Boron Oxides, Boric Acid, and Borates
Authors: Smith Robert A. , McBroom Robert B. 
Published: August 2004

AbstractThere are about 150 known boron‐containing minerals. In nature boron always occurs in chemical combination with oxygen in the form of borates. Borax (tincal), kernite, colemanite, ulexite, probertite, hydroboracite, inderite,datolite, and szaibelyite (ascharite) are the only borate minerals of commercial importance. Borax and colemanite are the most important. Borate production comes mostly from seven countries: the United States, Turkey, Russia, Kazakhstan, Argentina, China, Peru, and Chile. Boric oxide, B2O3, is the only commercially important oxide. Boric oxide is an excellent Lewis acid. The uses of boric oxide are as a flux in preparing many types of glass and porcelain enamels, as an acid catalyst in many organic reactions, or as a chemical intermediate in the production of boron halides, esters, carbide, nitride, and metallic borides. Vapor phases of BO, B2O3, and BO2have been the subject of a number of spectroscopic and mass spectrometric studies. The name boric acid is usually associated with orthoboric acid, which is the only commercially important form of boric acid and is found in nature as the mineral sassolite. Three crystalline modifications of metaboric acid also exist. The solubility of boric acid in water increases rapidly with temperature. Boric acid is quite soluble in many organic solvents. The majority of boric acid is produced by the reaction of inorganic borates with sulfuric acid in an aqueous medium. Boric acid has a surprising variety of applications in both industrial and consumer products. It serves as a source of B2O3in many fused products, including textile fiber glass, optical and sealing glasses, and porcelain enamels, among others. It also serves as a component of fluxes for welding and brazing. A number of boron chemicals including synthetic inorganic borate salts, boron phosphate, fluoborates, boron trihalides, and borate esters, are prepared directly from boric acid. The bacteriostatic and fungicidal properties of boric acid have led to its use as a preservative in natural products such as lumber, rubber latex emulsions, leather, and starch products. NF‐grade boric acid serves as a mild, nonirritating antiseptic in mouthwashes, hair rinse, talcum powder, eyewashes, and protective ointments. The sodium borates include disodium tetraborate decahydrate (borax decahydrate), disodium tetraborate pentahydrate (borax pentahydrate), disodium tetraborate tetrahydrate, disodium tetraborate (anhydrous borax), disodium octaborate tetrahydrate, sodium pentaborate pentahydrate, sodium metaborate tetrahydrate, sodium metaborate dihydrate, and sodium perborate hydrates (peroxyborates, commonly known as perborates). Only the mono‐ and tetrahydrate among this last group are of commercial importance, primarily as bleaching agents in laundry products. Cases of industrial intoxication on exposure to inorganic borates have not been reported. There is a large body of literature on the toxicology of boric acid and borax. Studies indicate no evidence of carcinogenic or mutagenic activity. Boric acid and borax are poorly absorbed through healthy skin and do not cause skin irritation. Gloves, goggles, and a simple dust mask should be used when handling sodium metaborate powder. Boron in the form of borate is an essential micronutrient for the healthy growth of plants and is present in the normal daily human diet at an estimated level of 3–40 mg as boron. It is not a proven essential micronutrient for animals. In the United States over 54% of the total B2O3consumption is for glass manufacture. Borates are used as fluxing agents for porcelain enamels and ceramic glazes. Other alkali metal and ammonium borates include dipotassium tetraborate tetrahydrate, potassium pentaborate tetrahydrate, diammonium tetraborate tetrahydrate, ammonium pentaborate tetrahydrate, and lithium borates. Few toxicological data are available on borates other than boric acid and borax. Most water‐soluble borates have the same toxicological effects as borax when adjusted to account for differences in B2O3content. Dipotassium tetraborate tetrahydrate is used to replace borax in applications where an alkali metal borate is needed but sodium salts cannot be used or where a more soluble form is required. The potassium compound is used as a solvent for casein, as a constituent in welding fluxes, and a component in diazotype developer solutions. Potassium pentaborate tetrahydrate is used in fluxes for welding and brazing of stainless steels for nonferrous metals. Diammonium tetraborate tetrahydrate is used where a highly soluble borate is desired but alkali metals cannot be tolerated, mostly as a neutralizing agent in the manufacture of urea–formaldehyde resins and as an ingredient in flameproofing formulations. Ammonium pentaborate tetrahydrate is used as a component of electrolytes for electrolytic capacitors, as an ingredient in flameproofing formulations, and in paper coatings. Calcium‐containing borates include dicalcium hexaborate pentahydrate, sodium calcium pentaborate octahydrate, and sodium calcium pentaborate pentahydrate. The alkaline‐earth metal borates of primary commercial importance are colemanite and ulexite. Colemanite,\documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${2{\rm{CaO}}{\rm{{\cdot{}}}}3{\rm{B}}{_{2}}{\rm{O}}{_{3}}{\rm{{\cdot{}}}}5{\rm{H}}{_{2}}{\rm{O}}}$\end{document}, is used in the production of boric acid and borax, as well as in several direct applications. It is a highly desirable material for the manufacture of the E‐glass used in textile glass fibers and plastic reinforcement (where sodium cannot be tolerated). Borate salts or complexes of virtually every metal have been prepared. Some have achieved commercial importance. Three hydrates of barium metaborate,\documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${{\rm{BaO}}{\rm{{\cdot{}}}}{\rm{B}}{_{2}}{\rm{O}}{_{3}}{\rm{{\cdot{}}}}x{\rm{H}}{_{2}}{\rm{O}}}$\end{document}, are known. Barium metaborate is used as an additive to impart fire‐retardant and mildew‐resistant properties to latex paints, plastics, textiles, and paper products. Copper, manganese, and cobalt borates are no longer produced commercially. A series of hydrated zinc borates have been developed for use as fire‐retardant additives and anticorrosive pigments. Boron phosphate, BPO4, is a white, infusible solid that vaporizes slowly above 1450°C, without apparent decomposition. Its principal application of boron phosphate has been as a heterogeneous acid catalyst.