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 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.

Not to be confused with Borat.

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:

Borate ions
  • The structure of the tetrahydroxyborate ion
    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
    The structure of the orthoborate ion ([BO3]3−). This anion has a trigonal planar molecular geometry.
  • The structure of the trimer of the metaborate ion
    The structure of the trimer of the metaborate ion ([B3O6]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
    The structure of the tetrahydroxytetraborate ion ([B4O5(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, and has a trigonal planar molecular geometry at the two tricoordinated boron atoms.
  • The structure of the tetrahydroxyborate ion
    The structure of the tetraborate ion ([B4O7]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
    The structure of the perborate ion ([B2O4(OH)4]2−). This anion is a cyclic molecule and has 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 in the alpha form of disodium octaborate
    The structure of the repeating unit of the octaborate ion ([B8O13]2−) in the alpha form of disodium octaborate (α-Na2[B8O13]).[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 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.

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

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

References
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