Borate

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A borate is any of a range of boron oxyanions, anions containing boron and oxygen, such as orthoborate BO3−3, metaborate BO−2, or tetraborate B4O2−7; or any salt of such anions, such as sodium metaborate, Na+[BO2]− and borax (Na+)2[B4O7]2−. The name also refers to esters of such anions, such as trimethyl borate B(OCH3)3.

Contents

Natural occurrence

Borate ions occur, alone or with other anions, in many borate and borosilicate minerals such as borax, boracite, ulexite (boronatrocalcite) and colemanite. Borates also occur in seawater, where they make an important contribution to the absorption of low frequency sound in seawater. [1]

Borates also occur in plants, including almost all fruits. [2]

Anions

The main borate anions are:

Preparation

In 1905, Burgess and Holt observed that fusing mixtures of boric oxide B2O3 and sodium carbonate Na2CO3 yielded on cooling two crystalline compounds with definite compositions, consistent with anhydrous borax Na2B4O7 (which can be written Na2O·2B2O3) and sodium octaborate Na2B8O13 (which can be written Na2O·4B2O3). [6]

Structures

Borate anions (and functional groups) consist of trigonal planar BO3 and/or tetrahedral BO4 structural units, joined together via shared oxygen atoms (corners) or atom pairs (edges) into larger clusters so as to construct various ions such as [B2O5]4−, [B3O8]7−, [B4O12]12−, [B5O6(OH)5]2−, [B6O13]8−, etc. These anions may be cyclic or linear in structure, and can further polymerize into infinite chains, layers, and tridimensional frameworks. [7] [8] The terminal (unshared) oxygen atoms in the borate anions may be capped with hydrogen atoms (−OH) or may carry a negative charge (−O−).

The planar BO3 units may be stacked in the crystal lattice so as to have π-conjugated molecular orbitals, which often results in useful optical properties such as strong harmonics generation, birefringence, and UV transmission. [8]

Polymeric borate anions may have linear chains of 2, 3 or 4 trigonal BO3 structural units, each sharing oxygen atoms with adjacent unit(s). [7] as in LiBO2, contain chains of trigonal BO3 structural units. Other anions contain cycles; for instance, NaBO2 and KBO2 contain the cyclic [B3O6]3− ion, [9] 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 BO3 and BO4 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. [10]

Reactions

Aqueous solution

In aqueous solution, boric acid B(OH)3 can act as a weak Brønsted acid, that is, a proton donor, with pKa ~ 9. However, it more often acts as a Lewis acid, accepting an electron pair from a hydroxyl ion produced by the water autoprotolysis: [11]

B(OH)3 + 2 H2O ⇌[B(OH)4]− + H3O+ (pK = 8.98) [12]

This reaction is very fast, with characteristic time less than 10 μs. [13] 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 [B4O7]2−, found in the mineral borax:

4 [B(OH)4]− + 2   H+ ⇌[B4O5(OH)4]2− + 7 H2O

Other anions observed in solution are triborate(1−) and pentaborate(1−), in equilibrium with boric acid and tetrahydroxyborate according to the following overall reactions: [13]

2B(OH)3 + [B(OH)4]−⇌[B3O3(OH)4]− + 3H2O (fast, pK = —1.92)
4B(OH)3 + [B(OH)4]−⇌[B5O6(OH)4]− + 6H2O (slow, pK = —2.05)

In the pH range 6.8 to 8.0, any alkali salts of "boric oxide" anions with general formula [BxOy(OH)z]((q−) where 3x+q = 2y + z will eventually equilibrate in solution to a mixture of B(OH)3, [B(OH)4]−, [B3O3(OH)4]−, and [B5O6(OH)4]−. [13]

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. They can be obtained by treating boric acid or boron oxides with metal oxides.[ citation needed ]

Mixed anion salts

Some chemicals contain another anion in addition to borate. These include borate chlorides, borate carbonates, borate nitrates, borate sulfates, borate phosphates.

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, boroselenates, borotellurates, boroantimonates, borophosphates, or boroselenites.

Borosilicate glass, also known as pyrex, can be viewed as a silicate in which some [SiO4]4− units are replaced by [BO4]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.

Uses

Borax crystals Borax crystals.jpg
Borax crystals

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 and shipped with twenty-mule teams from 1883 to 1889. In 1925, deposits were found at Boron, California on the edge of the Mojave Desert. The Atacama Desert in Chile also contains mineable borate concentrations.

Lithium metaborate, 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. [14]

Disodium octaborate tetrahydrate Na2B8O13·4H2O (commonly abbreviated DOT) is used as a wood preservative or fungicide. Zinc borate is used as a flame retardant.

Some borates with large anions and multiple cations, like K2Al2B2O7 and Cs3Zn6B9O21 have been considered for applications in nonlinear optics. [8]

Borate esters

Borate esters are organic compounds, which are conveniently prepared by the stoichiometric condensation reaction of boric acid with alcohols (or their chalcogen analogs [15] ).

Thin films

Metal borate thin films have been grown by a variety of techniques, including liquid-phase epitaxy (e.g. FeBO3, [16] β‐BaB2O4 [17] ), electron-beam evaporation (e.g. CrBO3, [18] β‐BaB2O4 [19] ), pulsed laser deposition (e.g. β‐BaB2O4, [20]  Eu(BO2)3 [21] ), and atomic layer deposition (ALD). Growth by ALD was achieved using precursors composed of the tris(pyrazolyl)borate ligand and either ozone or water as the oxidant to deposit CaB2O4, [22] SrB2O4, [23] BaB2O4, [24] Mn3(BO3)2, [25] and CoB2O4 [25] 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. [26]

See also

Related Research Articles

<span class="mw-page-title-main">Boron</span> Chemical element, symbol B and atomic number 5

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 three valence electrons for forming covalent bonds, resulting in many compounds such as boric acid, the mineral sodium borate, and the ultra-hard crystals of boron carbide and boron nitride.

<span class="mw-page-title-main">Boric acid</span> Weak acid with formula B(OH)₃

Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula B(OH)3. It may also be called hydrogen orthoborate, trihydroxidoboron or boracic acid. It is usually encountered as colorless crystals or a white powder, that dissolves in water, and occurs in nature as the mineral sassolite. It is a weak acid that yields various borate anions and salts, and can react with alcohols to form borate esters.

<span class="mw-page-title-main">Borax</span> Boron compound, a salt of boric acid

Borax and tincar is a salt, a hydrated or anhydrous borate of sodium, with the chemical formula Na2H20B4O17. It is a colorless crystalline solid that dissolves in water to make a basic solution.

Sodium borate is a generic name for any salt of sodium with an anion consisting of boron and oxygen, and possibly hydrogen, or any hydrate thereof. It can be seen as a hydrated sodium salt of the appropriate boroxy acid, although the latter may not be a stable compound.

<span class="mw-page-title-main">Lithium borate</span> Chemical compound

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.

<span class="mw-page-title-main">Borate mineral</span> Mineral which contains a borate anion group

The Borate Minerals are minerals which contain a borate anion group. The borate (BO3) 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. The [B(O,OH)4]− anion exists as well.

<span class="mw-page-title-main">Tetraborate</span>

In chemistry, tetraborate or pyroborate is an anion with formula B4O2−7; or a salt containing that anion, such as sodium tetraborate, Na2B4O7. It is one of the boron oxoacids, that is, a borate.

Sodium perborate is chemical compound whose chemical formula may be written NaH2BO4, Na2H4B2O8, or, more properly, [Na+]2[B2O4(OH)4]2−. Its name is sometimes abbreviated as PBS.

Lithium metaborate is a chemical compound of lithium, boron, and oxygen with elemental formula LiBO2. It is often encountered as a hydrate, LiBO2·nH2O, where n is usually 2 or 4. However, these formulas do not describe the actual structure of the solids.

<span class="mw-page-title-main">Boron compounds</span>

Boron compounds are compounds containing the element boron. In the most familiar compounds, boron has the formal oxidation state +3. These include oxides, sulfides, nitrides, and halides.

Tetrahydroxyborate is an inorganic anion with the chemical formula [BH4O4]− or [B(OH)4]−. It contributes no colour to tetrahydroxyborate salts. It is found in the mineral hexahydroborite, Ca(B(OH)4)2 · 2 H2O, originally formulated CaB2O4 · 6 H2O. It is one of the boron oxoanions, and acts as a weak base. The systematic names are tetrahydroxyboranuide (substitutive) and tetrahydroxidoborate(1−) (additive). It can be viewed as the conjugate base of boric acid.

<span class="mw-page-title-main">Disodium octaborate</span> Chemical compound

Disodium octaborate is a borate of sodium, a chemical compound of sodium, boron, and oxygen — a salt with elemental formula Na2B8O13 or (Na+)2[B8O13]2−, also written as Na2O·4B2O3. It is a colorless crystalline solid, soluble in water.

<span class="mw-page-title-main">Sodium metaborate</span> Chemical compound

Sodium metaborate is a chemical compound of sodium, boron, and oxygen with formula NaBO2. However, the metaborate ion is trimeric in the anhydrous solid, therefore a more correct formula is Na3B3O6 or (Na+)3[B3O6]3−. The formula can be written also as Na2O·B2O3 to highlight the relation to the main oxides of sodium and boron. The name is also applied to several hydrates whose formulas can be written NaBO2·nH2O for various values of n.

<span class="mw-page-title-main">Barium borate</span> Chemical compound

Barium borate is an inorganic compound, a borate of barium with a chemical formula BaB2O4 or Ba(BO2)2. It is available as a hydrate or dehydrated form, as white powder or colorless crystals. The crystals exist in the high-temperature α phase and low-temperature β phase, abbreviated as BBO; both phases are birefringent, and BBO is a common nonlinear optical material.

<span class="mw-page-title-main">Metaboric acid</span> Chemical compound

Metaboric acid is the name for a family of inorganic compounds with the same empirical formula HBO2. that differ in their molecular structure. They are colourless water-soluble solids formed by the dehydration or decomposition of boric acid.

<span class="mw-page-title-main">Metaborate</span> Boron-oxygen anion or functional group

A metaborate is a borate anion consisting of boron and oxygen, with empirical formula BO−2. Metaborate also refers to any salt or ester of such anion. Metaborate is one of the boron's oxyanions. Metaborates can be monomeric, oligomeric or polymeric.

<span class="mw-page-title-main">Foitite</span> Tourmaline mineral

Foitite is a mineral in the tourmaline group, it is a vacancy-dominant member of the group. Foitite is in the 'vacancy' group, due to the absence of atoms in the X site.

<span class="mw-page-title-main">Orthoborate</span> Chemical compound

In inorganic chemistry, an orthoborate is a polyatomic anion with formula [BO3]3− or a salt containing the anion; such as trisodium orthoborate (Na+)3[BO3]3−. It is one of several boron oxyanions, or borates.

Trisodium borate is a chemical compound of sodium, boron, and oxygen, with formula Na3BO3, or (Na+)3[BO3]3−. It is a sodium salt of the orthoboric acid B(OH)3.

<span class="mw-page-title-main">Sodium tetrahydroxyborate</span> Chemical compound

Sodium tetrahydroxyborate is a salt of with chemical formula NaH4BO4 or Na+[B(OH)4]−. It is one of several sodium borates. At room temperature it is a colorless crystalline solid.

References

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  2. ↑ Allen, A. H.; Tankard, A. R. (1904). "The Determination of Boric Acid in Cider, Fruits, etc". Analyst . 29 (October): 301–304. Bibcode:1904Ana....29..301A. doi:10.1039/an9042900301.
  3. ↑ "Tetraborate".
  4. ↑ Carrondo, M. A. A. F. de C. T.; Skapski, A. C. (1978). "Refinement of the X-ray crystal structure of the industrial bleaching agent disodium tetrahydroxo-di-μ-peroxo-diborate hexahydrate, Na2[B2(O2)2(OH)4]·6H2O". Acta Crystallogr B . 34: 3551. doi:10.1107/S0567740878011565.
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  8. 1 2 3 Miriding Mutailipu, Min Zhang, Xiaoyu Dong, Yanna Chen, and Shilie Pan (2016): "Effects of the Orientation of [B5O11]7– Fundamental Building Blocks on Layered Structures Based on the Pentaborates". Inorganic Chemistry, volume 55, issue 20, pages 10608–10616. doi : 10.1021/acs.inorgch
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  25. 1 2 Klesko, Joseph P.; Bellow, James A.; Saly, Mark J.; Winter, Charles H.; Julin, Jaakko; Sajavaara, Timo (September 2016). "Unusual stoichiometry control in the atomic layer deposition of manganese borate films from manganese bis(tris(pyrazolyl)borate) and ozone". Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films. 34 (5): 051515. Bibcode:2016JVSTA..34e1515K. doi: 10.1116/1.4961385 . ISSN   0734-2101.
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