Sodium metaborate
A boron compound used in glass, herbicides, and oil extraction.
Sodium metaborate is a chemical compound of sodium, boron, and oxygen with formula NaBO2, though the anhydrous solid contains the trimeric metaborate ion, making Na3B3O6 or (Na+)3[B3O6]3− more correct. It forms several hydrates and is used in the manufacture of borosilicate glasses, herbicides, and oil extraction.
- formula
- NaBO2 (anhydrous); Na3B3O6 (trimeric form)
- crystal_system
- Hexagonal (anhydrous); triclinic (dihydrate)
- density
- 2.348 g/cm³ (anhydrous); 1.905 g/cm³ (dihydrate)
- boiling_point
- 1434 °C
- transition_temperatures
- Tetrahydrate stable from −6 to 53.6 °C; dihydrate from 53.6 to 105 °C; hemihydrate from 105 °C to boiling point
Lore & Background
Sodium metaborate is prepared by fusion of sodium carbonate and boron oxide or borax, or by fusion of borax with sodium hydroxide at 700 °C. The anhydrous form is hygroscopic and crystallizes in a hexagonal system with space group R3̄c, containing planar six-membered [B3O6]3− rings. Its hydrates—tetrahydrate, dihydrate, and hemihydrate—crystallize from solutions at specific temperature ranges; a reported monohydrate has not been confirmed.
Reader's Guide
Sodium metaborate is significant for its role in producing borosilicate glasses, which resist thermal shock due to low thermal expansion. It is also used in herbicides and to raise pH in oil extraction fluids. The compound can be electrochemically converted to borax or reduced to sodium borohydride, a potential hydrogen storage material. Its vapor, studied by infrared spectroscopy and electron diffraction, shows isolated NaBO2 clusters with a linear O=B−O− anion. The compound's boiling point (1434 °C) is lower than that of boron oxide or borax, and it boils without composition change.
Did You Know?
- The anhydrous solid contains trimeric [B3O6]3− rings with alternating boron and oxygen atoms.
- The dihydrate remains metastable up to 106–110 °C before slowly changing.
- Electrolysis of a 20% NaBO2·4H2O solution with an inert anode converts metaborate to tetraborate, precipitating borax.
- Sodium metaborate can be reduced to sodium borohydride by ball milling with magnesium hydride at room temperature.
More in Sodium Compounds 1-24
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