Borates are the name for a large number of boron-containing oxyanions. The term "borates" may also refer to tetrahedral boron anions, or more loosely to chemical compounds which contain borate anions of either description. Larger borates are composed of trigonal planar BO3 or tetrahedral BO4 structural units, joined together via shared oxygen atoms and may be cyclic or linear in structure. Boron most often occurs in nature as borates, such as borate minerals and borosilicates.
The simplest borate anion, the orthoborate(3-) ion, [BO3]3-, is known in the solid state, for example in Ca3(BO3)2. In this it adopts a near 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 orbitals 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-.
All borates can be considered derivatives of boric acid, B(OH)3. Boric acid is a weak proton donor (pKa ~ 9) in the sense of 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 + 2H2O ⇌ [B(OH)4]− + [H3O]+ (pKa = 8.98)
At neutral pH boric acid undergoes condensation reactions to form polymeric oxyanions. Well-known polyborate anions include the triborate(1-), tetraborate(2-) and pentaborate(1-) anions. 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, as an octahydrate, Na2[B4O5(OH)4]·8H2O. The borax chemical formula is also commonly written in a more compact notation as Na2B4O7·10H2O.
A number of metal borates are known. They arise 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
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 of this substitution leads to imperfections, the material is slow to crystallise and forms a glass with low coefficient of thermal expansion and is resistant to cracking when heated, unlike soda glass.
Minerals and uses
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 transported out using the famous twenty-mule teams (1883 to 1889). Later (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 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. Borate fusion and energy dispersive X-ray fluorescence spectrometry with polarized excitation have been used in the analysis of contaminated soils.
Borate esters are organic compounds which are conveniently prepared by the stoichiometric condensation reaction of boric acid with alcohols. There are two main classes of borate esters: orthoborates, B(OR)3 and metaborates, B3O3(OR)3. Metaborates contain 6-membered boroxole rings.
- B(OH)3 + 3 ROH → B(OR)3 + 3 H2O
- 3 B(OH)3 + 3 ROH → B3O3(OR)3 + 6 H2O
A dehydrating agent, such as concentrated sulfuric acid is typically added. Borate esters are volatile and can be purified by distillation. This procedure is used for analysis of trace amounts of borate and for analysis of boron in steel. Like all boron compounds, alkyl borates burn with a characteristic green flame. This property is used to determine the presence of boron in qualitative analysis.
Borate esters form spontaneously when treated with diols such as sugars and the reaction with mannitol forms the basis of a titrimetric analytical method for boric acid.
Metaborate esters show considerable Lewis acidity and can initiate epoxide polymerization reactions. The Lewis acidity of orthoborate esters, as determined by the Gutmann-Beckett method, is relatively low.
- ArMgBr + B(OCH3)3 → MgBrOCH3 + ArB(OCH3)2
- ArB(OCH3)2 + 2H2O → ArB(OH)2 + 2HOCH3
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