Metaboric acid is the name for a family of inorganic compounds with the same empirical formula HBO2 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.
Metaboric acid occurs naturally as the mineral metaborite and its monoclinic dimorph clinometaborite.
Structure
The main forms of metaboric acid are:
Modification III, or trimeric, with the molecular formula . The molecule has C3h symmetry, with a six-member ring of alternating boron and oxygen atoms at the core, with groups attached to the borons. The crystal structure is orthorhombic with a sheet-like structure, similar to that of boric acid itself. It is obtained by heating orthoboric acid at 80-100 °C, with loss of water:
3 → + 3
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 The crystal structure is monoclinic. 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):
Cubic form. 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.
Reactions
When heated above about 170 °C, metaboric acid dehydrates, forming tetraboric acid, also called pyroboric acid (H2B4O7):
4 HBO2 → H2B4O7 +
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.
References
Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. .
H. J. Becher "Metaboric Acid" Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 791.
W. H. Zachariasen "The crystal structure of monoclinic metaboric acid" Acta Crystallogr. 1963, vol. 16, pp. 385-389.
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.
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.