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 ([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 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.
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.
The structure of the repeating unit of the octaborate ion ([B8O13]2−) in the alpha form of disodium octaborate (α-Na2[B8O13]). 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.
In 1905, Burgess and Holt observed that fusing mixtures of boric oxideB2O3 and sodium carbonateNa2CO3 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).
Borate anions (and functional groups) consist of trigonal planarBO3 and/or tetrahedralBO4 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−).
Polymeric borate anions may have linear chains of 2, 3 or 4 trigonal BO3 structural units, each sharing oxygen atoms with adjacent unit(s). 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, 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.
This reaction is very fast, with 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:
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]−.
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.
A number of metal borates are known. They can be obtained by treating boric acid or boron oxides with metal oxides.
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.
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.
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.
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