Sodium metaborate

Structure

Anhydrous

Solid anhydrous sodium metaborate has the hexagonal crystal system with space group ${\displaystyle R\overline{3}c}$. 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. 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/cm³. 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/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.

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.

A compound with an identical empirical formula but a distinct molecular and crystal structure is sodium tetrahydroxyborate (NaB(OH)₄).

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.

8 BO−2 → 2 B₄O2−7 + O₂ + 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):

Na⁺BO−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.