Borates are the name for a large number of boron-containing oxyanions. The term "borates" may also more loosely refer to chemical compounds which contain borate anions. 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 ion, BO33− 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 CO32−, 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 sp2 hybrid 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 BO33–
- B(OH)3 + 2H2O B(OH)−
4 + H3O+ (pKa = 8.98)
Under acid conditions boric acid undergoes condensation reactions to form polymeric oxyanions:
- 4 [B(OH)4]− + 2 H+ [B4O5(OH)4]2− + 7 H2O
The tetraborate anion occurs in the mineral borax, as an octahydrate, Na2B4O5(OH)4.8H2O. This compound 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 arise by treating boric acid or boron oxides with metal oxides. Examples include:
- diborate B2O54−, found in Mg2B2O5 (suanite)
- triborate B3O75−, found in CaAlB3O7 (johachidolite)
- tetraborate B4O96−, 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 B3O62– ion.
Borosilicate glass, also known as pyrex, can be viewed as a silicate in which some SiO44− units are replaced by BO45− centers, together with a cation to compensate for the difference in oxidation states of Si(IV) and B(III). Because of 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.
Minerals and uses
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 transported out using the famous twenty-mule teams (1883 to 1889). Later (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 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.
- B(OH)3 + 3 ROH → B(OR)3 +3 H2O
A dehydrating agent, such as concentrated sulfuric acid is typically added. Borate esters are volatile and can be purified by distillation. This procedure is used for analysis of trace amounts of borate and for analysis of boron in steel. Like all boron compounds, alkyl borates burn with a characteristic green flame. This property is used to determine the presence of boron in qualitative analysis.
Borate esters form more spontaneously when treated with diols such as sugars.
- ArMgBr + B(OCH3)3 → MgBrOCH3 + ArB(OCH3)2
- ArB(OCH3)2 + 2 H2O → ArB(OH)2 + 2 HOCH3
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