 Sodium metaboratemonomer | |
 Sodium metaboratetrimer | |
| Names | |
|---|---|
| IUPAC name Sodium metaborate | |
| Identifiers | |
3D model (JSmol) | |
| ChEBI | |
| ChemSpider | |
| ECHA InfoCard | 100.028.992 |
| EC Number |
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| RTECS number |
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| UNII | |
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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 inether,ethanol |
| Structure | |
| trigonal | |
| Thermochemistry | |
| 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) | 2330 mg/kg (rat, oral) |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Sodium metaborate is achemical compound ofsodium,boron, andoxygen withformulaNaBO2.[3] However, themetaborate ion is trimeric in the anhydrous solid, therefore a more correct formula isNa3B3O6 or(Na+)3[B3O6]3−. The formula can be written also asNa2O·B2O3 to highlight the relation to the mainoxides of sodium and boron.[2] The name is also applied to severalhydrates whose formulas can be writtenNaBO2·nH2O for various values ofn.
The anhydrous and hydrates are colorless crystalline solids. The anhydrous form ishygroscopic.[4]
The followinghydrates crystallize from solutions of the proper composition in various temperature ranges:[5]
Early reports of a monohydrateNaBO2·H2O have not been confirmed.[5]
Solid anhydrous sodium metaborate has thehexagonal crystal system withspace 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 sixoxygen atoms are evenly divided into two distinct structural sites, with different B–Obond lengths: B–O(external) 128.0pm and B–O(bridge) 143.3 pm.[6] The density is 2.348 ± 0.005g/cm3.[4] The approximate dimensions of the hexagonal cell area = 1275 pm,c = 733 pm.[4] However, the true unit cell isrhombohedral and has dimensions:ar= 776 pm, α = 110.6°,Z = 6 (5.98) molecules KB0[4]
The dihydrateNaBO2·2H2O crystallizes in thetriclinic crystal system, but is nearlymonoclinic, with both α and γ very close to 90°. The cell parameters area = 678 pm ,b = 1058A pm,c = 588 pm, α = 91.5°, β = 22.5°, γ = 89°,Z = 4, density 1.905 g/cm3. Therefractive indices at 25°C and wavelength 589.3nm 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 formulaNaBO2, and some dimers thereof.[8]Electron diffraction studies by Akishin and Spiridonov showed a structureO=B−O−Na with linear anionO=B−O− and angleB−O−Na of 90-110°. The atomic distances areO=B: 120 pm,B−O: 136 pm,O−Na: 214 pm[9]
Sodium metaborate is prepared by the fusion ofsodium carbonate andboron oxideB2O3[1][4] orboraxNa2B4O7. Another way to create the compound is by the fusion ofborax withsodium 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 oxideNa2O.[2]
The anhydrous salt can also be prepared from the tetraborate by heating to 270 °C in vacuum.[10]
Although not performed industrially,hydrolysis ofsodium borohydrideNa[BH4] with a suitablecatalyst gives sodium metaborate andhydrogen gas:[11]
When sodium metaborate is dissolved in water, the anion combines with two water molecules to form thetetrahydroxyborate anion[B(OH)4]−.[11]
Electrolysis of a concentrated aqueous solution of 20%[clarification needed]NaBO2·4H2O with ananion exchange membrane and inert anode (such asgold,palladium, orboron-doped diamond) converts the metaborate anion totetraborateB4O2−7, 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 variousreducing agents at high temperatures and pressure,[12] or withmagnesium hydrideMgH2 byball milling at room temperature, followed by extraction of theNa[BH4] withisopropylamine.[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 ofhydroxide:
Nanofiltration membranes can effectively separate the borohydride from the metaborate.[11]
Anhydrous sodium metaboraterefluxed withmethanol yields the correspondingsodium tetramethoxyborate (melting point: 253-258 °C,CAS number: 18024-69-6[14]):[15]
The analogous reaction withethanol yields thesodium tetraethoxyborate.[15]
Current and proposed applications of sodium metaborate include:
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