Boric oxide

Boric oxide, B2O3, is also referred to as anhydrous boric acid. The article of commerce is an amorphous material produced by fusing boric acid in a furnace. The structure of vitreous B2O3 is discussed above. Using a process 

Although boric oxide is extremely difficult to crystallize, several crystalline phases are known. However, these have no particular industrial importance. 

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Boric Oxide. Boric oxide, formula wt 69.62, is the only commercially important oxide. It is also known as diboron trioxide, boric... 

Boric oxide is an excellent Lewis acid. It coordinates even weak bases to form four-coordinate borate species. Reaction with sulfuric acid produces H[B(HSO 4] (18). At high (>1000° C) temperatures molten boric oxide dissolves most metal oxides and is thus very corrosive to metals in the presence of oxygen. 

Molten boric oxide reacts readily with water vapor above 1000°C to form metaboric acid in the vapor state. 

Table 4. Physical Properties of Vitreous Boric Oxide ...

The uses of boric oxide relate to its behavior as a flux, an acid catalyst, or a chemical iatermediate. The fluxing action of B2O2 is important ia preparing many types of glass, gla2es, frits, ceramic coatings, and porcelain enamels (qv). 

Boric oxide is used as a catalyst ia many organic reactions. It also serves as an iatermediate ia the production of boron haUdes, esters, carbide, nitride, and metallic borides. 

The name boric acid is usually associated with orthoboric acid, which is the only commercially important form of boric acid and is found ia nature as the mineral sassoflte. Three crystalline modifications of metaboric acid also exist. AH these forms of boric acid can be regarded as hydrates of boric oxide and formulated as B2O3 3H20 for orthoboric acid and B2O3 H20 for metaboric acid. 

Ammonium pentaborate tetrahydrate is very stable ia respect to ammonia loss. On heating from 100 to 230°C, it loses 75% of its water content but less than 1% of the ammonia. At 200°C, under reduced pressure, the water content drops to 1.15 mol, but only 2% of the ammonia is lost (61). At still higher temperatures all ammonia and water are expelled to give boric oxide (125). 

M2O/B2O2 from 0 to 2 mol) in boric oxide content, they become viscous on cooling and form glasses. 

From Boric Oxide and Alcohol. To avoid removing water, boric oxide, B2O3, can be used in place of boric acid. The water of reaction (eq. 4) is consumed by the oxide (eq. 5). Because boric acid reacts with borates at high temperatures, it is necessary to filter the reaction mixture prior to distillation of the product. Only 50% of the boron can be converted to ester by this method. In cases where this loss can be tolerated, the boric oxide method is convenient. This is particularly tme for methyl borate and ethyl borate preparation because formation of the undesirable azeotrope is avoided. 

The esterification of boric oxide does not require the removal of water. However, if high yields based on boron are desired, six or more moles of alcohol must be used and the water must be removed. 

For the most part boric acid esters are quantitated by hydrolysis in hot water followed by determination of the amount of boron by the mannitol titration (see Boron compounds, boric oxide, boric acid and borates). Separation of and measuring mixtures of borate esters can be difficult. Any water present causes hydrolysis and in mixtures, as a result of transesterification, it is possible to have a number of borate esters present. For some borate esters, such as triethanolamine borate, hydrolysis is sufftciendy slow that quantitation by hydrolysis and titration cannot be done. In these cases, a sodium carbonate fusion is necessary. 

Gas Fluxing. The methyl borate azeotrope is used as a gaseous flux for welding and brazing. The Gas Flux Co., Elyria, Ohio, manufactures the methyl borate azeotrope for their own use. The azeotrope acts as a volatile source of boric oxide and is introduced directly into the gas stream as a flux for the surfaces to be joined in the welding process. The European automobile industry is the primary user of this process, though there may be some usage for this purpose in the United States. 

Preparation. Boron carbide is most commonly produced by the reduction of boric oxide with carbon in an electric furnace between 1400 and 2300°C. In the presence of carbon, magnesium reduces boric oxide to boron carbide at 1400—1800°C. The reaction is best carried out in a hydrogen atmosphere in a carbon tube furnace. By-product magnesium compounds are removed by acid treatment. 

Hexagonal boron nitride is relatively stable in oxygen or chlorine up to 700°C, probably because of a protective surface layer of boric oxide. It is attacked by steam at 900°C, and rapidly by hot alkaU or fused alkaU carbonates. It is attacked slowly by many acids as well as alcohols (to form borate esters), acetone, and carbon tetrachloride. It is not wetted by most molten metals or many molten glasses. 

Preparation. Hexagonal boron nitride can be prepared by heating boric oxide with ammonia, or by heating boric oxide, boric acid, or its salts with ammonium chloride, alkaU cyanides, or calcium cyanamide at atmospheric pressure. Elemental nitrogen does not react with boric oxide even in the presence of carbon, though it does react with elemental boron at high temperatures. Boron nitride obtained from the reaction of boron trichloride or boron trifluoride with ammonia is easily purified. 

BF3 is used extensively as a catalyst in various industrial processes (p. 199) and can be prepared on a large scale by the fluorination of boric oxide or borates with fluorspar and concentrated H2SO4 ... 

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