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Borates are the name for a large number of boron-oxygen compounds usually 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 sp2hybrid 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-.
With different mannitol concentrations, the pK of B(OH)3 extends on five orders of magnitude (from 9 to 4). Greenwood and Earnshawn (1997) refer to a pK value of 5.15 while a pK value of 3.80 is also reported in the Vogel's book.
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, pKa = 3.80), releases three water molecules and one proton in water as follows:
? + 3 H2O + H+
(pKa 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 moderator to control the reactivity of the core.
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 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, or 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.
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):
Metaborates, such as LiBO2 contain chains of trigonal BO3 structural units, each sharing two oxygen atoms with adjacent units, whereas NaBO2 and KBO2 contain the cyclic [B3O6]2- ion.
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 and is resistant to cracking when heated, unlike soda glass.
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.
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