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 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):
Conversion of orthorhombic to monoclinic metaboric acid.
  • 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 + 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.