Borates are chemical compounds containing borate anions, that is, anions composed of boron and oxygen. There are various borate ions, the simplest of which is BO33−. These borate anions form salts with metallic elements. Many borates are readily hydrated to include structural hydroxide groups, and they should formally be considered hydroxoborates. In nature, borates are found as minerals.
Borates in the soil stimulate plant growth, as they supply boron, an essential micronutrient for plants. Borates are generally nontoxic for humans and most animals, but they are toxic for insects and they inhibit bacterial and fungal growth.
The various types of borates are useful for a number of different applications. For example, they may be added to fertilizers to increase crop yields, to laundry detergents to raise their ability to remove stains, to glass to increase its resistance to heat and chemicals, to personal care products to prevent bacterial growth, or to polymers to act as flame retardants. In addition, wood structures may be protected from fungal and insect attack by borate treatments.
Some of the naturally occurring borate minerals are borax, ulexite, and colemanite. Borax, also known as sodium borate or sodium tetraborate, can be found in evaporite deposits produced by the repeated evaporation of seasonal lakes. It occurs in several forms that differ in their content of water of crystallization: Anhydrous sodium borate (Na2B4O7); sodium borate pentahydrate (Na2B4O7•5H2O); and sodium borate decahydrate (Na2B4O7•10H2O). From a commercial standpoint, the most important deposits of borax are located in Turkey, the United States (such as near Boron, California, and other parts of the southwestern U.S.), Chile (Atacama desert), and Tibet.
Ulexite (NaCaB5O9·8H2O) (hydrated sodium calcium borate hydroxide) is usually found as evaporite deposits along with borax. It occurs in the form of white, rounded crystalline masses or as closely packed fibrous crystals. In the latter form, it is known as "TV rock" or "TV stone," because it has unusual optical characteristics.
Colemanite (CaB3O4(OH)3·H2O) is a secondary mineral that is formed by the alteration of borax and ulexite. It is found in evaporite deposits in parts of Turkey, the United States (particularly Death Valley, Argentina, and Kazakhastan, as well as other parts of the globe.
As noted above, many borates are known, with various borate anions. A borate anion is also called a boron oxyanion, because it is composed of boron and oxygen atoms. The simplest borate contains the ion BO33−, in which the atoms are arranged in a trigonal planar arrangement. In this and other borate ions, the oxidation state of boron is +3.
Several borates are good buffers, balancing acidity and alkalinity. Also, borates interact with surfaces of materials containing iron, forming a coating that prevents corrosion. When present in glass, borates enhance its strength and resistance to heat and chemicals.
Polymeric ions in anhydrous compounds
A number of polymeric borate ions are known in anhydrous compounds, which are made by reacting B(OH)3 or B2O3 with metal oxides. Examples include:
- Diborate B2O54−, as in Mg2B2O5 (suanite)
- Triborate B3O75, as in CaAlB3O7 (johachidolite)
- Tetraborate B4O96− as in Li6B4O9
- Metaborates containing the linear [BO2−]n with three coordinate boron, as in LiBO2, CaB2O4
- Metaborates containing 3 and four coordinate boron, which are often high-pressure modifications.
In aqueous solution, borate exists in many forms. In acidic and near-neutral conditions, it is boric acid, commonly written as H3BO3 but more correctly B(OH)3. The pKa of boric acid is 9.14 at 25 °C. Boric acid does not dissociate in aqueous solution, but is acidic due to its interaction with water molecules, forming tetrahydroxyborate:
- B(OH)3 + H2O ⇌ B(OH)4− + H+
- Ka = 5.8x10−10 mol/l; pKa = 9.24.
- B(OH)3 + H2O ⇌ B(OH)4− + H+
Polymeric anions containing structural OH units (polyhydroxoborates) are formed at pH 7–10 if the boron concentration is higher than about 0.025 mol/L. The best known of these is the ion, found in the mineral borax:
- 4B(OH)4− + 2H+ ⇌ B4O5(OH)4 2− + 7H2O
Although boric acid adds hydroxide to form B(OH)4−, it may be easier to use the fictitious ions for certain calculations (as when determining pKa values). Thus, for a typical polyprotic acid, one may write the deprotonation series as dihydrogen borate [H2BO3−;], hydrogen borate [HBO32−], and borate [BO33−], as pH increases.
Common borate salts
Common borate salts include sodium metaborate, NaBO2, and sodium tetraborate, Na2B4O7. The latter is usually encountered as borax, mentioned above. The so-called decahydrate actually contains the hydroxoborate ion, B4O5(OH)42−, and it is formulated Na2[B4O5(OH)4]·8H2O.
Borate esters are organic compounds of the type B(OR)3 where R is an organic residue (such as alkyl or aryl). Borate esters include trimethyl borate, B(OCH3)3, which is used as a precursor to boronic esters.
Effects on living organisms
Boron is an essential micronutrient for plant growth, and plants obtain it from borates in the soil. People regularly consume borates that may be present in water and plant foods, but it is unclear whether boron is an essential ingredient for the human diet. In any case, borates are generally nontoxic (or have very low toxicity) for humans and many animals. By contrast, borates inhibit the growth of bacteria and fungi, and they are toxic for insects such as carpenter ants and termites.
Borates are useful for a variety of applications, some of which are noted below.
- Farmers may use borate fertilizers to compensate for insufficient boron concentrations in the soil, thereby boosting crop yields.
- Sodium borate pentahydrate (Na2B4O7 • 5H2O) is used in large amounts in making insulating fiberglass and sodium perborate bleach.
- Sodium borate decahydrate (Na2B4O7 • 10H2O, or borax) is used in laundry detergents, antiseptics, adhesives, and anti-corrosion systems, among other products.
- Borosilicate glass is used for heat-resistant cookware.
- Borates are used in ceramic and enamel glazes to increase their durability and shine.
- Various forms of borate (such as disodium octaborate tetrahydrate) are useful as fungicides and wood preservatives, protecting wood from termite attack.
- Borates may be added to cosmetics and other personal care products to control the growth of bacteria.
- Zinc borates are added to polymers to serve as flame retardants.
- Cornelis Klein and Cornelius S. Hurlbut, Jr., Manual of Mineralogy (after James D. Dana), 20th ed. (New York: Wiley, 1985, ISBN 0471805807).
- Handbook of Mineralogy, Colemanite. Retrieved January 12, 2009.
- Arnold Frederick Holleman, Egon Wiberg, and Nils Wiberg, Inorganic Chemistry (San Diego, CA: Academic Press, 2001, ISBN 0123526515).
- IMA Europe, Borates. Retrieved January 11, 2009.
- Chang, Raymond. Chemistry, 9th ed. New York: McGraw-Hill Science/Engineering/Math, 2006. ISBN 0073221031
- Cotton, F. Albert, and Geoffrey Wilkinson. Advanced Inorganic Chemistry, 4th ed. New York: Wiley, 1980. ISBN 0471027758
- Farndon, John. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books, 2006. ISBN 0754815412
- Greenwood, N.N., and A. Earnshaw. Chemistry of the Elements, 2nd ed. Burlington, MA: Butterworth-Heinemann, Elsevier Science, 1998. ISBN 0750633654
- Holleman, Arnold Frederick, Egon Wiberg, and Nils Wiberg. Inorganic Chemistry. San Diego, CA: Academic Press, 2001. ISBN 0123526515
- Klein, Cornelis, and Barbara Dutrow. Manual of Mineral Science, 23rd ed. New York: John Wiley, 2007. ISBN 0471721573
All links retrieved June 17, 2016.
- Non-CCA Wood Preservatives: Borates. National Pesticide Information Center.
- Colemanite. mindat.org.
- Colemanite. Webmineral.com.
- Handbook of Mineralogy.
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