Metaboric acid
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Through Molecular geometry, Dehydration and Crystal structure
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. It is obtained by heating orthoboric acid at 80-100 °C, with loss of water:
- 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. 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.
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Reactions
When heated above about 170 °C, metaboric acid dehydrates, forming tetraboric acid, also called pyroboric acid (H2B4O7):
- 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.