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Sodium metaborate

Sodium metaborate monomer
Sodium metaborate trimer
Names
IUPAC name Sodium metaborate
Identifiers
CAS Number	7775-19-1
3D model (JSmol)	Interactive image
ChEBI	CHEBI:75227
ECHA InfoCard	100.028.992
EC Number	231-891-6
PubChem CID	145326
RTECS number	ED4640000
UNII	Z6Q395A23R
CompTox Dashboard (EPA)	DTXSID2034386
InChI InChI=1S/BO2.Na/c2-1-3;/q-1;+1Key: NVIFVTYDZMXWGX-UHFFFAOYSA-N
SMILES B(=O).
Properties
Chemical formula	NaBO2
Molar mass	65.80 g·mol
Appearance	Colorless crystals
Odor	Odorless
Density	2.464 g/cm (anhydrous)
Melting point	966 °C (1,771 °F; 1,239 K)
Boiling point	1,434 °C (2,613 °F; 1,707 K)
Solubility in water	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
Crystal structure	trigonal
Thermochemistry
Heat capacity (C)	65.94 J/(mol·K)
Std molar entropy (S298)	73.39 J/(mol·K)
Std enthalpy of formation (ΔfH298)	−1059 kJ/mol
Hazards
Lethal dose or concentration (LD, LC):
LD50 (median dose)	2330 mg/kg (rat, oral)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). Infobox references

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

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

Hydrates and solubility

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

• 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.

Structure

Anhydrous

Solid anhydrous sodium metaborate has the hexagonal crystal system with space group ${\displaystyle R{\bar {3}}c}$. It actually contains a six-membered rings with the formula [B₃O₆], consisting of alternating boron and oxygen atoms with one negatively charged extra oxygen atom attached to each boron atom. All nine atoms lie on a plane. 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. The density is 2.348 ± 0.005 g/cm. The approximate dimensions of the hexagonal cell are a = 1275 pm, c = 733 pm. However, the true unit cell is rhombohedral and has dimensions: a_r= 776 pm, α = 110.6°, Z = 6 (5.98) molecules KB0

Dihydrate

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/cm. 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.

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.

Vapor

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. 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

Preparation

Sodium metaborate is prepared by the fusion of sodium carbonate and boron oxide B₂O₃ 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.

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

Although not performed industrially, hydrolysis of sodium borohydride Na[BH₄] with a suitable catalyst gives sodium metaborate and hydrogen gas:

Na[BH₄] + 2 H₂O → NaBO₂ + 4 H₂ (ΔH = −217 kJ/mol)

Reactions

With water

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

Electrochemical conversion to borax

Electrolysis of a concentrated aqueous solution of 20% 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.

BO2−2 + 2 OH → B₄O2−7 + H₂O + 4 e

Reduction to sodium borohydride

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

NaBO₂ + 2 MgH₂ → Na[BH₄] + 2 MgO

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

BO−2 + 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.

Reaction with alcohols

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

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

The analogous reaction with ethanol yields the sodium tetraethoxyborate.

Uses

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.
• Raising the pH of injected fluids for oil extraction.

See also

• Borate

References

1. ^ Fang, Ssu-Mien (1938). "The Crystal Structure of Sodium Metaborate Na₃ (B₃ O₆)". Zeitschrift für Kristallographie - Crystalline Materials. 99 (1–6): 1–8. doi:10.1524/zkri.1938.99.1.1.
2. ^ Cole, Sandford S.; Taylor, Nelson W. (1935). "THE SYSTEM Na₂O-B₂O₃, IV Vapor Pressures of Boric Oxide, Sodium Metaborate, and Sodium Diborate between 1150°C and 1400°C". Journal of the American Ceramic Society. 18 (1–12): 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. ^ Zachariasen, W. H. (1937). "The Crystal Structure of Potassium Metaborate, K3(B3O6)". The Journal of Chemical Physics. 5 (11): 919–922. doi:10.1063/1.1749962.
5. ^ Nies, Nelson P.; Hulbert, Richard W. (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 & Engineering Data. 12 (3): 303–313. doi:10.1021/je60034a005.
6. ^ Marezio, M.; Plettinger, H. A.; Zachariasen, W. H. (1963). "The bond lengths in the sodium metaborate structure". Acta Crystallographica. 16 (7): 594–595. Bibcode:1963AcCry..16..594M. doi:10.1107/S0365110X63001596.
7. ^ Krc, John (1951). "Crystallographic Data. 44. Sodium Metaborate Dihydrate". Analytical Chemistry. 23 (5): 806. doi:10.1021/ac60053a043.
8. Alfred Büchler and Edward P. Marram (1963). "Gaseous Metaborates. II. Infrared Spectraof Alkali Metaborate Vapors". Journal of Chemical Physics. 39: 292. doi:10.1063/1.173424439 (inactive 1 November 2024).{{cite journal}}: CS1 maint: DOI inactive as of November 2024 (link)
9. Akishin, P. A.; Spiridonov, V. P. (1962). "Electron diffraction study of the structure of LiBO2 and NaBO2 metaborate molecules in the vapor state". Journal of Structural Chemistry. 3 (3): 251–253. doi:10.1007/BF01151477.
10. ^ Kong, Lingyan; Cui, Xinyu; Jin, Huazi; Wu, Jie; Du, Hao; Xiong, Tianying (2009). "Mechanochemical Synthesis of Sodium Borohydride by Recycling Sodium Metaborate". Energy & Fuels. 23 (10): 5049–5054. doi:10.1021/ef900619y.
11. ^ Atiyeh, H.; Davis, B. (2007). "Separation of sodium metaborate from sodium borohydride using nanofiltration membranes for hydrogen storage application". International Journal of Hydrogen Energy. 32 (2): 229–236. Bibcode:2007IJHE...32..229A. doi:10.1016/j.ijhydene.2006.06.003.
12. ^ Park, E.; Ukjeong, S.; Hojung, U.; Kim, S.; Lee, J.; Woonam, S.; Hoonlim, T.; Junpark, Y.; Hoyu, Y. (2007). "Recycling of sodium metaborate to borax". International Journal of Hydrogen Energy. 32 (14): 2982–2987. Bibcode:2007IJHE...32.2982P. doi:10.1016/j.ijhydene.2007.03.029.
13. Li, Z.P.; Liu, B.H.; Arai, K.; Morigazaki, N.; Suda, S. (2003). "Protide compounds in hydrogen storage systems". Journal of Alloys and Compounds. 356–357: 469–474. doi:10.1016/S0925-8388(02)01241-0.
14. "Product". sigmaaldrich.com.
15. ^ Kemmitt, T.; Gainsford, G.J. (2009). "Regeneration of sodium borohydride from sodium metaborate, and isolation of intermediate compounds". International Journal of Hydrogen Energy. 34 (14): 5726–5731. Bibcode:2009IJHE...34.5726K. doi:10.1016/j.ijhydene.2009.05.108.
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17. Chen, Fuzhen; Jiang, Hanqiao; Bai, Xiaohu; Zheng, Wei (2013). "Evaluation the performance of sodium metaborate as a novel alkali in alkali/Surfactant/Polymer flooding". Journal of Industrial and Engineering Chemistry. 19 (2): 450–457. doi:10.1016/j.jiec.2012.08.029.

• Borates
• Sodium compounds