|Molar mass||131.45 g mol−1|
|Density||1.784 g cm-3|
176 °C, 449 K, 349 °F
|trigonal at B|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)|
Heating of boric acid at 80-100 °C releases one equivalent of water to give orthorhombic metaboric acid:
This form is molecular, consisting of discrete trimers. This molecule has C3h symmetry and forms a sheet-like structure, similar to that of boric acid itself. It is also called "modification III" of the metaboric acids.
Upon heating at 130-140 °C in a sealed ampoule (to prevent dehydration), orthorhombic metaboric acid converts to the monoclinic form:
This material, called modification II, has a polymeric structure, and a higher melting point (201 °C) and density (2.045 g/cm3). The structure of this species resembles its precursor except that the rings are connected and 1/3 of the boron centres are tetrahedral.
Above 140 °C, boric acid or the other forms of metaboric acid convert to cubic metaboric acid.
Metaborates are derivatives of BO2-. Like metaboric acid, the metaborates exist in disparate structures. Sodium and potassium metaborates are salts formed by deprotonation of orthorhombic metaboric acid. Calcium metaborate is a derivative of the polymer (BO2-)n.