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.
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:
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:
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:
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 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.
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.
All links retrieved February 18, 2013.
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