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 monomer
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Sodium metaborate trimer
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Names | |
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IUPAC name
Sodium metaborate
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Identifiers | |
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3D model (JSmol)
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ChEBI |
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ECHA InfoCard | 100.028.992 |
EC Number |
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PubChem CID
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UNII |
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CompTox Dashboard (EPA)
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Properties | |
NaBO2 | |
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 |
Structure | |
trigonal | |
Thermochemistry | |
Heat capacity (C)
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65.94 J/(mol·K) |
Std molar
entropy (S⦵298) |
73.39 J/(mol·K) |
Std enthalpy of
formation (ΔfH⦵298) |
−1059 kJ/mol |
Hazards | |
Lethal dose or concentration (LD, LC): | |
LD50 (median dose)
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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
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The anhydrous and hydrates are colorless crystalline solids. The anhydrous form is hygroscopic.[4]
The following hydrates crystallize from solutions of the proper composition in various temperature ranges:[5]
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.[6] 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 rhombohedral 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.[7]
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.[7]
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.[8] 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[9]
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:
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.[10]
Although not performed industrially, hydrolysis of sodium borohydride Na[BH4] with a suitable catalyst gives sodium metaborate and hydrogen gas:[11]
When sodium metaborate is dissolved in water, the anion combines with two water molecules to form the tetrahydroxyborate anion [B(OH)4]−.[11]
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.[12]
Sodium metaborate can be converted to sodium borohydride by several methods, including the reaction with various reducing agents at high temperatures and pressure,[12] or with magnesium hydride MgH2 by ball milling at room temperature, followed by extraction of the Na[BH4] with isopropylamine.[13][10]
Another method is the electrolytic reduction of a concentrated sodium metaborate solution,[10] namely
However, this method is not efficient since it competes with the reduction of hydroxide:
Nanofiltration membranes can effectively separate the borohydride from the metaborate.[11]
Anhydrous sodium metaborate refluxed with methanol yields the corresponding sodium tetramethoxyborate (melting point: 253-258 °C, CAS number: 18024-69-6[14]):[15]
The analogous reaction with ethanol yields the sodium tetraethoxyborate.[15]
Current and proposed applications of sodium metaborate include: