Chemistry: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, (Na+
)
2[B
4O
5(OH)
4]2− · 8H2O = Na
2B
4H
20O
17.
The ternary phase diagram of the Na
2O–B
2O
3–H
2O phase diagram in the 0–100 °C temperature range contains 13 unique hydrated crystalline sodium borates, including five important industrial products.[1]
Sodium borates, as well as boroxy acids, are often described as mixtures xNa
2O · yB
2O
3 · zH
2O = Na
2xB
2yH
2zO
x+3y+z, with x, y, and z chosen to fit the elemental formula, or a multiple thereof. Thus, for example, borax Na
2B
4H
20O
17 would be 1Na
2O · 2B
2O
3 · 10H2O, and boric acid B(OH)
3 would be 0Na
2O · 1B
2O
3 · 1H2O = 2[B(OH)
3].
The elemental formula was often interpreted as a z-hydrate of an "anhydrous" salt without any hydrogen, namely Na
2xB
2yO
3y · zH
2O. However, later research uncovered that many borates have hydroxyl groups HO−
bound covalently to the boron atoms in the anion. Thus borax, for example, is still often described as a decahydrate Na
2B
4O
7 · 10H2O, with the implied anion [B
4O
7]2−, whereas the correct formula is Na
2B
4O
5(OH)
4 · 8H2O, with anion [B
4O
5(OH)
4]2−.
The following table gives some of the crystalline sodium borates in this family.[2] The column x/(x+y) is the formal mole fraction of Na
2O in the "anhydrous" version.
| x:y:z | x/(x+y) | Elemental formula | As hydrate | Correct formula | Name |
|---|---|---|---|---|---|
| 3:1:0 | 0.750 | Na 6B 2O 6 = Na 3BO 3 |
Na 3BO 3 |
trisodium orthoborate (anhydrous) | |
| 1:1:0 | 0.500 | Na 2B 2O 4 = NaBO 2 |
Na 3B 3O 6 |
sodium metaborate | |
| 1:1:1 | 0.500 | Na 2H 2B 2O 5 |
Na 2B 2O 4 · H2O |
? | ?[2] |
| 1:1:4 | 0.500 | Na 2H 8B 2O 8 = NaBH 4O 4 |
NaBO 2 · 2H2O |
NaB(OH) 4 |
sodium tetrahydroxyborate[2] |
| 1:1:8 | 0.500 | Na 2H 16B 2O 12 = NaBH 8O 6 |
NaBO 2 · 4H2O |
? | ?[2] |
| 1:2:0 | 0.333 | Na 2B 4O 7 |
Na 2B 4O 7 |
borax (anhydrous) | |
| 1:2:4 | 0.333 | Na 2H 8B 4O 11 |
Na 2B 4O 7 · 4H2O |
Na 2B 4O 5(OH) 4 |
borax "tetrahydrate" |
| 1:2:5 | 0.333 | Na 2H 10B 4O 12 = NaB 2H 5O 6 |
Na 2B 4O 7 · 5H2O |
Na 2B 4O 5(OH) 4 · H2O |
borax "pentahydrate"[2] |
| 1:2:10 | 0.333 | Na 2H 20B 4O 17 |
Na 2B 4O 7 · 10H2O |
Na 2B 4O 5(OH) 4 · 8H2O |
borax "decahydrate"[2] |
| 1:3:0 | 0.250 | Na 2B 6O 10 = NaB 3O 5 |
NaB 3O 5 |
? | |
| 1:4:0 | 0.200 | Na 2B 8O 13 |
Na 2B 8O 13 |
disodium octaborate (anhydrous) | |
| 2:9:11 | 0.182 | Na 4H 22B 18O 31 |
Na 4B 18O 29 · 11H2O |
Na 2[B 8O 11(OH) 4] · [B(OH) 3] · 2H2O [1] |
disodium enneaborate |
| 1:5:2 | 0.167 | Na 2H 4B 10O 18 = NaH 2B 5O 9 |
NaB 5O 8 · H2O |
? | sodium pentaborate "monohydrate"[1] |
| 1:5:4 | 0.167 | Na 2H 8B 10O 20 = NaH 4B 5O 10 |
NaB 5O 8 · 2H2O |
Na[B 5O 7(OH) 2] · H2O |
sodium pentaborate "dihydrate"[3] |
| 1:5:10 | 0.167 | Na 2H 20B 10O 26 = NaH 10B 5O 13 |
NaB 5O 8 · 5H2O |
? | sodium pentaborate "pentahydrate"[4] |
| 3:5:4 | 0.125 | Na 6H 8B 10O 22 = Na 3H 4B 5O 11 |
Na 3B 5O 7 · 2H2O |
Na 3B 5O 8(OH) 2 · H2O |
trisodium pentaborate[5] |
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.[6]
Some sodium borates hower cannot be analyzed as combinations xNa
2O · yB
2O
3 · zH
2O of the three ordinary oxides. The most important example is sodium perborate, originally described as NaBO
3 · H2O but actually (Na+
)
2[B
2O
4(OH)
4]2−. The anion of this compound has two peroxide bridges –O–O– which make it oxygen-rich compared to the general family above.
References
- ↑ 1.0 1.1 1.2 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. doi:10.1021/acs.inorgchem.7b00823
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 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. doi:10.1021/je60034a005
- ↑ 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. doi:10.1016/j.saa.2008.04.021
- ↑ 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. doi:10.1016/j.mtcomm.2022.103157
- ↑ Silvio Menchetti and Cesare Sabelli (1977): "The crystal structure of synthetic sodium pentaborate monohydrate". Acta Crystallographica Section B, volume B33, pages 3730-3733. doi:10.1107/S0567740877011959
- ↑ 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.doi:10.1098/rspl.1904.0112
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