Metaboric acid


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.

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:
  • 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 and density It is obtained by heating the trimeric form at 130-140 °C in a sealed ampoule, orthorhombic metaboric acid converts to the monoclinic form :
  • 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 :

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, Ca2, which contains the chain polymeric ion n.