Metaboric acid
Names
IUPAC name
Oxoborinic acid
Other names
Metaboric acid
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.033.313
EC Number
  • 236-659-8
121829
  • InChI=1S/BHO2/c2-1-3/h2H checkY
    Key: VGTPKLINSHNZRD-UHFFFAOYSA-N checkY
  • O=BO
  • OB1OB(O)OB(O)O1
Properties
B3H3O6
Molar mass 131.45 g·mol−1
Appearance white solid
Density 1.784 g cm−3
Melting point 176 °C (349 °F; 449 K)
Acidity (pKa) 9.236
Conjugate base Metaborate
Structure
trigonal at B
Hazards
GHS labelling:[1]
GHS07: Exclamation mark
Warning
H315, H319, H335
P261, P305+P351+P338
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Metaboric acid is the name for a family of inorganic compounds with the same empirical formula HBO2.[2] that differ in their molecular structure. They are colourless water-soluble solids formed by the dehydration or decomposition of boric acid.

Metaboric acid is formally the parent acid of the metaborate anions.

Structure

The main forms of metaboric acid are:

  • Modification III, or trimeric, with the molecular formula H3B3O6. The molecule has C3h symmetry, with a six-member ring of alternating boron and oxygen atoms at the core, with OH groups attached to the borons. The crystal structure is orthorhombic with a sheet-like structure, similar to that of boric acid itself.[3] It is obtained by heating orthoboric acid at 80-100 °C, with loss of water:[3]
3 B(OH)3 → (BOH)3O3 + 3 H2O
  • Modification II. A polymer with structure similar to modification III, except that the rings are connected and 1/3 of the boron centres are tetrahedral. The molecular formula is therefore HO[−B(BOH)2O3O−]nH The crystal structure is monoclinic.[4] This form has a higher melting point (201 °C) and density (2.045 g/cm3) It is obtained by heating the trimeric form at 130-140 °C in a sealed ampoule (to prevent dehydration), orthorhombic metaboric acid converts to the monoclinic form (II):[4]
Conversion of orthorhombic to monoclinic metaboric acid.
  • Cubic form.[5] It is a white solid and is only slightly soluble in water that melts at about 236 °C. It is obtained by heating either modification II or III above 140 °C.[5]

Reactions

When heated above about 170 °C, metaboric acid dehydrates, forming tetraboric acid, also called pyroboric acid (H2B4O7):[6][7]

4 HBO2 → H2B4O7 + H2O

Metaborates

Metaborates are derivatives of BO2−. Like metaboric acid, the metaborates exist with disparate structures. Examples are sodium and potassium metaborates, salts formed by deprotonation of orthorhombic metaboric acid containing the cyclic B3O63− ion and calcium metaborate, Ca(BO2)2, which contains the chain polymeric ion (BO2−)n.[2]

References

  1. ↑ GHS: Sigma-Aldrich 413453
  2. 1 2 Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
  3. 1 2 H. J. Becher "Metaboric Acid" Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 791.
  4. 1 2 W. H. Zachariasen "The crystal structure of monoclinic metaboric acid" Acta Crystallogr. 1963, vol. 16, pp. 385-389. doi:10.1107/S0365110X6300102X
  5. 1 2 Freyhardt, C. C.; Wiebcke, M.; Felsche, J. (2000). "The monoclinic and cubic phases of metaboric acid (precise redeterminations)". Acta Crystallogr C. 56 (3): 276–278. doi:10.1107/S0108270199016042. PMID 10777918.
  6. ↑ Gurwinder Kaur, Shagun Kainth, Rohit Kumar, Piyush Sharma and O. P. Pandey (2021): "Reaction kinetics during non-isothermal solid-state synthesis of boron trioxide via boric acid dehydration." Reaction Kinetics, Mechanisms and Catalysis, volume 134, pages 347–359. doi:10.1007/s11144-021-02084-8
  7. ↑ Siavash Aghili, Masoud Panjepour, and Mahmood Meratian (2018): "Kinetic analysis of formation of boron trioxide from thermal decomposition of boric acid under non-isothermal conditions." Journal of Thermal Analysis and Calorimetry, volume 131, pages 2443–2455. doi:10.1007/s10973-017-6740-3
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