Metaboric acid
Chemical compound
From Wikipedia, the free encyclopedia
Metaboric acid is the name for a family of inorganic compounds with the same empirical formula HBO2[2] that differ in their molecular structure. They are colourless water-soluble solids formed by the dehydration or decomposition of boric acid.
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| Names | |||
|---|---|---|---|
| IUPAC name
Oxoborinic acid | |||
| Other names
Metaboric acid | |||
| Identifiers | |||
3D model (JSmol) |
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| ChemSpider | |||
| ECHA InfoCard | 100.033.313 | ||
| EC Number |
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| 121829 | |||
PubChem CID |
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CompTox Dashboard (EPA) |
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| Properties | |||
| B3H3O6 | |||
| Molar mass | 131.45 g·mol−1 | ||
| Appearance | white solid | ||
| Density | 1.784 g/cm3 | ||
| Melting point | 176 °C (349 °F; 449 K) | ||
| Acidity (pKa) | 9.236 | ||
| Conjugate base | Metaborate | ||
| Structure | |||
| trigonal at B | |||
| Hazards | |||
| GHS labelling:[1] | |||
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| Warning | |||
| H315, H319, H335 | |||
| P261, P305+P351+P338 | |||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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 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.[3] It is obtained by heating orthoboric acid at 80-100 °C, with loss of water:[3]
- 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.[4] 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):[4]
Reactions
When heated above about 170 °C, metaboric acid dehydrates, forming tetraboric acid, also called pyroboric acid (H2B4O7):[6][7]
- 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.[2]