{{Short description|Boron-oxygen anion {{chem2|BO2-}} or functional group}} A '''metaborate''' is a [[boron]] [[oxyanion]]s, [[anion|negative ion]]s consisting of boron and [[oxygen]], with [[empirical formula]] {{chem2|BO2-}} or any [[salt (chemistry)|salt]] with such anions, such as [[sodium metaborate]], {{chem2|[Na+][BO2-]}} or [[calcium metaborate]] {{chem2|[Ca(2+)][BO2-]2}}. ==Structures== Borate anions or functional groups consist of [[trigonal planar molecular geometry|trigonal planar]] BO3 or [[tetrahedral molecular geometry|tetrahedral]] BO4 structural units, joined together via shared oxygen atomsWiberg E. and Holleman A.F. (2001) ''Inorganic Chemistry'', Elsevier {{ISBN|0-12-352651-5}} and may be cyclic or linear in structure. [[File:Boric-acid-2D.png|thumb|150px|Structure of [[boric acid]], illustrating [[trigonal planar molecular geometry]]]] The simplest borate anion, the orthoborate(3−) ion, [BO3]3−, is known in the solid state, for example, in Ca3(BO3)2,{{cite journal |last1=Vegas |first1=A. |title=New description of the Ca3(BO3)2 structure |journal=Acta Crystallographica Section C |volume=41 |issue=11 |year=1985 |pages=1689–1690 |issn=0108-2701 |doi=10.1107/S0108270185009052|doi-access=free }} where it adopts a nearly trigonal planar structure. It is a structural analogue of the [[carbonate]] anion [CO3]2−, with which it is [[isoelectronic]]. Simple bonding theories point to the trigonal planar structure. In terms of [[valence bond theory]], the bonds are formed by using sp2 [[hybrid orbital]]s on boron. Some compounds termed orthoborates do not necessarily contain the trigonal planar ion, for example, gadolinium orthoborate GdBO3 contains the polyborate [B3O9]9− ion, whereas the high-temperature form contains planar [BO3]3−.{{cite journal |last1=Ren |first1=M. |last2=Lin |first2=J. H. |last3=Dong |first3=Y. |last4=Yang |first4=L. Q. |last5=Su |first5=M. Z. |last6=You |first6=L. P. |title=Structure and Phase Transition of GdBO3 |journal=Chemistry of Materials |volume=11 |issue=6 |year=1999 |pages=1576–1580 |issn=0897-4756 |doi=10.1021/cm990022o}} ==Boric acid== [[File:Tetrahydroxyborate-2D-dimensions.png|thumb|150px|The structure of the tetrahydroxyborate anion.]] All borates can be considered derivatives of [[boric acid]], B(OH)3. Boric acid is a weak proton donor ([[Acid dissociation constant|p''K''a ~ 9]]) in the sense of [[Brønsted–Lowry acid–base theory|Brønsted acid]], but is a [[Lewis acid]], i.e., it can accept an [[electron pair]]. In water, it behaves as a Lewis acid, accepting the electron pair of a [[hydroxyl ion]] produced by the water [[autoprotolysis]]. B(OH)3 is acidic because of its reaction with OH− from [[water (molecule)|water]], forming the [[tetrahydroxyborate]] complex [B(OH)4]− and releasing the corresponding proton left by the water [[autoprotolysis]]:{{cite book |title = Inorganic Chemistry |edition = 5th |last = Atkins |year = 2010 |publisher = Oxford University Press |isbn = 9780199236176 |page = 334 |display-authors=etal}} : B(OH)3 + 2 H2O ⇌ [B(OH)4]− + [H3O]+ (p''K''a = 8.98)Ingri N. (1962) ''Acta Chem. Scand.'', '''16''', 439. In the presence of ''[[Cis–trans isomerism|cis]]''-[[Diol#Vicinal_diols|vicinal diols]], such as [[mannitol]], [[sorbitol]], [[glucose]] and [[glycerol]], the acidity of the boric acid solution is increased, and the p''K''a can be lowered to about 4 if enough mannitol is added.{{VogelQuantitative6th|page=357}}. With different [[mannitol]] concentrations, the pK of B(OH)3 extends on 5 orders of magnitude (from 9 to 4).{{Cite book |publisher = U.S. Government Printing Office |title = NIST Special Publication |date = 1969}} Greenwood and Earnshawn (1997){{Greenwood&Earnshaw}} refer to a pK value of 5.15, while a pK value of 3.80 is also reported in the Vogel's book.{{VogelQuantitative6th|page=357}}. The formation of the complex (more exactly, in fact an [[ester]]) between one B(OH)3 molecule and two mannitol (C6H14O6) molecules (sometimes referred as mannitoborate, the conjugated base of the mannitoboric acid, p''K''a = 3.80), releases three water molecules and one proton in water as follows: : {{overset|(mannitoboric acid)|{{overset|boric acid|B(OH)3}} + 2 {{overset|mannitol|C6H14O6}} }} ⇌ {{overset|mannitoborate complex|[B(C6H8O2(OH)4)2]−}} + 3 H2O + H+ : (p''K''a ranging from 4 to 9, depending on the mannitol concentration) The solution obtained after the complexation/esterification reaction – involving also the release of a proton, from there, the ancient name of mannitoboric acid – is then sufficiently acid to be titrated by a strong base as NaOH. The equivalence point can then be determined by potentiometric titration using an automated titrator in order to assay the borate content present in aqueous solution. This method is often used to determine the boron content in the water of the primary circuit of [[light-water reactor]], in which boric acid is added as a [[neutron poison]] to control the reactivity of the core by absorbing excess neutrons early in the refueling cycle. As reactivity decreases, boric acid is slowly removed from the reactor coolant to maintain criticality. ==Polymeric ions== [[File:Tetraborate-xtal-3D-balls.png|thumb|Tetraborate (borax) ion structure: pink, boron; red, oxygen; white, hydrogen. This tetrameric boron structure comprises two boron atoms in tetrahedral configuration sharing one common oxygen atom and linked by other oxygens to two other boron atoms present in trigonal configuration. Three cycles are also visible: two with 3 boron atoms and one with 4 boron atoms.]] At neutral pH boric acid undergoes condensation reactions to form polymeric [[oxyanion]]s. Well-known polyborate anions include the triborate(1−), tetraborate(2−) and pentaborate(1−) anions. 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. The condensation reaction for the formation of tetraborate(2−) is as follows: : 2 B(OH)3 + 2 [B(OH)4]− ⇌ [B4O5(OH)4]2− + 5 H2O The tetraborate anion ([[tetramer]]) includes two tetrahedral and two trigonal boron atoms symmetrically assembled in a fused bicyclic structure. The two tetrahedral boron atoms are linked together by a common oxygen atom, and each also bears a negative net charge brought by the supplementary OH− groups laterally attached to them. This intricate molecular anion also exhibits three rings: two fused distorted hexagonal (boroxole) rings and one distorted octagonal ring. Each ring is made of a succession of alternate boron and oxygen atoms. Boroxole rings are a very common structural motif in polyborate ions. The tetraborate anion occurs in the mineral [[borax]] (sodium tetraborate octahydrate) with the formula Na2[B4O5(OH)4]·8H2O. The borax chemical formula is also commonly written in a more compact notation as Na2B4O7·10H2O. Sodium borate can be obtained in high purity and so can be used to make a [[standard solution]] in titrimetric analysis.{{VogelQuantitative6th|page=316}}. A number of metal borates are known. They are produced by treating boric acid or boron oxides with metal oxides. Examples hereafter include linear chains of 2, 3 or 4 trigonal BO3 structural units, each sharing only one oxygen atom with adjacent unit(s): * diborate [B2O5]4−, found in Mg2B2O5 ([[suanite]]), * triborate [B3O7]5−, found in CaAlB3O7 ([[johachidolite]]), * tetraborate [B4O9]6−, found in Li6B4O9. Metaborates, such as [[lithium metaborate|LiBO2]], contain chains of trigonal BO3 structural units, each sharing two oxygen atoms with adjacent units, whereas [[Sodium metaborate|NaBO2]] and KBO2 contain the cyclic [B3O6]2− ion.{{Greenwood&Earnshaw2nd|page=205}} ==Borosilicates== [[Borosilicate|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== [[File:Borax crystals.jpg|thumb|left|150px|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 team]]s from 1883 to 1889. In 1925, deposits were found at [[Boron, California|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 [[X-ray fluorescence|XRF]], [[atomic absorption spectroscopy|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.{{cite journal |last=Hettipathirana |first=Terrance D. |year=2004 |title=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 |journal=Spectrochimica Acta Part B: Atomic Spectroscopy |volume=59 |issue=2 |pages=223–229 |doi=10.1016/j.sab.2003.12.013 |bibcode = 2004AcSpe..59..223H}} [[Disodium octaborate tetrahydrate]] (commonly abbreviated DOT) is used as a [[Wood preservation#Borate preservatives|wood preservative]] or fungicide. [[Zinc borate]] is used as a [[flame retardant]]. == Borate esters== [[Borate ester]]s are [[organic compound]]s, which are conveniently prepared by the stoichiometric condensation reaction of boric acid with alcohols. ==Mixed anion compounds== Some chemicals contain another anion in addition to borate. These include [[borate chloride]]s, [[borate carbonate]]s, [[borate nitrate]]s, [[borate sulfate]]s, [[borate phosphate]]s. More complex anions can be formed by condensing borate triangles or tetrahedra with other [[oxyanion]]s to yield materials such as [[borosulfates]], [[boroselenates]], [[borotellurates]], [[boroantimonates]], [[borophosphates]], or [[boroselenites]]. == Thin films == Metal borate thin films have been grown by a variety of techniques, including liquid-phase [[epitaxy]] (e.g. FeBO3,{{Cite journal |last1=Yagupov |first1=S. |last2=Strugatsky |first2=M. |last3=Seleznyova |first3=K. |last4=Mogilenec |first4=Yu. |last5=Milyukova |first5=E. |last6=Maksimova |first6=E. |last7=Nauhatsky |first7=I. |last8=Drovosekov |first8=A. |last9=Kreines |first9=N. |date=November 2016 |title=Iron borate films: Synthesis and characterization |journal=Journal of Magnetism and Magnetic Materials |volume=417 |pages=338–343 |doi=10.1016/j.jmmm.2016.05.098 |bibcode=2016JMMM..417..338Y|url=https://hal.archives-ouvertes.fr/hal-01392749/file/YagupovJMMM2016_Postprint.pdf }} β‐BaB2O4{{Cite journal |last1=Liu |first1=Junfang |last2=He |first2=Xiaoming |last3=Xia |first3=Changtai |last4=Zhou |first4=Guoqing |last5=Zhou |first5=Shengming |last6=Xu |first6=Jun |last7=Yao |first7=Wu |last8=Qian |first8=Liejia |date=July 2006 |title=Preparation of crystalline beta barium borate thin films on Sr2+-doped alpha barium borate substrates by liquid phase epitaxy |journal=Thin Solid Films |volume=510 |issue=1–2 |pages=251–254 |doi=10.1016/j.tsf.2005.12.205 |bibcode=2006TSF...510..251L}}), [[Electron-beam physical vapor deposition|electron-beam evaporation]] (e.g. CrBO3,{{Cite journal |last1=Jha |first1=Menaka |last2=Kshirsagar |first2=Sachin D. |last3=Ghanashyam Krishna |first3=M. |last4=Ganguli |first4=Ashok K. |date=June 2011 |title=Growth and optical properties of chromium borate thin films |journal=Solid State Sciences |volume=13 |issue=6 |pages=1334–1338 |doi=10.1016/j.solidstatesciences.2011.04.002 |bibcode=2011SSSci..13.1334J}} β‐BaB2O4{{Cite journal |last1=Maia |first1=L. J. Q. |last2=Feitosa |first2=C. A. C. |last3=De Vicente |first3=F. S. |last4=Mastelaro |first4=V. R. |last5=Li |first5=M. Siu |last6=Hernandes |first6=A. C. |date=September 2004 |title=Structural and optical characterization of beta barium borate thin films grown by electron beam evaporation |journal=Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films |volume=22 |issue=5 |pages=2163–2167 |doi=10.1116/1.1778409 |issn=0734-2101 |bibcode=2004JVST...22.2163M}}), [[pulsed laser deposition]] (e.g. β‐BaB2O4,{{Cite journal |last1=Xiao |first1=R.‐F. |last2=Ng |first2=L. C. |last3=Yu |first3=P. |last4=Wong |first4=G. K. L. |date=1995-07-17 |title=Preparation of crystalline beta barium borate (β‐BaB2O4) thin films by pulsed laser deposition |journal=Applied Physics Letters |volume=67 |issue=3 |pages=305–307 |doi=10.1063/1.115426 |issn=0003-6951 |bibcode=1995ApPhL..67..305X}}  Eu(BO2)3{{Cite journal |last1=Aleksandrovsky |first1=A. S. |last2=Krylov |first2=A. S. |last3=Potseluyko |first3=A. M. |last4=Seredkin |first4=V. A. |last5=Zaitsev |first5=A. I. |last6=Zamkov |first6=A. V. |date=2006-02-09 |editor-last=Konov |editor-first=Vitaly I. |editor2-last=Panchenko |editor2-first=Vladislav Y. |editor3-last=Sugioka |editor3-first=Koji |editor4-last=Veiko |editor4-first=Vadim P. |title=Pulsed laser deposition of europium borate glass films and their optical and magneto-optical properties |journal=Society of Photo-Optical Instrumentation Engineers (Spie) Conference Series |volume=6161 |url=http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1280542 |pages=61610A–61610A–7 |doi=10.1117/12.675020 |bibcode=2006SPIE.6161E..0AA|s2cid=136530746 }}), and [[atomic layer deposition]] (ALD). Growth by ALD was achieved using [[Precursor (chemistry)|precursors]] composed of the [[Trispyrazolylborate|tris(pyrazolyl)borate]] [[ligand]] and either ozone or water as the [[Oxidizing agent|oxidant]] to deposit CaB2O4,{{Cite journal |last1=Saly |first1=Mark J. |last2=Munnik |first2=Frans |last3=Winter |first3=Charles H. |date=2010 |title=Atomic layer deposition of CaB2O4 films using bis(tris(pyrazolyl)borate)calcium as a highly thermally stable boron and calcium source |journal=Journal of Materials Chemistry |volume=20 |issue=44 |pages=9995 |doi=10.1039/c0jm02280b |issn=0959-9428}} SrB2O4,{{Cite journal |last1=Saly |first1=Mark J. |last2=Munnik |first2=Frans |last3=Winter |first3=Charles H. |date=June 2011 |title=The Atomic Layer Deposition of SrB2O4 Films Using the Thermally Stable Precursor Bis(tris(pyrazolyl)borate)strontium |journal=Chemical Vapor Deposition |volume=17 |issue=4–6 |pages=128–134 |doi=10.1002/cvde.201006890}} BaB2O4,{{Cite journal |last1=Saly |first1=Mark J. |last2=Munnik |first2=Frans |last3=Baird |first3=Ronald J. |last4=Winter |first4=Charles H. |date=2009-08-25 |title=Atomic Layer Deposition Growth of BaB2O4 Thin Films from an Exceptionally Thermally Stable Tris(pyrazolyl)borate-Based Precursor |journal=Chemistry of Materials |volume=21 |issue=16 |pages=3742–3744 |doi=10.1021/cm902030d |issn=0897-4756}} Mn3(BO3)2,{{Cite journal |last1=Klesko |first1=Joseph P. |last2=Bellow |first2=James A. |last3=Saly |first3=Mark J. |last4=Winter |first4=Charles H. |last5=Julin |first5=Jaakko |last6=Sajavaara |first6=Timo |date=September 2016 |title=Unusual stoichiometry control in the atomic layer deposition of manganese borate films from manganese bis(tris(pyrazolyl)borate) and ozone |journal=Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films |volume=34 |issue=5 |pages=051515 |doi=10.1116/1.4961385 |issn=0734-2101 |bibcode=2016JVSTA..34e1515K|doi-access=free }} and CoB2O4 films. ==See also== {{Portal|Earth sciences}} * [[Nanochannel glass materials]] * [[Porous glass]] * [[Vycor|Vycor glass]] * [[Silly Putty]] * [[Slime (toy)]] * [[Tris(2,2,2-trifluoroethyl) borate]] ==References== {{Reflist}} ==External links== {{Commons category|Borates}} * [http://webmineral.com/data/Suanite.shtml Suanite at webmineral] * [http://webmineral.com/data/Johachidolite.shtml Johachidolite at webmineral] * [http://npic.orst.edu/hottopic/AltCCA.pdf Non-CCA Wood Preservatives: Guide to Selected Resources - National Pesticide Information Center] {{Borates}} [[Category:Borate minerals]] [[Category:Borates| ]] [[Category:Industrial minerals]] [[Category:Inorganic compounds]] [[Category:Neutron poisons]] [[Category:Oxyanions]] [[Category:Pesticides]] [[Category:Preservatives]] [[Category:Rheology]] [[Category:Testicular toxicants]]