Metaboric acid
# Metaboric acid

Metaboric acid is the name for a family of inorganic compounds with the same empirical formula $HBO_2.$ 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 $H_3B_3O_6.$ The molecule has $C_{3h}$ 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 $B(OH)_3$ → $(BOH)_3O_3$ + 3 $H_2O$
* 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 $HO[−B(BOH)_2O_3O−]_nH$ The crystal structure is monoclinic. This form has a higher melting point (201 °C) and density (2.045 $g/cm^3)$ 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 $(H_2B_4O_7):$

    4 $HBO_2$ → $H_2B_4O_7$ + $H_2O$

## Metaborates

Metaborates are derivatives of $BO_2^−.$ 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 $B_3O_6^{3−}$ ion and calcium metaborate, $Ca(BO_2)_2,$ which contains the chain polymeric ion $(BO_2^−)_n.$

