Metaboric acid | |
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Identifiers | |
CAS number | 13460-51-0 |
PubChem | 24492 |
ChemSpider |
22900 ![]() |
EC number | 236-659-8 |
Gmelin Reference | 121829 |
Properties | |
Molecular formula | H3B3O6 |
Molar mass | 131.45 g mol−1 |
Appearance | white solid |
Density | 1.784 g cm-3 |
Melting point |
176 °C, 449 K, 349 °F |
Structure | |
Coordination geometry |
trigonal at B |
Hazards | |
EU classification |
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R-phrases | R36/37/38 |
S-phrases | S26, S37 |
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) | |
Infobox references |
Metaboric acid is the name for a family of inorganic compounds formed by the dehydration of boric acid. These are colourless solids with the empirical formula HBO2.[1]
Heating of boric acid at 80-100 °C releases one equivalent of water to give orthorhombic metaboric acid:[2]
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.[3]
Above 140 °C, boric acid or the other forms of metaboric acid convert to cubic metaboric acid.[4]
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.[1]