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[B4O5(OH)4]2−·8H2O = Na2B4H20O17.

The ternary phase diagram of the Na2O–B2O3–H2O 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 xNa2O·yB2O3·zH2O = Na2xB2yH2zOx+3y+z, with x, y, and z chosen to fit the elemental formula, or a multiple thereof. Thus, for example, borax Na2B4H20O17 would be 1Na2O·2B2O3·10H2O, and boric acid B(OH)3 would be 0Na2O·1B2O3·1H2O = 2[B(OH)3].

The elemental formula was often intrepreted as a z-hydrate of an "anhydrous" salt without any hydrogen, namely Na2xB2yO3y·zH2O. 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 Na2B4O7·10H2O, with the implied anion [B4O7]2−, whereas the correct formula is Na2B4O5(OH)4·8H2O, with anion [B4O5(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 Na2O in the "anhydrous" version.

x:y:zx/(x+y)Elemental formulaAs hydrateCorrect formulaName
3:1:00.750Na6B2O6 = Na3BO3Na3BO3trisodium orthoborate (anhydrous)
1:1:00.500Na2B2O4 = NaBO2       Na3B3O6sodium metaborate
1:1:10.500Na2H2B2O5Na2B2O4·H2O ? ?[2]
1:1:40.500Na2H8B2O8 = NaBH4O4NaBO2·2H2ONaB(OH)4sodium tetrahydroxyborate[2]
1:1:80.500Na2H16B2O12 = NaBH8O6NaBO2·4H2O ? ?[2]
1:2:00.333Na2B4O7Na2B4O7borax (anhydrous)
1:2:40.333Na2H8B4O11Na2B4O7·4H2ONa2B4O5(OH)4borax "tetrahydrate"
1:2:50.333Na2H10B4O12 = NaB2H5O6Na2B4O7·5H2ONa2B4O5(OH)4·H2Oborax "pentahydrate"[2]
1:2:100.333Na2H20B4O17Na2B4O7·10H2ONa2B4O5(OH)4·8H2Oborax "decahydrate"[2]
1:3:00.250Na2B6O10 = NaB3O5NaB3O5 ?
1:4:00.200Na2B8O13Na2B8O13disodium octaborate (anhydrous)
2:9:110.182Na4H22B18O31                    Na4B18O29·11H2ONa2[B8O11(OH)4]·[B(OH)3]·2H2O [1]disodium enneaborate
1:5:20.167Na2H4B10O18 = NaH2B5O9NaB5O8·H2O ?sodium pentaborate "monohydrate"[1]
1:5:40.167Na2H8B10O20 = NaH4B5O10  NaB5O8·2H2ONa[B5O7(OH)2]·H2Osodium pentaborate "dihydrate"[3]
1:5:100.167Na2H20B10O26 = NaH10B5O13NaB5O8·5H2O ?sodium pentaborate "pentahydrate"[4]
3:5:40.125Na6H8B10O22 = Na3H4B5O11Na3B5O7·2H2ONa3B5O8(OH)2·H2Otrisodium 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 xNa2O·yB2O3·zH2O of the three ordinary oxides. The most important example is sodium perborate, originally described as NaBO3·H2O but actually (Na+)2[B2O4(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. 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. 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
  3. 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
  4. 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
  5. 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
  6. 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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