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Through Oxygen, Salt (chemistry) and Mineral
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.
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.
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Anions
The main borate anions are:
- tetrahydroxyborate [B(OH)4]−, found in sodium tetrahydroxyborate Na[B(OH)4].
- orthoborate [BO3]3−, found in trisodium orthoborate Na3[BO3]
- [BO
4]5−
, found in the calcium yttrium borosilicate oxyapatite Ca
3Y
7BO
4(SiO
4)
5O - perborate [B2O4(OH)4]2−, as in sodium perborate Na2[H4B2O8]
- metaborate [BO2]− or its cyclic trimer [B3O6]3−, found in sodium metaborate Na3[B3O6]
- diborate [B2O5]4−, found in magnesium diborate (suanite) Mg2[B2O5] ,
- triborate [B3O7]5−, found in calcium aluminium triborate (johachidolite) Ca[AlB3O7] ,
- tetraborate [B4O7]2−, found in anhydrous borax Na2[B4O7]
- tetrahydroxytetraborate [B4O5(OH)4]2−, found in borax "decahydrate" Na2[B4O5(OH)4]·8H2O
- tetraborate(6-) [B4O9]6−, found in lithium tetraborate(6-) Li6[B4O9]
- pentaborate [B5O8]− or [B10O16]2−, found in sodium pentaborate Na2[B10O16]·10H2O
- octaborate [B8O13]2−, found in disodium octaborate Na2[B8O13]
Borate ions
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).
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. 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 to have π-conjugated molecular orbitals, which often results in useful optical properties such as strong harmonics generation, birefringence, and UV transmission.
Polymeric borate anions, such as LiBO2, may have linear chains of 2, 3 or 4 trigonal BO3 structural units, each sharing oxygen atoms with adjacent unit(s). Other anions contain cycles; for instance, NaBO2 and KBO2 contain the cyclic [B3O6]3− ion, 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.
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:
- B(OH)3 + 2 H2O ⇌ [B(OH)4]− + H3O+ (pK = 8.98)
This reaction is very fast, with a characteristic time less than 10 μs. 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:
Other anions observed in solution are triborate(1−) and pentaborate(1−), in equilibrium with boric acid and tetrahydroxyborate according to the following overall reactions:
- 2 B(OH)3 + [B(OH)4]− ⇌ [B3O3(OH)4]− + 3 H2O (fast, pK = −1.92)
- 4 B(OH)3 + [B(OH)4]− ⇌ [B5O6(OH)4]− + 6 H2O (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]−.
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.
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
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.
Disodium octaborate tetrahydrate Na2B8O13·4H2O (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 K2Al2B2O7 and Cs3Zn6B9O21, have been considered for applications in nonlinear optics.
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).
Thin films
Metal borate thin films have been grown by a variety of techniques, including liquid-phase epitaxy (e.g. FeBO3, β-BaB2O4), electron-beam evaporation (e.g. CrBO3, β-BaB2O4), pulsed laser deposition (e.g. β-BaB2O4, Eu(BO2)3), 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, SrB2O4, BaB2O4, Mn3(BO3)2, and CoB2O4 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.