Metaboric acid

Names
IUPAC name Oxoborinic acid
Other names Metaboric acid
Identifiers
CAS Number	13460-51-0 N
3D model (JSmol)	Interactive image Interactive image
ChemSpider	22900 Y
ECHA InfoCard	100.033.313
EC Number	236-659-8
Gmelin Reference	121829
PubChem CID	24492
CompTox Dashboard (EPA)	DTXSID6065478
InChI InChI=1S/BHO2/c2-1-3/h2H Y Key: VGTPKLINSHNZRD-UHFFFAOYSA-N Y
SMILES O=BO OB1OB(O)OB(O)O1
Properties
Chemical formula	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
Coordination geometry	trigonal at B
Hazards
GHS labelling:[1]
Pictograms
Signal word	Warning
Hazard statements	H315, H319, H335
Precautionary statements	P261, P305+P351+P338
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

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.

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 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.^[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]

• Cubic form.^[5] 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.^[5]

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]