Sodium borate

Sodium borate is a generic name for any salt of sodium with an anion consisting of boron and oxygen, and possibly hydrogen, or any hydrate thereof. It can be seen as a hydrated sodium salt of the appropriate boroxy acid, although the latter may not be a stable compound.

Many sodium borates have important industrial and household applications; the best known being borax, Template:Chem2 = Template:Chem2.

The ternary phase diagram of the Template:Chem2–Template:Chem2–Template:Chem2 phase diagram in the 0–100 °C temperature range contains 13 unique hydrated crystalline sodium borates, including five important industrial products.<ref name=nein2017/>

Sodium borates, as well as boroxy acids, are often described as mixtures Template:Chem2 = Template:Chem2, with x, y, and z chosen to fit the elemental formula, or a multiple thereof. Thus, for example, borax Template:Chem2 would be Template:Chem2, and boric acid Template:Chem2 would be Template:Chem2 = Template:Chem2.

The elemental formula was often interpreted as a z-hydrate of an "anhydrous" salt without any hydrogen, namely Template:Chem2. However, later research uncovered that many borates have hydroxyl groups Template:Chem2 bound covalently to the boron atoms in the anion. Thus borax, for example, is still often described as a decahydrate Template:Chem2, with the implied anion Template:Chem2, whereas the correct formula is Template:Chem2, with anion Template:Chem2.

The following table gives some of the crystalline sodium borates in this family.<ref name=nies1967/> The column x/(x+y) is the formal mole fraction of Template:Chem2 in the "anhydrous" version.

x:y:z	x/(x+y)	Elemental formula	As hydrate	Correct formula	Name
3:1:0	0.750	Template:Chem2 = Template:Chem2		Template:Chem2	trisodium orthoborate (anhydrous)
1:1:0	0.500	Template:Chem2 = Template:Chem2		Template:Chem2	sodium metaborate
1:1:1	0.500	Template:Chem2	Template:Chem2	?	?<ref name=nies1967/>
1:1:4	0.500	Template:Chem2 = Template:Chem2	Template:Chem2	Template:Chem2	sodium tetrahydroxyborate<ref name=nies1967/>
1:1:8	0.500	Template:Chem2 = Template:Chem2	Template:Chem2	?	?<ref name=nies1967/>
1:2:0	0.333	Template:Chem2		Template:Chem2	borax (anhydrous)
1:2:4	0.333	Template:Chem2	Template:Chem2	Template:Chem2	borax "tetrahydrate"
1:2:5	0.333	Template:Chem2 = Template:Chem2	Template:Chem2	Template:Chem2	borax "pentahydrate"<ref name=nies1967/>
1:2:10	0.333	Template:Chem2	Template:Chem2	Template:Chem2	borax "decahydrate"<ref name=nies1967/>
1:3:0	0.250	Template:Chem2 = Template:Chem2		Template:Chem2	?
1:4:0	0.200	Template:Chem2		Template:Chem2	disodium octaborate (anhydrous)
2:9:11	0.182	Template:Chem2	Template:Chem2	Template:Chem2 <ref name=nein2017/>	disodium enneaborate
1:5:2	0.167	Template:Chem2 = Template:Chem2	Template:Chem2	?	sodium pentaborate "monohydrate"<ref name=nein2017/>
1:5:4	0.167	Template:Chem2 = Template:Chem2	Template:Chem2	Template:Chem2	sodium pentaborate "dihydrate"<ref name=mary2008/>
1:5:10	0.167	Template:Chem2 = Template:Chem2	Template:Chem2	?	sodium pentaborate "pentahydrate"<ref name=seno2022/>
3:5:4	0.125	Template:Chem2 = Template:Chem2	Template:Chem2	Template:Chem2	trisodium pentaborate<ref name=menc1977/>

Some of the borates above may have more than one isomeric or crystalline form. Some may decompose when dissolved in water. Note that the anion of the "anhydrous borax" is different from that of its "hydrates".

Some of the anhydrous borates above can be crystallized from molten mixtured of sodium oxide and boric oxide.<ref name=burg1905/>

Some sodium borates hower cannot be analyzed as combinations Template:Chem2 of the three ordinary oxides. The most important example is sodium perborate, originally described as Template:Chem2 but actually Template:Chem2. The anion of this compound has two peroxide bridges Template:Chem2 which make it oxygen-rich compared to the general family above.

References

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<ref name=mary2008>S. Stella Mary, S. Shahil Kirupavathy, P. Mythili, R. Gopalakrishnan (2008): "Growth and characterization of sodium pentaborate [Na(H₄B₅O₁₀)] single crystals". Spectrochimica Acta Part A, volume 71, issue 4, 15 pages 1311-1316. {{#invoke:doi|main}}</ref>

<ref name=seno2022>Taha Cagri Senocak, Taha Alper Yilmaz, Hasan Feyzi Budak, Gokhan Gulten, Ahmet Melik Yilmaz, Kadri Vefa Ezirmik, Yasar Totikc (2022): "Influence of sodium pentaborate (B₅H₁₀NaO₁₃) additive in plasma electrolytic oxidation process on WE43 magnesium alloys". Materials Today Communications, volume 30, article 103157. {{#invoke:doi|main}}</ref>

<ref name=nein2017>Doinita Neiner, Yulia V. Sevryugina, Larry S. Harrower, and David M. Schubert (2017): "Structure and Properties of Sodium Enneaborate, Na₂[B₈O₁₁(OH)₄]·B(OH)₃·2H₂O". Inorganic Chemistry, volume 56, issue 12, pages 7175–7181. {{#invoke:doi|main}}</ref>

<ref name=nies1967>Nelson P. Nies and Richard W. Hulbert (1967): "Solubility isotherms in the system sodium oxide-boric oxide-water. Revised solubility-temperature curves of boric acid, borax, sodium pentaborate, and sodium metaborate". Journal of Chemical and Engineering Data, volume 12, issue 3, pages 303–313. {{#invoke:doi|main}}</ref>

<ref name=burg1905>Charles Hutchens Burgess and Alfred Holt (1905): "Some physical characters of the sodium borates, with a new and rapid method for the determination of melting points." Proceedings of the Royal Society of London, volume 74, pages 285–295.{{#invoke:doi|main}}</ref>

<ref name=menc1977>Silvio Menchetti and Cesare Sabelli (1977): "The crystal structure of synthetic sodium pentaborate monohydrate". Acta Crystallographica Section B, volume B33, pages 3730-3733. {{#invoke:doi|main}}</ref>

</references>