Metaboric acid

Metaboric acid
Names
	IUPAC name Oxoborinic acid
	Other names Metaboric acid
Identifiers
CAS Number	13460-51-0;
3D model (JSmol)	Interactive image; Interactive image;
ChemSpider	22900 ;
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 Key: VGTPKLINSHNZRD-UHFFFAOYSA-N ;
	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 cm−3
Melting point	176 °C (349 °F; 449 K)
Acidity (pKa)	9.236
Conjugate base	Metaborate
Structure
Coordination geometry	trigonal at B
Hazards
EU classification (DSD) (outdated)	Xi
R-phrases (outdated)	R36/37/38
S-phrases (outdated)	S26, S37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 two forms of metaboric acid, all are white solids. One form of metaboric acid is molecular, and the other forms are polymers.

Preparation

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 C_3h 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

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]

References

Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
H. J. Becher "Metaboric Acid" Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 791.
W. H. Zachariasen "The crystal structure of monoclinic metaboric acid" Acta Crystallogr. 1963, vol. 16, pp. 385-389. doi:10.1107/S0365110X6300102X
Freyhardt, C. C.; Wiebcke, M.; Felsche, J. (2000). "The monoclinic and cubic phases of metaboric acid (precise redeterminations)". Acta Crystallogr C. 56 (3): 276–278. doi:10.1107/S0108270199016042. PMID 10777918.

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[Greenwood-1] Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.

[Brauer-2] 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 (3): 276–278. doi:10.1107/S0108270199016042. PMID 10777918.