Boron trioxide

Crystal structure
__ B 3+      __ O 2−
General
Surname	Boron trioxide
other names	Diboron trioxide Borosesquioxide Boric anhydride Boric acid glass Boron (III) oxide
Ratio formula	B 2 O 3
Brief description	colorless, hygroscopic mass
External identifiers / databases
CAS number 1303-86-2 EC number 215-125-8 ECHA InfoCard 100,013,751 PubChem 518682 ChemSpider 452485 Wikidata Q411076
properties
Molar mass	69.62 g mol −1
Physical state	firmly
density	2.56 g cm −3 (crystalline) 1.83 g cm −3 (amorphous)
Melting point	475 ° C (crystalline)
boiling point	2250 ° C
solubility	little in water (36 g l −1 at 25 ° C)
safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary danger H and P phrases H: 360FD P: 201-308 + 313
Authorization procedure under REACH	of particular concern : toxic for reproduction ( CMR )
MAK	Switzerland: 10 mg m −3 (measured as inhalable dust )
Toxicological data	3163 mg kg −1 ( LD 50 ,  mouse ,  oral )
Thermodynamic properties
ΔH f 0	−1273.5 (14) kJ / mol
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Boron trioxide or diboron trioxide is a chemical compound of the elements boron and oxygen with the empirical formula B ₂ O ₃ .

history

Boron trioxide was the starting point for the first presentation of boron . In 1808, the French chemists Joseph Louis Gay-Lussac and Louis Jacques Thénard succeeded for the first time in the preparation of impure boron from boron trioxide by reduction with potassium . The first high-purity boron was then obtained in 1909 by the American chemist W. Weintraub by reducing gaseous boron trioxide with hydrogen in an electric arc .

Extraction and presentation

If boric acid is annealed, a colorless, glassy- amorphous mass ( boron oxide glass ) is obtained, which is difficult to crystallize :

${\displaystyle \mathrm {2\ H_{3}BO_{3}+190,5\ kJ\rightleftharpoons B_{2}O_{3}+3\ H_{2}O} }$

By slowly dehydrating boric acid at 150–250 ° C, crystalline boron trioxide can be produced.

In 2007 around 3.8 million tons of boron trioxide were produced worldwide. The main producers are Turkey , Argentina and Chile .

properties

Boron trioxide reacts acidic and forms boric acid on contact with water. Both the amorphous and the crystalline form are hygroscopic. When reduced with magnesium , potassium , hydrogen and others, boron is formed.

${\displaystyle \mathrm {B_{2}O_{3}+3\ Mg\rightleftharpoons 2\ B+3\ MgO\ \ \Delta H=-533kJ} }$

Boron trioxide (α-boron trioxide) crystallizes trigonally in the space group P 3 ₁ 21 (space group no. 152) or P 3 ₂ 21 (no. 154) with the lattice parameters a  = 434 pm and c  = 834 pm and six formula units per unit cell . In the crystal structure , each boron atom is surrounded in a planar manner by three oxygen atoms, the individual [BO ₃ ] ³⁻ units are connected to neighboring boron atoms via all three oxygen atoms, resulting in a three-dimensional network. This network can be described with the Niggli formula , which results in a boron-to-oxygen ratio of 1: 1.5 or 2: 3, which is reflected in the chemical formula B ₂ O ₃ . When viewed correctly, the compound B ₂ O ₃ is therefore not a trioxide, but a sesquioxide . ${\displaystyle \mathrm {{}_{\infty }^{3}\lbrace [BO_{3/2}^{e}]} \rbrace }$

In addition to the α-form and the amorphous form, there is also a high-temperature variant (β-boron trioxide).

Boron trioxide has been classified as a " Substance of Very High Concern " since June 2012 because of its reproductive toxicity .

use

Boron trioxide is a starting material for the production of other boron compounds (e.g. pure boron, boron carbide , trimethyl borate by dissolving in methanol , diborane by hydrogenation ). It is used in practice as a flux or as a component ( borosilicate glass , boron phosphate glass) in enamels and glasses . In hot-pressed boron nitride - ceramic it can in amounts of 2-6% as a binder to act. Boron trioxide is also used as a fire extinguishing agent in metal fires .

Individual evidence

1. ↑ Entry on boron trioxide. In: Römpp Online . Georg Thieme Verlag, accessed on July 17, 2014.
2. ^ ^{A b c d e f} A. F. Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 1104.
3. ↑ ^{a b c} Entry on diboron trioxide in the GESTIS substance database of the IFA , accessed on February 20, 2017(JavaScript required) .
4. ↑ Entry on Diboron trioxide in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on February 1, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .
5. ↑ Entry in the SVHC list of the European Chemicals Agency , accessed on July 17, 2014.
6. ↑ Swiss Accident Insurance Fund (Suva): Limit values ​​- current MAK and BAT values (search for 1303-86-2 or boron trioxide ), accessed on November 2, 2015.
7. ↑ Data sheet boron trioxide from AlfaAesar, accessed on February 3, 2010 ( PDF )(JavaScript required) . .
8. ↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, CODATA Key Values ​​for Thermodynamics, pp. 5-1.
9. ^ E. Pilgrim: Discovery of the elements , Mundus Verlag, Stuttgart 1950, page 190.
10. ↑ Boron (PDF; 88 kB). In: US Geological Survey, Mineral Commodity Summaries, January 2009.
11. ↑ by Karl-Heinz Lautenschläger: Taschenbuch der Chemie - Karl-Heinz Lautenschläger . Harri Deutsch Verlag, 2007, ISBN 978-3-8171-1760-4 , pp. 218 ( limited preview in Google Book search).
12. ↑ Eberhard Gerdes: Qualitative Inorganic Analysis . Springer DE, 2001, ISBN 3-642-59021-7 , p. 230 ( limited preview in Google Book search).
13. ↑ Herta Effenberger, Christian L. Lengauer, Erwin Parth: Trigonal B ₂ O ₃ with Higher Space-Group Symmetry: Results of a Reevaluation. In: Monthly books for chemistry / Chemical Monthly. 132, pp. 1515-1517, doi: 10.1007 / s007060170008 .
14. ↑ v. a. Mukhanov, OO Kurakevych, VL Solozhenko: On the hardness of boron (III) oxide: Results of a reevaluation. In: Journal of Superhard Materials. 30, pp. 71-72, arxiv : 1101.2965 , doi : 10.1007 / s11961-008-1009-6 (currently unavailable) .
15. ↑ Gisbert Rodewald, Alfons Rempe, Kohlhammer: Feuerlöschmittel 7th edition, ISBN 3-17-018492-X .