OVERVIEW
Boric acid (BORE-ik ASS-id) is a colorless, odorless, white or colorless powder or crystalline material with a slightly oily feeling that slowly decomposes with heat, changing first to metaboric acid (HBO2), then to pyroboric acid (H2B4O7), and eventually to boric oxide (B2O3). The compound's solubility is very much a factor of temperature. In cold water, about 5 grams (0.2 ounce) of boric acid dissolve in 100 mL (3.4 ounces) of water, while at 100°C (212°F), its solubility increases to 25 grams (0.9 ounce) in 100 mL (3.4 ounces) of water.
KEY FACTS
OTHER NAMES:
Orthoboric acid; hydrogen orthoborate; boracic acid
FORMULA:
H3BO3
ELEMENTS:
Hydrogen, boron, oxygen
COMPOUND TYPE:
Inorganic acid
STATE:
Solid
MOLECULAR WEIGHT:
61.83 g/mol
MELTING POINT:
170.9°C (339.6°F)
BOILING POINT:
Decomposes above its melting point
SOLUBILITY:
Somewhat soluble in water, ethyl alcohol, and glycerol
Boric acid occurs naturally in a number of locations where it has precipitated out of hot springs. It may occur then in the form of the mineral sassolite. Given its abundance in nature, it is not surprising that the compound has been known to and used by humans for many centuries. For example, the Greeks are known to have used boric acid as an antiseptic, to preserve foods, and as a cleaning agent. The first person to prepare boric acid in Europe is thought to be the German chemist Wilhelm Homberg (1652–1715), who in 1702 treated natural borax (Na2B4O7·10H2O) with acid to obtain a product he called sedative salt, probably a form of boric acid. The chemical structure of the compound was determined in 1808 by the French researchers Louis-Joseph Gay-Lussac (1778–1850) and Louis-Jacques Thénard (1777–1857).
HOW IT IS MADE
The most common method of producing boric acid is by treating the relatively abundant borax with hydrochloric or sulfuric acid and crystallizing out the boric acid that forms in the reaction. A less common method of preparation involves the treatment of borax brine solutions with a chelating agent that binds to the borates present in the brine, which can then be converted to boric acid.
COMMON USES AND POTENTIAL HAZARDS
By far the greatest amount of boric acid is used in the production of heat-resistant (borosilicate) glass, glass fibers, porcelain enamels, crockery, laboratory glassware, and other specialized types of glass and ceramic materials. In 2004, about 80 percent of all the boric acid used in the United States was applied to these purposes. Although accounting for a much smaller amount of boric acid, another well-known use of the compound is as an antiseptic in eyewashes, ointments, and mouthwashes. It is also used as a preservative in foods and as a fungicide on citrus fruit crops. In the United States, a residue of no more than 8 parts per million is permitted on fruits treated with boric acid.
Interesting Facts
- The most common sources of boric acid are hot springs and volcanic sites such as the mineral springs at Vichy and Aix-la-Chapelle in France and Wiesbaden in Germany, around the volcanic regions of Tuscany in Italy, and in dry lakes of California and Nevada, such as California's Borax Lake.
- Boric acid was once used as an ingredient in talcum and diaper powders and in salves for diaper rashes because of its antiseptic properties. The compound was eventually banned for such uses, however, when regulators decided that it was too toxic if it accidentally entered the body through an open wound or by being swallowed.
Some other important applications of boric acid include the following:
- In the production of flame retardant materials;
- As a pesticide for the treatment of co*ckroaches, black carpet beatles, termites, fleas, fire ants, centipedes, grasshoppers, and slugs;
- As a flux in welding and brazing;
- In the synthesis of many boron compounds;
- To provide the finishing touches on the production of leather and fur products;
- In the manufacture of latex paints; and
- In the nickel plating of metallic products.
Boric acid is toxic if swallowed. It causes nausea, vomiting, diarrhea, and stomach cramps. In extreme cases, it can cause the collapse of the circulatory system, delirium, convulsions, coma, and death. Ingestion of no more than 5 grams (0.2 ounce) of boric acid can cause death in an infant. Ingestion of 15 to 20 grams (0.5 to 0.7 ounce) by an adult can also be fatal. Boric acid can also cause irritation of the skin, which, in extreme cases, can result in a condition known as borism. Borism is characterized by dry skin, eruptions of the skin and mucous membranes, and gastric disturbances.
Words to Know
- BRINE SOLUTION
- A solution that is saturated with or nearly saturated with sodium chloride or other inorganic salts.
- CHELATING AGENT
- An organic compound that binds ("grabs on to") some specific compound or compounds present in a mixture.
- FLUX
- A material that lowers the melting point of another substance or mixture of substances or that is used in cleaning a metal.
FOR FURTHER INFORMATION
"Borax; Boric Acid, and Borates." IPM of Alaska. http://www.ipmofalaska.com/files/Borates.html (accessed on October 12, 2005).
"Boric Acid: Technical Description." Manufacturas Los Andes. http://www.mandes.com.ar/technic-boric-acid.php (accessed on October 12, 2005).
Potter, Mike. "co*ckroach Elimination." University of Kentucky Entomology. http://www.uky.edu/Agriculture/Entomology/entfacts/struct/ef614.htm (accessed on October 12, 2005).
