoanions are determined by the octet rule...

s. The equilibrium
4 [B(OH)4]- + 2 H+ [B4O5(OH)4]2− + 7 H2O

illustrates the process. The tetrameric anion is present in the mineral borax
Borax, also known as sodium borate, sodium tetraborate, or disodium tetraborate, is an important boron compound, a mineral, and a salt of boric acid. It is usually a white powder consisting of soft colorless crystals that dissolve easily in water.Borax has a wide variety of uses...

, as a octahydrate, Na2B4O5(OH)4.8H2O. This compound can be obtained in high purity and so can be used to make a standard solution
Standard solution
In analytical chemistry, a standard solution is a solution containing a precisely known concentration of an element or a substance i.e, a known weight of solute is dissolved to make a specific volume. It is prepared using a standard substance, such as a primary standard. Standard solutions are used...

 in titrimetric analysis.

Polymeric ions

A number of polymeric borate ions are known. They may be made by reacting B(OH)3 or B2O3 with metal oxides. Examples include:
  • diborate B2O54− e.g. in Mg2B2O5 (suanite)

  • triborate B3O75- in CaAlB3O7 (johachidolite)

  • tetraborate B4O96− in e.g. Li6B4O9

  • metaborates, such as LiBO2
    Lithium metaborate
    Lithium metaborate is a chemical compound.-Uses:Lithium metaborate or 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, ICP-AES and ICP-MS....

     contain long chains of trigonal BO3 structural units, each sharing two oxygen atoms with adjacent units.

  • borates containing 3 and four coordinate boron, including the anion in borax.

  • borosilicate Borosilicate glass, also known as pyrex
    Pyrex is a brand name for glassware, introduced by Corning Incorporated in 1915.Originally, Pyrex was made from borosilicate glass. In the 1940s the composition was changed for some products to tempered soda-lime glass, which is the most common form of glass used in glass bakeware in the US and has...

    , is useful because it has a very low coefficient of thermal expansion and is resistant to cracking when heated, unlike soda glass. Structurally it is a silicate
    A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...

     in which some SiO4 units are replaced by BO4, together with a cation to compensate for the difference in oxidation states of Si(IV) and B(III).

Minerals and uses

Common borate salts include sodium metaborate, NaBO2, and borax
Borax, also known as sodium borate, sodium tetraborate, or disodium tetraborate, is an important boron compound, a mineral, and a salt of boric acid. It is usually a white powder consisting of soft colorless crystals that dissolve easily in water.Borax has a wide variety of uses...

. Borax is quite soluble in water, so mineral deposits only occur in places with very low rainfall. Extensive deposits were found in Death Valley
Death Valley
Death Valley is a desert valley located in Eastern California. Situated within the Mojave Desert, it features the lowest, driest, and hottest locations in North America. Badwater, a basin located in Death Valley, is the specific location of the lowest elevation in North America at 282 feet below...

 and transported out using the famous twenty-mule teams (1883 to 1889). Later (1925), deposits were found at Boron
Boron, California
Boron is a census-designated place in Kern County, California, United States. Boron is located east-southeast of Castle Butte, at an elevation of 2467 feet . The population was 2,253 at the 2010 census, up from 2,025 at the 2000 census...

, California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

 on the edge of the Mojave Desert
Mojave Desert
The Mojave Desert occupies a significant portion of southeastern California and smaller parts of central California, southern Nevada, southwestern Utah and northwestern Arizona, in the United States...

. The Atacama Desert
Atacama Desert
The Atacama Desert is a plateau in South America, covering a strip of land on the Pacific coast, west of the Andes mountains. It is, according to NASA, National Geographic and many other publications, the driest desert in the world...

 in Chile
Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...

 also contains mineable borate concentrations. Gerry
Gerry, New York
Gerry is a town in Chautauqua County, New York, United States. The population was 2,054 at the 2000 census. The town is named after Elbridge Gerry, a Vice-President of the U.S....

, New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...

 has been described by Agapito Associates Inc. as a "very nice place to mine for [borate]" although it is uncertain how much damage that might do to the surrounding area.

Lithium metaborate
Lithium metaborate
Lithium metaborate is a chemical compound.-Uses:Lithium metaborate or 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, ICP-AES and ICP-MS....

 or lithium tetraborate, or a mixture of both, can be used in borate fusion sample preparation of various samples for analysis by
X-ray fluorescence
X-ray fluorescence is the emission of characteristic "secondary" X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays...

Atomic absorption spectroscopy
Atomic absorption spectroscopy is a spectroanalytical procedure for the qualitative and quantitative determination of chemical elements employing the absorption of optical radiation by free atoms in the gaseous state. In analytical chemistry the technique is used for determining the concentration...

Inductively coupled plasma atomic emission spectroscopy , also referred to as inductively coupled plasma optical emission spectrometry , is an analytical technique used for the detection of trace metals...

 and ICP-MS
Inductively coupled plasma mass spectrometry is a type of mass spectrometry that is highly sensitive and capable of the determination of a range of metals and several non-metals at concentrations below one part in 1012...

. Borate fusion and energy dispersive X-ray fluorescence spectrometry with polarized excitation have been used in the analysis of contaminated soils.

Disodium octaborate tetrahydrate
Disodium octaborate tetrahydrate
Disodium octaborate tetrahydrate, an alkaline salt, is produced in two forms. One is a clear, liquid concentrate, which can be mistaken for corn syrup if repackaged and not properly labeled or it is packaged as a white, odorless, powdered chemical substance that is not flammable, combustible, or...

 is used as wood
Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

A preservative is a naturally occurring or synthetically produced substance that is added to products such as foods, pharmaceuticals, paints, biological samples, wood, etc. to prevent decomposition by microbial growth or by undesirable chemical changes....

s or fungicide. Zinc borate
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....

 is used as a flame retardant
Flame retardant
Flame retardants are chemicals used in thermoplastics, thermosets, textiles and coatings that inhibit or resist the spread of fire. These can be separated into several different classes of chemicals:...


Borate esters

Borate esters are organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

s of the type B(OR)3 where R is an organic residue (for example alkyl or aryl
In the context of organic molecules, aryl refers to any functional group or substituent derived from an aromatic ring, be it phenyl, naphthyl, thienyl, indolyl, etc....

). They are conveniently prepared by a reaction such as
B(OH)3 + 3ROH → B(OR)3 +3H2O

in the presence of a dehydrating agent, such as concetrated sulphuric acid. The borate ester is volatile and can be removed from the reaction mixture by distillation. This procedure is used for analysis of trace amounts of borate and for analysis of boron in steel. Alkyl borates burn with a characteristic green flame. This property is used to determine the presence of boron in qualitative analysis.

Trimethyl borate
Trimethyl borate
Trimethyl borate, or boron trimethoxide, has formula B3. It is a clear liquid. It melts at −34 °C and boils at 68-69 °C. It decomposes in contact with water...

, B(OCH3)3, is used as a precursor to boronic esters for Suzuki couplings. Borate esters were shown to serve as effective coupling agents in the synthesis of amides from either carboxylic acids or primary amides.

External links

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