Sodium metaborate

The following hydrates crystallize from solutions of the proper composition in various temperature ranges: • tetrahydrate from −6 to 53.6 °C • dihydrate from 53.6 °C to 105 °C • hemihydrate from 105 °C to the boiling point. Early reports of a monohydrate have not been confirmed. ==Structure==

Anhydrous Solid anhydrous sodium metaborate has the hexagonal crystal system with space group R\bar3 c. It actually contains a six-membered rings with the formula , 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: ar= 776 pm, α = 110.6°, Z = 6 (5.98) molecules KB0 Dihydrate The dihydrate 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 , and some dimers thereof. Electron diffraction studies by Akishin and Spiridonov showed a structure with linear anion and angle of 90-110°. The atomic distances are : 120 pm, : 136 pm,: 214 pm ==Preparation==

Sodium metaborate is prepared by the fusion of sodium carbonate and boron oxide or borax . 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 . 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 with a suitable catalyst gives sodium metaborate and hydrogen gas: : (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 .): : 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==