Metaboric acid is the name for a family of inorganic compounds with the same empirical formula HBO<sub>2</sub> 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 C<sub>3h</sub> 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 ðC, with loss of water:
:3 â + 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 ðC) and density (2.045 g/cm<sup>3</sup>) It is obtained by heating the trimeric form at 130-140 ðC 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 ðC. It is obtained by heating either modification II or III above 140 ðC.
Reactions
When heated above about 170 ðC, metaboric acid dehydrates, forming tetraboric acid, also called pyroboric acid (H<sub>2</sub>B<sub>4</sub>O<sub>7</sub>):
4 HBO<sub>2</sub> â H<sub>2</sub>B<sub>4</sub>O<sub>7</sub> +
Metaborates
Metaborates are derivatives of BO<sub>2</sub><sup>âÂÂ</sup>. 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 B<sub>3</sub>O<sub>6</sub><sup>3âÂÂ</sup> ion and calcium metaborate, Ca(BO<sub>2</sub>)<sub>2</sub>, which contains the chain polymeric ion (BO<sub>2</sub><sup>âÂÂ</sup>)<sub>n</sub>.
References