Metaboric acid

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Metaboric acid
Metaboronic acid molecule.png
Metaboric acid Ball and Stick.png
Metaboric acid Space Fill.png
IUPAC name
Oxoborinic acid
Other names
Metaboric acid
ECHA InfoCard 100.033.313
EC Number 236-659-8
Molar mass 131.45 g·mol−1
Appearance white solid
Density 1.784 g cm−3
Melting point 176 °C (349 °F; 449 K)
Acidity (pKa) 9.236
trigonal at B
Irritant Xi
R-phrases (outdated) R36/37/38
S-phrases (outdated) S26, S37
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 formed by the dehydration of boric acid. Metaboric acids are colourless solids with the empirical formula HBO2.[1] There are three forms of metaboric acid, all are white solids. One form of metaboric acid is molecule, and the other forms are polymers.


Heating of boric acid at 80-100 °C releases water to give orthorhombic metaboric acid:[2] 3 B(OH)3 → (BOH)3O3 + 3 H2O

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.[2]

Upon heating at 130-140 °C in a sealed ampoule (to prevent dehydration), orthorhombic metaboric acid converts to the monoclinic form:

(BOH)3O3 → B3O4(OH)(H2O)

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]

Conversion of orthorhombic to monoclinic metaboric acid.

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 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.[1]


  1. ^ a b Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
  2. ^ a b H. J. Becher "Metaboric Acid" Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 791.
  3. ^ W. H. Zachariasen "The crystal structure of monoclinic metaboric acid" Acta Crystallogr. 1963, vol. 16, pp. 385-389. doi:10.1107/S0365110X6300102X
  4. ^ Freyhardt, C. C.; Wiebcke, M.; Felsche, J. (2000). "The monoclinic and cubic phases of metaboric acid (precise redeterminations)". Acta Crystallogr C. 56: 276–278. doi:10.1107/S0108270199016042.