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Metaboric Acid (Wiki)

Metaboric acid

From Wikipedia, the free encyclopedia
  (Redirected from Sodium metaborate)
Metaboric acid
CAS number 13460-51-0
PubChem 24492
ChemSpider 22900 Yes
EC number 236-659-8
Gmelin Reference 121829
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

trigonal at B
EU classification Irritant Xi
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]

B(OH)3 → 1/3 (BOH)3O3 + 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.

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 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]


  1. ^ a b Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
  2. ^ 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 Cryst. 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 Cryst C56: 276–278. doi:10.1107/S0108270199016042.





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