Borate

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 B
4O2−
7; or any salt of such anions, such as sodium metaborate, Na+
[BO
2]−
and borax (Na+

2[B

7]2−. The name also refers to esters of such anions, such as trimethyl borate B(OCH

## Contents

* 1 Natural occurrence
* 2 Anions
* 3 Preparation
* 4 Structures
* 5 Reactions
* 5.1 Aqueous solution
* 5.2 Borate salts
* 5.3 Mixed anion salts
* 5.4 Complex oxyanions containing boron
* 6 Uses
* 7 Borate esters
* 8 Thin films
* 9 Physiology
* 10 See also
* 11 References
* 12 External links

## 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, contributing to the absorption of low-frequency sound in seawater.[1]

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.

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

## Anions

The main borate anions are:

* tetrahydroxyborate [B(OH)
4]−
, found in sodium tetrahydroxyborate Na[B(OH)
4].
* orthoborate [BO
3]3−, found in trisodium orthoborate Na
3[BO

* [BO4]5−, found in the calcium yttrium borosilicate oxyapatite Ca3Y7BO4(SiO4)5O
* perborate [B

4(OH)
4]2−, as in sodium perborate Na
2[H

* metaborate [BO
2]−
or its cyclic trimer [B

6]3−, found in sodium metaborate Na
3[B

* diborate [B

5]4−, found in magnesium diborate (suanite) Mg
2[B

5] ,
* triborate [B

7]5−, found in calcium aluminium triborate (johachidolite) Ca[AlB

7] ,
* tetraborate [B

7]2−, found in anhydrous borax Na
2[B

* tetrahydroxytetraborate [B

5(OH)
4]2−, found in borax "decahydrate" Na
2[B

5(OH)
4] * 8H2O
* tetraborate(6-) [B

9]6−, found in lithium tetraborate(6-) Li
6[B

* pentaborate [B

8]−
or [B
10O
16]2−, found in sodium pentaborate Na
2[B
10O
16] * 10H2O
* octaborate [B

13]2−, found in disodium octaborate Na
2[B

13]

Borate ions

* The structure of the tetrahydroxyborate ion ([B(OH)
4]−
). This anion has a tetrahedral molecular geometry at the boron atom.

* The structure of the orthoborate ion ([BO
3]3−). This anion has a trigonal planar molecular geometry.

* The structure of the trimer of the metaborate ion ([B

6]3−). This anion is a cyclic molecule and has a trigonal planar molecular geometry at the boron atoms. All nine atoms of this anion lie on the same plane.

* The structure of the tetrahydroxytetraborate ion ([B

5(OH)
4]2−). This anion is a bridged bicyclic molecule, contains oxygen atoms bridging the boron atoms, which are linked to four hydroxyl groups (–OH), one per each boron atom. The anion has a tetrahedral molecular geometry at the two tetracoordinated boron atoms. It has a trigonal planar molecular geometry at the two tricoordinated boron atoms.

* The structure of the tetraborate ion ([B

7]2−). This anion has the same topology as the tetrahydroxytetraborate ion, but without the hydroxyl groups and all boron atoms have a trigonal planar molecular geometry.[3]

* The structure of the perborate ion ([B

4(OH)
4]2−). This anion is a cyclic molecule with a tetrahedral molecular geometry at the boron atoms. It contains two bridging peroxide groups (–O–O–) and four hydroxyl groups (–OH) attached to boron atoms, two per each boron. The ring has a chair conformation.[4]

* The structure of the repeating unit of the octaborate ion ([B

13]2−) in the alpha form of disodium octaborate (α-Na
2[B

13]).[5] This anion is cyclic and polymeric. It has a tetrahedral molecular geometry at the negatively charged boron atoms and a trigonal planar molecular geometry at the neutral boron atoms.

## Preparation

In 1905, Burgess and Holt observed that fusing mixtures of boric oxide B

3 and sodium carbonate Na
2CO
3 yielded on cooling two crystalline compounds with definite compositions, consistent with anhydrous borax Na

7 (which can be written Na
2O * 2B

3) and sodium octaborate Na

13 (which can be written Na
2O * 4B

3).[6]

## Structures

Borate anions (and functional groups) consist of trigonal planar BO
3 and/or tetrahedral BO
4 structural units, joined together via shared oxygen atoms (corners) or atom pairs (edges) into larger clusters so as to construct various ions such as [B

5]4−, [B

8]7−, [B

12]12−, [B

6(OH)
5]2−, [B

13]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 BO
3 units may be stacked in the crystal lattice 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, such as LiBO
2, may have linear chains of 2, 3 or 4 trigonal BO
3 structural units, each sharing oxygen atoms with adjacent unit(s).[7] Other anions contain cycles; for instance, NaBO
2 and KBO
2 contain the cyclic [B

6]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 BO
3 and BO
4 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 hydroxide ion produced by the water autoprotolysis:[11]

B(OH)
3 \+ 2 H
2O ⇌ [B(OH)
4]−
\+ H
3O+
(pK = 8.98)[12]

This reaction is very fast, with a 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 [B

7]2−, found in the mineral borax:

4 [B(OH)
4]−
\+ 2  H+ ⇌ [B

5(OH)
4]2− \+ 7 H

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]

2 B(OH)
3 \+ [B(OH)
4]−
⇌ [B

3(OH)
4]− \+ 3 H
2O (fast, pK = −1.92)

4 B(OH)
3 \+ [B(OH)
4]−
⇌ [B

6(OH)
4]− \+ 6 H
2O (slow, pK = −2.05)

In the pH range 6.8 to 8.0, any alkali salts of "boric oxide" anions with general formula [B

y(OH)
z]((q−
) where 3x \+ q = 2y \+ z will eventually equilibrate in solution to a mixture of B(OH)
3, [B(OH)
4]−, [B

3(OH)
4]−, and [B

6(OH)
4]−.[13]

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.[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. It forms a glass with a low coefficient of thermal expansion, thus resistant to cracking when heated, unlike soda glass.

## Uses

Borax crystals

Lithium metaborate, lithium tetraborate, or a mixture of the two, can be used to prepare borate fusion 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 to analyse contaminated soils.[14]

Disodium octaborate tetrahydrate Na

13 * 4H2O (commonly abbreviated DOT) is used as a wood preservative and a fungicide. Zinc borate is used as a flame retardant.

Some borates with large anions and multiple cations, such as K
2Al

7 and Cs
3Zn

21, have been considered for applications in nonlinear optics.[8]

## Borate esters

Borate esters are organic compounds which can be 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 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.[26]

## See also

* Nanochannel glass materials
* Porous glass
* Vycor glass
* Silly Putty
* Slime (toy)
* Tris(2,2,2-trifluoroethyl) borate
* Triethanolamine borate

## References

1. ↑ "Underlying Physics and Mechanisms for the Absorption of Sound in Seawater". National Physical Laboratory. http://resource.npl.co.uk/acoustics/techguides/seaabsorption/physics.html.
2. ↑ Allen, A. H.; Tankard, A. R. (1904). "The Determination of Boric Acid in Cider, Fruits, etc". Analyst 29 (October): 301–304. doi:10.1039/an9042900301. Bibcode: 1904Ana....29..301A. https://zenodo.org/record/1429686.
3. ↑ "Tetraborate". https://pubchem.ncbi.nlm.nih.gov/compound/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.
5. ↑ Bubnova, R. S.; Shepelev, Ju. F.; Sennova, N. A.; Filatov, S. K. (2002). "Thermal behaviour of the rigid boron-oxygen groups in the α-Na2B8O13 crystal structure". Zeitschrift für Kristallographie – Crystalline Materials 217 (9): 444–450. doi:10.1524/zkri.217.9.444.22881. Bibcode: 2002ZK....217..444B. https://www.degruyter.com/document/doi/10.1524/zkri.217.9.444.22881/html.
6. ↑ 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. doi:10.1098/rspl.1904.0112
7. ↑ 7.0 7.1 Wiberg E. and Holleman A.F. (2001) Inorganic Chemistry, Elsevier ISBN 0-12-352651-5
8. ↑ 8.0 8.1 8.2 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
9. ↑ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 205. ISBN 978-0-08-037941-8.
10. ↑ 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. doi:10.1002/pssb.200880253
11. ↑ Atkins (2010). Inorganic Chemistry (5th ed.). Oxford University Press. p. 334. ISBN 9780199236176.
12. ↑ Ingri, N. (1962). "Equilibrium Studies of Polyanions. 8. On the First Equilibrium Steps in the Hydrolysis of Boric Acid, a Comparison between Equilibria in 0.1 M and 3.0 M NaClO4.". Acta Chemica Scandinavica 16 (2): 439–448. doi:10.3891/acta.chem.scand.16-0439. ISSN 0904-213X.
13. ↑ 13.0 13.1 13.2 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. doi:10.1021/ic50052a026
14. ↑ Hettipathirana, Terrance D. (2004). "Simultaneous determination of parts-per-million level Cr, As, Cd and Pb, and major elements in low level contaminated soils using borate fusion and energy dispersive X-ray fluorescence spectrometry with polarized excitation". Spectrochimica Acta Part B: Atomic Spectroscopy 59 (2): 223–229. doi:10.1016/j.sab.2003.12.013. Bibcode: 2004AcSpB..59..223H.
15. ↑ "Esters". The IUPAC Compendium of Chemical Terminology. 2014. doi:10.1351/goldbook.E02219. https://goldbook.iupac.org/terms/view/E02219.
16. ↑ Yagupov, S.; Strugatsky, M.; Seleznyova, K.; Mogilenec, Yu.; Milyukova, E.; Maksimova, E.; Nauhatsky, I.; Drovosekov, A. et al. (November 2016). "Iron borate films: Synthesis and characterization". Journal of Magnetism and Magnetic Materials 417: 338–343. doi:10.1016/j.jmmm.2016.05.098. Bibcode: 2016JMMM..417..338Y. https://hal.archives-ouvertes.fr/hal-01392749/file/YagupovJMMM2016_Postprint.pdf.
17. ↑ Liu, Junfang; He, Xiaoming; Xia, Changtai; Zhou, Guoqing; Zhou, Shengming; Xu, Jun; Yao, Wu; Qian, Liejia (July 2006). "Preparation of crystalline beta barium borate thin films on Sr2+-doped alpha barium borate substrates by liquid phase epitaxy". Thin Solid Films 510 (1–2): 251–254. doi:10.1016/j.tsf.2005.12.205. Bibcode: 2006TSF...510..251L.
18. ↑ Jha, Menaka; Kshirsagar, Sachin D.; Ghanashyam Krishna, M.; Ganguli, Ashok K. (June 2011). "Growth and optical properties of chromium borate thin films". Solid State Sciences 13 (6): 1334–1338. doi:10.1016/j.solidstatesciences.2011.04.002. Bibcode: 2011SSSci..13.1334J.
19. ↑ Maia, L. J. Q.; Feitosa, C. A. C.; De Vicente, F. S.; Mastelaro, V. R.; Li, M. Siu; Hernandes, A. C. (September 2004). "Structural and optical characterization of beta barium borate thin films grown by electron beam evaporation". Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 22 (5): 2163–2167. doi:10.1116/1.1778409. ISSN 0734-2101. Bibcode: 2004JVSTA..22.2163M.
20. ↑ Xiao, R.-F.; Ng, L. C.; Yu, P.; Wong, G. K. L. (1995-07-17). "Preparation of crystalline beta barium borate (β-BaB2O4) thin films by pulsed laser deposition". Applied Physics Letters 67 (3): 305–307. doi:10.1063/1.115426. ISSN 0003-6951. Bibcode: 1995ApPhL..67..305X.
21. ↑ Aleksandrovsky, A. S.; Krylov, A. S.; Potseluyko, A. M.; Seredkin, V. A.; Zaitsev, A. I.; Zamkov, A. V. (2006-02-09). Konov, Vitaly I.; Panchenko, Vladislav Y.; Sugioka, Koji et al.. eds. "Pulsed laser deposition of europium borate glass films and their optical and magneto-optical properties". Society of Photo-Optical Instrumentation Engineers (Spie) Conference Series. SPIE Proceedings 6161: 61610A–61610A–7. doi:10.1117/12.675020. Bibcode: 2006SPIE.6161E..0AA. https://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1280542.
22. ↑ Saly, Mark J.; Munnik, Frans; Winter, Charles H. (2010). "Atomic layer deposition of CaB2O4 films using bis(tris(pyrazolyl)borate)calcium as a highly thermally stable boron and calcium source". Journal of Materials Chemistry 20 (44): 9995. doi:10.1039/c0jm02280b. ISSN 0959-9428.
23. ↑ Saly, Mark J.; Munnik, Frans; Winter, Charles H. (June 2011). "The Atomic Layer Deposition of SrB2O4 Films Using the Thermally Stable Precursor Bis(tris(pyrazolyl)borate)strontium". Chemical Vapor Deposition 17 (4–6): 128–134. doi:10.1002/cvde.201006890.
24. ↑ Saly, Mark J.; Munnik, Frans; Baird, Ronald J.; Winter, Charles H. (2009-08-25). "Atomic Layer Deposition Growth of BaB2O4 Thin Films from an Exceptionally Thermally Stable Tris(pyrazolyl)borate-Based Precursor". Chemistry of Materials 21 (16): 3742–3744. doi:10.1021/cm902030d. ISSN 0897-4756.
25. ↑ 25.0 25.1 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. doi:10.1116/1.4961385. ISSN 0734-2101. Bibcode: 2016JVSTA..34e1515K.
26. ↑ 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.

## External links

* Suanite at webmineral
* Johachidolite at webmineral
* Non-CCA Wood Preservatives: Guide to Selected Resources - National Pesticide Information Center

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