Sodium metaborate

Sodium metaborate is a colorless solid chemical compound of sodium, boron, and oxygen with formula NaBO
2.[3] The formula can be written also as Na
2O·B
2O
3 to highlight the relation to the main oxides of sodium and boron.[2]

Sodium metaborate
Identifiers
3D model (JSmol)
ECHA InfoCard 100.028.992
EC Number
  • 231-891-6
RTECS number
  • ED4640000
Properties
NaBO2
Molar mass 65.80 g/mol
Appearance colorless crystals
Odor odorless
Density 2.464 g/mL (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
Structure
trigonal
Thermochemistry
65.94 J/mol K
73.39 J/mol K
-1059 kJ/mol
Hazards
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).
Infobox references

Structure

Solid anhydrous sodium metaborate crystallizes in the hexagonal space group. It actually contains the trimeric anion [B
3O
6]3−
.[1] The six oxygen atoms are evenly divided into two distinct structural sites, with different B–O bond lengths (about 128 and 143 pm, respectively).[4]

Hydrates and solubility

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

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

Early reports of a monohydrate NaBO
2·H
2O have not been confirmed.[5]

The anhydrous salt can be prepared from the tetraborate by heating to 270 °C in vacuum.[6]

Preparation

Sodium metaborate is prepared by the fusion of sodium carbonate and boron oxide B
2O
3[1] or borax Na
2B
4O
7. Another way to create the compound is by the fusion of borax with sodium hydroxide at 700° C:

B
2O
3 + 2 NaOH → 2 NaBO
2 + 2 H
2O

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 Na
2O.[2]

Reactions

Electrochemical conversion to borax

Electrolysis of a concentrated solution of 20% NaBO
2·4H
2O with an anion exchange membrane and inert anode (such as gold, palladium, or boron-doped diamond) converts the metaborate anion to tetraborate B
4O2−
7, and the sodium salt of the later (borax) precipitates as a white powder.[7]

BO2−
2 + 2HO−
 → B
4O2−
7 + H
2O + 4 e−

Reduction to sodium borohydride

Sodium metaborate is also a byproduct of hydrolysis of sodium borohydride NaBH
4, a proposed hydrogen storage material for hydrogen-fueled vehicles that is safer (stable in dry air) and more efficient on a weight basis than most other alternatives.[7][8] The reaction is

NaBH
4 + 2 H
2O → NaBO
2 + 4 H
2

and requires a catalyst.

To be economical, that approach would require a cheap and efficient method to recycle the metaborate to the borohydride. Several methods have been studied, such as the reaction with various reducing agents at high temperatures and pressure,[7] or with magnesium hydride MgH
2 by ball milling at room temperature, followed by extraction of the NaBH
4 with isopropylamine.[8][6]

NaBO
2 + 2 MgH
2 → NaBH
4 + 2 MgO

Another alternative that has been considered is the electrolytic reduction of a concentrated sodium metaborate solution,[6] namely

BO2−
2 + 6 H
2O + 8 e− → BH−
4 + 8 HO−

However, this method is not efficient since it competes with the reduction of hydroxide, 4 HO−
 → 2 H
2O + O
2 + 4 e−

Conversion to sodium alkoxides

Anhydrous sodium metaborate refluxed with methanol yields the corresponding sodium methoxyborate:[9]

Na+
[BO
2]−
 + 4 CH
3OH → Na+
[B(OCH
3)
4]−
 + 2 H
2O

The analogous reaction with ethanol yields the ethoxyborate.[9]

Uses

Sodium metaborate is used in the manufacturing of borosilicate glasses. It is also a component of herbicides and antifreeze products.

See also

References
  1. Ssu-Mien Fang (1938): "The Crystal Structure of Sodium Metaborate Na
    3    (B
    3    O
    6    )    ". Zeitschrift für Kristallographie - Crystalline Materials, volume 99, issue 1-6, pages 1–8, doi:10.1524/zkri.1938.99.1.1
  2. Sandford S. Cole andNelson W. Taylor, "The system Na
    2    O    -B
    2    O
    3    , 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 at Chemister
  4. 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
  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
  6. 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
  7. 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
  8. 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
  9. 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
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