Sodium metaborate
Sodium metaborate is a chemical compound of sodium, boron, and oxygen with formula Template:Chem2.[1] However, the metaborate ion is trimeric in the anhydrous solid, therefore a more correct formula is Template:Chem2 or Template:Chem2. The formula can be written also as Template:Chem2 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 Template:Chem2 for various values of n.
The anhydrous and hydrates are colorless crystalline solids. The anhydrous form is hygroscopic.[3]
Hydrates and solubility
The following hydrates crystallize from solutions of the proper composition in various temperature ranges:[4]
- tetrahydrate Template:Chem2 from −6 to 53.6 °C
- dihydrate Template:Chem2 from 53.6 °C to 105 °C
- hemihydrate Template:Chem2 from 105 °C to the boiling point.
Early reports of a monohydrate Template:Chem2 have not been confirmed.[4]
Structure
Anhydrous
Solid anhydrous sodium metaborate has the hexagonal crystal system with space group . It actually contains a six-membered rings with the formula Template:Chem2, consisting of alternating boron and oxygen atoms with one negatively charged extra oxygen atom attached to each boron atom.[5][6] All nine atoms lie on a plane.[3] 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.[5] The density is 2.348 ± 0.005 g/cm3.[3] The approximate dimensions of the hexagonal cell are a = 1275 pm, c = 733 pm.[3] However, the true unit cell is rhombohedral and has dimensions: ar= 776 pm, α = 110.6°, Z = 6 (5.98) molecules KB0[3]
Dihydrate
The dihydrate Template:Chem2 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]
Vapor
Infrared spectroscopy of the vapor from anhydrous sodium metaborate, heated to between 900 °C and 1400 °C, shows mostly isolated clusters with formula Template:Chem2, and some dimers thereof.[8] Electron diffraction studies by Akishin and Spiridonov showed a structure Template:Chem2 with linear anion Template:Chem2 and angle Template:Chem2 of 90-110°. The atomic distances are Template:Chem2: 120 pm, Template:Chem2: 136 pm,Template:Chem2: 214 pm[9]
Preparation
Sodium metaborate is prepared by the fusion of sodium carbonate and boron oxide Template:Chem2[6][3] or borax Template:Chem2. 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 Template:Chem2.[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 Template:Chem2 with a suitable catalyst gives sodium metaborate and hydrogen gas:[11]
- Template:Chem2 (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 Template:Chem2.[11]
Electrochemical conversion to borax
Electrolysis of a concentrated aqueous solution of 20%Template:Cln Template:Chem2 with an anion exchange membrane and inert anode (such as gold, palladium, or boron-doped diamond) converts the metaborate anion to tetraborate Template:Chem2, and the sodium salt of the later (borax) precipitates as a white powder.[12]
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,[12] or with magnesium hydride Template:Chem2 by ball milling at room temperature, followed by extraction of the Template:Chem2 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]
Reaction with alcohols
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]
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.[16]
- Raising the pH of injected fluids for oil extraction.[17]
See also
References
- ↑ "Sodium metaborate" Substance page at the Chemister website. Accessed on 2022-06-28.
- ↑ 2.0 2.1 Template:Cite journal
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Template:Cite journal
- ↑ 4.0 4.1 Template:Cite journal
- ↑ 5.0 5.1 Template:Cite journal
- ↑ 6.0 6.1 Template:Cite journal
- ↑ 7.0 7.1 Template:Cite journal
- ↑ Template:Cite journal
- ↑ Template:Cite journal
- ↑ 10.0 10.1 10.2 Template:Cite journal
- ↑ 11.0 11.1 11.2 Template:Cite journal
- ↑ 12.0 12.1 Template:Cite journal
- ↑ Template:Cite journal
- ↑ Template:Cite web
- ↑ 15.0 15.1 Template:Cite journal
- ↑ "BareSpot Monobor-Chlorate". Product safety data sheet at the BareSpot company website.Retrieved 2022-06-28.
- ↑ Template:Cite journal