page_title: Borate
section_title: Polymeric ions
hierarchical_section_title: Borate / Polymeric ions
caption_attribution_description: Ball-and-stick model of the tetraborate anion, [B4O5(OH)4]2−, in crystalline borax, Na2[B4O5(OH)4]·8H2O. Neutron diffraction data from H. A. Levy and G. C. Lisensky&#32;(December 1978).&#32;"Crystal structures of sodium sulfate decahydrate (Glauber's salt) and sodium tetraborate decahydrate (borax). Redetermination by neutron diffraction". Acta. Cryst.&#32;B 34&#32;(12): 3502-3510. DOI:10.1107/S0567740878011504.
context_page_description: Borates are boron-oxygen compounds, which form boron oxyanions. These can be trigonal 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.
context_section_description: 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 (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): 
  diborate [B₂O₅]⁴⁻, found in Mg₂B₂O₅ (suanite),
  triborate [B₃O₇]⁵⁻, found in CaAlB₃O₇ (johachidolite),
  tetraborate [B₄O₉]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 [B₃O₆]²⁻ ion.
caption: 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.
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