Borates are boron-oxygen compounds, which form boron oxyanions. These can be al or tetrahedral in structure, or more loosely can consist of chemical mixtures which contain borate anions of either description. The element boron most often occurs in nature as borates, such as borate minerals and borosilicates.
Structures
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. , illustrating trigonal planar molecular geometry The simplest borate anion, the orthoborate ion, 3−, is known in the solid state, for example, in Ca32, where it adopts a nearly trigonal planar structure. It is a structural analogue of the carbonate anion 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 9− ion, whereas the high-temperature form contains planar 3−.
Boric acid
All borates can be considered derivatives of boric acid, B3. Boric acid is a weak proton donor 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. B3 is acidic because of its reaction with OH− from water, forming the tetrahydroxyborate complex − and releasing the corresponding proton left by the water autoprotolysis: In the presence of cis-vicinal diols, such as mannitol, sorbitol, glucose and glycerol, the acidity of the boric acid solution is increased, and the pKa can be lowered to about 4 if enough mannitol is added. With different mannitol concentrations, the pK of B3 extends on 5 orders of magnitude. Greenwood and Earnshawn 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 between one B3 molecule and two mannitol molecules, releases three water molecules and one proton in water as follows: 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.
Polymeric ions
At neutral pH boric acid undergoes condensation reactions to form polymeric oxyanions. Well-known polyborate anions include the triborate, tetraborate and pentaborate anions. The condensation reaction for the formation of tetraborate is as follows: The tetraborate anion 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 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 with the formula Na2·8H2O. 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:
diborate 4−, found in Mg2B2O5,
triborate 5−, found in CaAlB3O7,
tetraborate 6−, found in Li6B4O9.
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 2− ion.
Borosilicates
, also known as pyrex, can be viewed as a silicate in which some 4− units are replaced by 5− centers, together with additional cations to compensate for the difference in valence states of Si and B. 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.
