Sodium metaborate

Sodium metaborate is a chemical compound of sodium, boron, and oxygen with formula NaBO2.[3] However, the metaborate ion is trimeric in the anhydrous solid, therefore a more correct formula is Na3B3O6 or (Na+)3[B3O6]3−. The formula can be written also as Na2O·B2O3 to highlight the relation to the main oxides of sodium and boron.[2] The name is also applied to several hydrates whose formulas can be written NaBO2·nH2O for various values of n.

Sodium metaborate
Sodium metaborate monomer with bent anion.png
Sodium metaborate monomer
Sodium metaborate trimer.png
Sodium metaborate trimer
IUPAC name
Sodium metaborate
3D model (JSmol)
ECHA InfoCard 100.028.992 Edit this at Wikidata
EC Number
  • 231-891-6
RTECS number
  • ED4640000
  • InChI=1S/BO2.Na/c2-1-3;/q-1;+1
  • B(=O)[O-].[Na+]
Molar mass 65.80 g·mol−1
Appearance Colorless crystals
Odor Odorless
Density 2.464 g/cm3 (anhydrous)[1]
Melting point 966 °C (1,771 °F; 1,239 K)
Boiling point 1,434[2] °C (2,613 °F; 1,707 K)
16.4 g/(100 mL) (0 °C)
28.2 g/(100 mL) (25 °C)
125.2 g/(100 mL) (100 °C)
Solubility insoluble in ether, ethanol
65.94 J/(mol·K)
73.39 J/(mol·K)
−1059 kJ/mol
Lethal dose or concentration (LD, LC):
2330 mg/kg (rat, oral)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

The anhydrous and hydrates are colorless crystalline solids. The anhydrous form is hygroscopic.[4]

Hydrates and solubilityEdit

The following hydrates crystallize from solutions of the proper composition in various temperature ranges:[5]

  • tetrahydrate NaBO2·4H2O from −6 to 53.6 °C
  • dihydrate NaBO2·2H2O from 53.6 °C to 105 °C
  • hemihydrate NaBO2·0.5H2O from 105 °C to the boiling point.

Early reports of a monohydrate NaBO2·H2O have not been confirmed.[5]



Solid anhydrous sodium metaborate has the hexagonal crystal system with space group  . It actually contains a six-membered rings with the formula [B3O6]3−, consisting of alternating boron and oxygen atoms with one negatively charged extra oxygen atom attached to each boron atom.[6][1] All nine atoms lie on a plane.[4] The six oxygen atoms are evenly divided into two distinct structural sites, with different B–O bond lengths: B–O(external) 128.0 pm and B–O(bridge) 143.3 pm.[7] The density is 2.348 ± 0.005 g/cm3.[4] The approximate dimensions of the hexagonal cell are a = 1275 pm, c = 733 pm.[4] However, the true unit cell is rhomboedral and has dimensions: ar= 776 pm, α = 110.6°, Z = 6 (5.98) molecules KB0[4]


The dihydrate NaBO2·2H2O crystallizes in the triclinic crystal system, but is nearly monoclinic, with both α and γ very close to 90°. The cell parameters are a = 678 pm , b = 1058A pm, c = 588 pm, α = 91.5°, β = 22.5°, γ = 89°, Z = 4, density 1.905 g/cm3. The refractive indices at 25°C and wavelength 589.3 nm are α = 1.439, β = 1.473, γ = 1.484. The dispersion is strong, greater at red than at violet.[8]

The transition temperature between the dihydrate and the hemihydrate is 54 °C. However, the crystalline dihydrate will remain metastable until 106 °C to 110 °C, and change slowly above that temperature.[8]


Infrared spectroscopy of the vapor from anhydrous sodium metaborate, heated to between 900 °C and 1400 °C, shows mostly isolated clusters with formula NaBO2, and some dimers thereof.[9] Electron diffraction studies by Akishin and Spiridonov showed a structure O=B−O−Na with linear anion O=B−O and angle B−O−Na of 90-110°. The atomic distances are O=B: 120 pm, B−O: 136 pm,O−Na: 214 pm[10]


Sodium metaborate is prepared by the fusion of sodium carbonate and boron oxide B2O3[1][4] or borax Na2B4O7. Another way to create the compound is by the fusion of borax with sodium hydroxide at 700 °C:

B2O3 + 2 NaOH → 2 NaBO2 + H2O

The boiling point of sodium metaborate (1434 °C) is lower than that of boron oxide (1860 °C) and borax (1575 °C) In fact, while the metaborate boils without change of composition, borax gives off a vapor of sodium metaborate with a small excess of sodium oxide Na2O.[2]

The anhydrous salt can also be prepared from the tetraborate by heating to 270 °C in vacuum.[11]


With waterEdit

When sodium metaborate is dissolved in water, the anion combines with two water molecules to form the tetrahydroxyborate anion [B(OH)4].[12]

Electrochemical conversion to boraxEdit

Electrolysis of a concentrated aqueous solution of 20%[clarification needed] NaBO2·4H2O with an anion exchange membrane and inert anode (such as gold, palladium, or boron-doped diamond) converts the metaborate anion to tetraborate B4O2−7, and the sodium salt of the later (borax) precipitates as a white powder.[13]

BO2−2 + 2 OH → B4O2−7 + H2O + 4 e[clarification needed]

Hydrolysis of sodium borohydrideEdit

Hydrolysis of sodium borohydride Na[BH4] with a suitable catalyst gives sodium metaborate:

Na[BH4] + 2 H2O → NaBO2 + 4 H2H = −217 kJ/mol)[12]

Reduction to sodium borohydrideEdit

Sodium metaborate can be converted to sodium borohydride by several methods, including the reaction with various reducing agents at high temperatures and pressure,[13] or with magnesium hydride MgH2 by ball milling at room temperature, followed by extraction of the Na[BH4] with isopropylamine.[14][11]

NaBO2 + 2 MgH2 → Na[BH4] + 2 MgO

Another method is the electrolytic reduction of a concentrated sodium metaborate solution,[11] namely

BO2 + 6 H2O + 8 e → [BH4] + 8 OH

However, this method is not efficient since it competes with the reduction of hydroxide:

4 OH → 2 H2O + O2 + 4 e

Nanofiltration membranes can effectively separate the borohydride from the metaborate.[12]

Reaction with alcoholsEdit

Anhydrous sodium metaborate refluxed with methanol yields the corresponding sodium tetramethoxyborate (melting point: 253-258 °C, CAS number: 18024-69-6[15]):[16]

Na+BO2 + 4 CH3OH → Na+[B(OCH3)4] + 2 H2O

The analogous reaction with ethanol yields the sodium tetraethoxyborate.[16]


Current and proposed applications of sodium metaborate include:

  • Manufacture of borosilicate glasses, which are resistant to uneven or fast heating because of their small coefficient of thermal expansion.
  • Composition of herbicides.[17]
  • Raising the pH of injected fluids for oil extraction.[18]

See alsoEdit


  1. ^ a b c Ssu-Mien Fang (1938): "The Crystal Structure of Sodium Metaborate Na
    ". Zeitschrift für Kristallographie - Crystalline Materials, volume 99, issue 1-6, pages 1–8, doi:10.1524/zkri.1938.99.1.1
  2. ^ a b c Sandford S. Cole and Nelson W. Taylor, "The system Na
    , IV: Vapor Pressures of Boric Oxide, Sodium Metaborate, and Sodium Diborate between 1150°C and 1400°C". Journal of the American Ceramic Society, volume 18, issue 1‐12, pages 82-85 doi:10.1111/j.1151-2916.1935.tb19358.x
  3. ^ "Sodium metaborate" Substance page at the Chemister website. Accessed on 2022-06-28.
  4. ^ a b c d e f W. H. Zachariasen (1937): "The Crystal Structure of Potassium Metaborate, K3(B3O6)". Journal of Chemical Physics, volume 5, issue 11, page 919. doi:10.1063/1.1749962
  5. ^ a b 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
  6. ^ M. Marezio, H. A. Plettinger and W. H. Zachariasen (1963): "The bond lengths in the sodium metaborate structure", Acta Crystallographica, volume 16, pages 594-595. doi:10.1107/S0365110X63001596
  7. ^ M. Marezio, H. A. Plettinger and W. H. Zachariasen (1963): "The bond lengths in the sodium metaborate structure", Acta Crystallographica, volume 16, pages 594-595. doi:10.1107/S0365110X63001596
  8. ^ a b John Krc, Jr. (1951): "Crystallographic Data. 44. Sodium Metaborate Dihydrate". Analytical Chemistry, volume 23, issue 5, page 806. doi:10.1021/ac60053a043
  9. ^ Alfred Büchler and Edward P. Marram (1963): "Gaseous Metaborates. II. Infrared Spectraof Alkali Metaborate Vapors". Journal of Chemical Physics, volume 39, page 292. doi:10.1063/1.173424439
  10. ^ P. A. Akishin and V. P. Spirtdonov (1962): "Electron Diffraction Study of the Structure Of LiBO2 and NaBO2 Metaborate Molecules in the Vapor State". Zhumal Struktumoi Khimii, volume 3, issue 3, pages 267-269. doi:10.1007/BF01151477
  11. ^ a b c Lingyan Kong, Xinyu Cui, Huazi Jin, Jie Wu, Hao Du, and Tianying Xiong (2009): "Mechanochemical Synthesis of Sodium Borohydride by Recycling Sodium Metaborate". Energy Fuels, volume 23, issue 10, pages 5049-5054. doi:10.1021/ef900619y
  12. ^ a b c Hasan K. Atiyeh and Boyd R. Davis (2007): "Separation of sodium metaborate from sodium borohydride using nanofiltration membranes for hydrogen storage application". International Journal of Hydrogen Energy, volume 32, issue 2, pages 229-236. doi:10.1016/j.ijhydene.2006.06.003
  13. ^ a b Eun Hee Park, Seong Uk Jeong, Un Ho Jung, Sung Hyun Kim, Jaeyoung Lee, Suk Woo Nam, Tae Hoon Lim, Young Jun Park, Yong Ho Yuc (2007): "Recycling of sodium metaborate to borax". International Journal of Hydrogen Energy, volume 32, issue 14, pages 2982-2987. doi:10.1016/j.ijhydene.2007.03.029
  14. ^ Z. P. Li, B. H. Liu. K. Arai, N. Morigazaki, S. Suda (2003): "Protide compounds in hydrogen storage systems". Journal of Alloys and Compounds, volumes 356–357, pages 469-474. doi:10.1016/S0925-8388(02)01241-0
  15. ^
  16. ^ a b T. Kemmitt and G. J. Gainsford (2009): "Regeneration of sodium borohydride from sodium metaborate, and isolation of intermediate compounds" International Journal of Hydrogen Energy, volume 34, issue 14, pages 5726-5731. doi:10.1016/j.ijhydene.2009.05.108
  17. ^ "BareSpot Monobor-Chlorate". Product safety data sheet at the BareSpot company website. Accessed on 2022-06-28.
  18. ^ Fuzhen Chen, Hanqiao Jiang, Xiaohu Bai, Wei Zheng (2013): "Evaluation the performance of sodium metaborate as a novel alkali in alkali/surfactant/polymer flooding". Journal of Industrial and Engineering Chemistry, volume 19, issue 2, 25 March , Pages 450-457. doi:10.1016/j.jiec.2012.08.029