Sodium metaborate is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one ele ...

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

, boron, and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

with formula . However, the

metaborate A metaborate is an anion (negative ion) consisting of boron and oxygen, with empirical formula ; or any salt with such anions, such as sodium metaborate, or calcium metaborate . It is one of the boron oxoanions or borates In aqueous solut ...

ion is trimeric in the anhydrous solid, therefore a more correct formula is or . The formula can be written also as · to highlight the relation to the main oxides of sodium and boron. The name is also applied to several hydrates whose formulas can be written for various values of ''n''. The anhydrous and hydrates are colorless crystalline solids. The anhydrous form is

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...

Hydrates and solubility

The following hydrates crystallize from solutions of the proper composition in various temperature ranges: * tetrahydrate ·4 from −6 to 53.6 °C * dihydrate ·2 from 53.6 °C to 105 °C * hemihydrate ·0.5 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 In crystallography, the hexagonal crystal family is one of the six crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the trigonal crysta ...

with

space group In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it ...

R\bar3 c

. It actually contains the trimeric anion . The nine atoms lie on a plane. The six

atoms are evenly divided into two distinct structural sites, with different B–O

bond length In molecular geometry, bond length or bond distance is defined as the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest ...

s (about 128 and 143 pm, respectively). 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 rhomboedral and has dimensions: ''a''_r= 776 pm, α = 110.6°, ''Z'' = 6 (5.98) molecules KB0

Dihydrate

The dihydrate crystallizes in the

triclinic crystal system 180px, Triclinic (a ≠ b ≠ c and α ≠ β ≠ γ ) In crystallography, the triclinic (or anorthic) crystal system is one of the 7 crystal systems. A crystal system is described by three basis vectors. In the triclinic system, the crystal is ...

, but is nearly

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...

, 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.

Vapor

Infrared spectroscopy Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or function ...

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 Electron diffraction refers to the bending of electron beams around atomic structures. This behaviour, typical for Wave (physics), waves, is applicable to electrons due to the wave–particle duality stating that electrons behave as both particle ...

studies by Akishin and Spiridonov showed a structure with linear anion and angle of 90-110°. The atomic distances were : 120 pm, : 136 pm,: 214 pm

Preparation

Sodium metaborate is prepared by the fusion of sodium carbonate and boron oxide or

borax Borax is a salt (chemistry), salt (ionic compound), a hydration (chemistry), hydrated borate of sodium, with chemical formula often written . It is a colorless crystalline solid, that dissolves in water to make a base (chemistry), basic aqueo ...

. Another way to create the compound is by the fusion of

with

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkal ...

at 700 °C: : + 2 → 2 + 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.

Reactions

With water

When sodium metaborate is dissolved in water, the anion combines with two water molecules to form the tetrahydroxyborate anion .

Electrochemical conversion to borax

Electrolysis of a concentrated solution of 20% ·4 with an

anion exchange membrane An anion exchange membrane (AEM) is a semipermeable membrane generally made from ionomers and designed to conduct anions but reject gases such as oxygen or hydrogen. Applications Anion exchange membranes are used in electrolytic cells and fuel ...

and inert anode (such as

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile ...

palladium Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself nam ...

, or boron-doped diamond) converts the metaborate anion to

tetraborate In chemistry, tetraborate or pyroborate is an anion (negative ion) with formula ; or a salt containing that anion, such as sodium tetraborate, . It is one of the boron oxoacids, that is, a borate. The name is also applied to the hydrated ion ...

, and the sodium salt of the later (

) precipitates as a white powder. : + 2 → + + 4 e⁻

Hydrolysis of sodium borohydride

Hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysis ...

sodium borohydride Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula Na BH4. This white solid, usually encountered as an aqueous basic solution, is a reducing agent that finds applic ...

with a suitable

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

gives sodium metaborate: : + 2 → + 4 (Δ''H'' = -217 kJ/ mol)

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 by

ball mill A ball mill is a type of grinder used to grind or blend materials for use in mineral dressing processes, paints, pyrotechnics, ceramics, and selective laser sintering. It works on the principle of impact and attrition: size reduction is done ...

ing at room temperature, followed by extraction of the with

isopropylamine Isopropylamine (monoisopropyl amine, MIPA, 2-Propylamine) is an organic compound, an amine. It is a hygroscopic colorless liquid with ammonia-like odor. It is miscible with water and flammable. It is a valuable intermediate in chemical industry. ...

. : + 2 → + 2 MgO Another method is the electrolytic reduction of a concentrated sodium metaborate solution, namely : + 6 + 8 e⁻ → + 8 However, this method is not efficient since it competes with the reduction of

hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water ...

, 4 → 2 + + 4 e⁻

Nanofiltration membrane Nanofiltration is a membrane filtration process used most often to soften and disinfect water. Overview Nanofiltration is a membrane filtration-based method that uses nanometer sized pores through which particles smaller than 10 nanometers pas ...

s can effectively separate the borohydride from the metaborate.

Reaction with alcohols

Anhydrous sodium metaborate

reflux Reflux is a technique involving the condensation of vapors and the return of this condensate to the system from which it originated. It is used in industrial and laboratory distillations. It is also used in chemistry to supply energy to reaction ...

ed with methanol yields the corresponding sodium tetramethoxyborate: : + 4 → + 2 The analogous reaction with

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a h ...

yields the tetraethoxyborate.

Uses

Current and proposed applications of sodium metaborate include: *Manufacture of

borosilicate glass Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...

es, 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 An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturate ...

extraction.

References

P. A. Akishin and V. P. Spirtdonov (1962): "Electron Diffraction Study of the Structure Of LiBO2 and NaBO2 Metaborate Molecules in the Vapor State". ''Zhumal Struktumoi Khimii'', volume 3, issue 3, pages 267-269. Alfred Büchler and Edward P. Marram (1963): "Gaseous Metaborates. II. Infrared Spectraof Alkali Metaborate Vapors". ''Journal of Chemical Physics'', volume 39, page 292. Hasan K. Atiyeh and Boyd R. Davis (2007): "Separation of sodium metaborate from sodium borohydride using nanofiltration membranes for hydrogen storage application". ''International Journal of Hydrogen Energy'', volume 32, issue 2, pages 229-236. Fuzhen Chen, Hanqiao Jiang, Xiaohu Bai, Wei Zheng (2013): "Evaluation the performance of sodium metaborate as a novel alkali in alkali/surfactant/polymer flooding". ''Journal of Industrial and Engineering Chemistry'', volume 19, issue 2, 25 March , Pages 450-457. John Krc, Jr. (1951): "Crystallographic Data. 44. Sodium Metaborate Dihydrate". ''Analytical Chemistry'', volume 23, issue 5, page 806. BareSpot Monobor-Chlorate
. Product safety data sheet at the BareSpot company website. Accessed on 2022-06-28. Sodium metaborate
Substance page at the Chemister website. Accessed on 2022-06-28. Z. P. Li, B. H. Liu. K. Arai, N. Morigazaki, S. Suda (2003): "Protide compounds in hydrogen storage systems". ''Journal of Alloys and Compounds'', volumes 356–357, pages 469-474. T. Kemmitt and G. J. Gainsford (2009): "Regeneration of sodium borohydride from sodium metaborate, and isolation of intermediate compounds" ''International Journal of Hydrogen Energy'', volume 34, issue 14, pages 5726-5731. Lingyan Kong, Xinyu Cui, Huazi Jin, Jie Wu, Hao Du, and Tianying Xiong (2009): "Mechanochemical Synthesis of Sodium Borohydride by Recycling Sodium Metaborate". ''Energy Fuels'', volume 23, issue 10, pages 5049-5054. Eun Hee Park, Seong Uk Jeong, Un Ho Jung, Sung Hyun Kim, Jaeyoung Lee, Suk Woo Nam, Tae Hoon Lim, Young Jun Park, Yong Ho Yuc (2007): "Recycling of sodium metaborate to borax". ''International Journal of Hydrogen Energy'', volume 32, issue 14, pages 2982-2987. Nelson P. Nies and Richard W. Hulbert (1967): "Solubility isotherms in the system sodium oxide-boric oxide-water. Revised solubility-temperature curves of boric acid, borax, sodium pentaborate, and sodium metaborate". ''Journal of Chemical and Engineering Data'', volume 12, issue 3, pages 303-313. Ssu-Mien Fang (1938): "The Crystal Structure of Sodium Metaborate ". ''Zeitschrift für Kristallographie - Crystalline Materials'', volume 99, issue 1-6, pages 1–8, M. Marezio, H. A. Plettinger and W. H. Zachariasen (1963): "The bond lengths in the sodium metaborate structure", ''Acta Crystallographica'', volume 16, pages 594-595. Sandford S. Cole and Nelson W. Taylor, "The system -, IV: Vapor Pressures of Boric Oxide, Sodium Metaborate, and Sodium Diborate between 1150°C and 1400°C". ''Journal of the American Ceramic Society'', volume 18, issue 1‐12, pages 82-85 W. H. Zachariasen (1937): "The Crystal Structure of Potassium Metaborate, K3(B3O6)". ''Journal of Chemical Physics'', volume 5, issue 11, page 919. {{Borates Borates Sodium compounds