Editing Borax (section)

== Chemistry ==
[[File:Borax-unit-cell-3D-balls.png|thumb|upright|left|The structure of borax according to [[X-ray crystallography]].]]

From a chemical perspective, borax contains the [B<sub>4</sub>O<sub>5</sub>(OH)<sub>4</sub>]<sup>2−</sup> ion. In this structure, there are two four-coordinate boron centers and two three-coordinate boron centers.

It is a [[proton conductor]] at temperatures above 21&nbsp;°C. Conductivity is maximum along the ''b-''axis.<ref name=mari1962/>

Borax is also easily converted to [[boric acid]] and other [[borate]]s, which have many applications. Its reaction with hydrochloric acid to form boric acid is:

:{{chem2|Na2B4O7*10H2O + 2 HCl → 4 H3BO3 + 2 NaCl + 5 H2O}}
Borax is sufficiently stable to find use as a [[primary standard]] for acid-base [[titrimetry]].<ref name=vogel/>{{rp|p.316}}

Molten borax dissolves many metal oxides to form glasses. This property is important for its uses in metallurgy and for the [[borax bead test]] of qualitative chemical analysis.

Borax is soluble in a variety of solvents; however, it is notably insoluble in ethanol.<ref name="NIMH.borax"/>

{| class="wikitable sortable mw-collapsible"
|+ Solubility of borax in some solvents<ref name=BorCom.borax/>
! Organic solvent
! Temperature<br>°C (°F)
! Borax % by weight<br>in saturated solution
|-
|Glycerol 98.5%
|20 (68)
|52.60
|-
|Glycerol 86.5%
|20 (68)
|47.19
|-
|Ethylene glycol
|25 (77)
|41.60
|-
|Diethylene glycol
|25 (77)
|18.60
|-
|Methanol
|25 (77)
|19.90
|-
|Aqueous ethanol 46.5%
|15.5 (60)
|2.48
|-
|Acetone
|25 (77)
|0.60
|-
|Ethyl acetate
|25 (77)
|0.14
|}

{| class="wikitable"
|+ Solubility of borax in water<ref name=BorCom.borax/>
! Temperature<br>°C (°F)
! Borax % by weight in saturated solution
|-
|0 (32)
|1.99
|-
|5 (41)
|2.46
|-
|10 (50)
|3.09
|-
|15 (59)
|3.79
|-
|20 (68)
|4.70
|-
|25 (77)
|5.80
|-
|30 (86)
|7.20
|-
|35 (95)
|9.02
|-
|40 (104)
|11.22
|-
|45 (113)
|14.21
|-
|50 (122)
|17.91
|-
|55 (131)
|23.22
|-
|60 (140)
|30.32
|-
|65 (149)
|33.89
|-
|70 (158)
|36.94
|-
|75 (167)
|40.18
|-
|80 (176)
|44.31
|-
|85 (185)
|48.52
|-
|90 (194)
|53.18
|-
|95 (203)
|58.94
|-
|100 (212)
|65.63
|}

The term ''borax'' properly refers to the so-called "decahydrate" {{chem2|Na2B4O7*10H2O}}, but that name is not consistent with its structure. It is actually octahydrate. The [[anion]] is not [[tetraborate]] {{chem2|[B4O7](2-)}} but [[tetraborate|tetrahydroxy tetraborate]] {{chem2|[B4O5(OH)4](2-)}},<ref name=mari1962/> so the more correct formula should be {{chem2|Na2B4O5(OH)4*8H2O}}. However, the term may be applied also to the related compounds. Borax "pentahydrate" has the formula {{chem2|Na2B4O7*5H2O}}, which is actually a trihydrate {{chem2|Na2B4O5(OH)4*3H2O}}. It is a colorless solid with a density of 1.880&nbsp;kg/m<sup>3</sup> that crystallizes from water solutions above 60.8&nbsp;°C in the [[rhombohedral crystal system]]. It occurs naturally as the mineral [[tinkhanite]].<ref name=akgu2021/> It can be obtained by heating the "decahydrate" above 61&nbsp;°C.<ref name=sahin2002/> Borax "dihydrate" has the formula {{chem2|Na2B4O7*2H2O}}, which is actually anhydrous, with the correct formula {{chem2|Na2B4O5(OH)4}}. It can be obtained by heating the "decahydrate" or "pentahydrate" to above 116-120&nbsp;°C.<ref name=sahin2002/> Anhydrous borax is sodium tetraborate proper, with formula {{chem2|Na2B4O7}}. It can be obtained by heating any hydrate to 300&nbsp;°C.<ref name=sahin2002/> It has one amorphous (glassy) form and three crystalline forms – α, β, and γ, with melting points of 1015, 993 and 936 K respectively. α-{{chem2|Na2B4O7}} is the stable form.<ref name=sahin2002/>