Sodium Tetraborate

OVERVIEW

Sodium tetraborate (SO-dee-um tet-ruh-BOR-ate) is a term used for either the anhydrous or hydrated form of the compound with the formula Na₂B₄O₇. The decahydrate (Na₂B₄O₇·10H₂O) is also referred to as borax. Borax also occurs without water of hydration and in that form is known as anhydrous borax.

KEY FACTS

OTHER NAMES:

Sodium borate; sodium pyroborate; disodium tetraborate; borax

FORMULA:

Na₂B₄O₇ or Na₂B₄O₇·10H₂O; see Overview

ELEMENTS:

Sodium, boron, oxygen

COMPOUND TYPE:

Salt (inorganic)

STATE:

Solid

MOLECULAR WEIGHT:

Na₂B₄O₇: 201.22 g/mol; Na₂B₄O₇·10H₂O: 381.37 g/mol

MELTING POINT:

Na₂B₄O₇: 743°C (1370°F); Na₂B₄O₇·10H₂O: decomposes at about 75°C (170°F)

BOILING POINT:

Na₂B₄O₇: 1575°C (2867°F); Na₂B₄O₇·10H₂O: not applicable

SOLUBILITY:

Soluble in water

Some historians think that borax may have been known as long as 4,000 years ago. Since ancient people did not use the term, however, there is considerable doubt as to the authenticity of these claims. The compound was certainly in use as far back as about 800 bce when the compound was being used in the Chinese and Islamic civilization for making glass and in jewelry work. The substance was very expensive, however, and it was not widely used in Europe until the Middle Ages. Borax became more commonly used after extensive deposits of its naturally occurring form were found in the United States. The first of those deposits was discovered in Nevada in 1879, although the largest deposits were later found in the desert regions of southern California. Today, the largest reserves of borax are found near the town of Boron, California, and at Borax Lake, California. The compound is also mined in Tibet, Russia, Chile, and Turkey.

Sodium tetraborate is an odorless white crystalline solid or powder. The hydrated form loses its water of hydration when heated and then fuses (melts) to form a glass-like solid at higher temperatures.

HOW IT IS MADE

Sodium tetraborate occurs naturally as the minerals tincal (pronounced "tinkle;" Na₂B₄O₇·10H₂O) and kernite (Na₂B₄O₇·4H₂O). Ores containing these minerals are crushed, washed, and processed to obtain the decahydrate of high purity. Anhydrous sodium tetraborate can be obtained by heating the decahydrate. Sodium tetraborate can also be obtained by processing other minerals that contain borates, such as ulexite (NaCaB₅O₉·8H₂O) and colemanite (Ca₂B₆O₁₁·5H₂O).

COMMON USES AND POTENTIAL HAZARDS

The primary use of sodium tetraborate is in the manufacture of glass products. About 43 percent of all the compound used in the world goes to this application. Glass made with sodium tetraborate is very strong and heat resistant. The well-known Pyrex® brand of glass is made with sodium tetraborate. Today, the largest single use of borax glass is in the manufacture of fiberglass insulation and fiberglass textiles.

Interesting Facts

• Perhaps the best known commercial form of sodium tetraborate is called Twenty-Mule-Team® Borax. The name comes from the fact that the first borax mines in California were located 165 miles from the nearest train station in Mojave, California. The mined borax was transported that distance in wagons pulled by 20 mules. Each wagon cost $900 to build, weighed 14,500 kilograms (32,000 pounds), and had wheels 2 meters (7 feet) in diameter. The trip took about twenty days, often in temperatures as high as 45°C (113°F). Over the six years during which mule teams were used, about nine million kilograms (20 million pounds) of borax were moved from mine to railway station. In 1896 the Pacific Coast Borax Company (later, U.S. Borax) took the twenty-mule teams as their corporate symbol.

The next most important use of sodium tetraborate is in the manufacture of soaps, detergents, and personal care products. Some well known products that contain borax include 20-Mule-Team® Borax all-purpose cleaner, 20-Mule-Team® Borax Laundry Booster, Borateem® stain remover, and Boraxo® powdered hand soap. Some other uses of sodium tetraborate include:

• As a flame-retardant and fungicide for wood products;
• In the production of enamel, porcelain, glazes, enamels, and frits (specialized types of glass);
• In the manufacture of fertilizers and herbicides;
• As additives for certain kinds of polymers;
• As a flux for smelting and soldering metals;
• In the preparation of rust inhibitors; and
• In certain photographic processes.

Sodium tetraborate is a mild irritant to the skin, eyes, and respiratory tract. It can cause inflammation of the skin, eyes, nose, throat, and lungs. If ingested, it can cause nausea, vomiting, diarrhea, and abdominal pain. There is some evidence that ingestion of sodium tetraborate can cause reproductive problems in laboratory animals, although similar effects have not been seen in humans. Swallowing large quantities of sodium tetraborate can have serious health consequences, especially for young children. The fatal dose of sodium tetraborate for young children is about five grams (0.2 ounce) of the compound.

Words to Know

ANHYDROUS

Describing a compound that lacks any water of hydration.

DECAHYDRATE

Form of a compound that exists with ten molecules of water.

FLUX

A material that aids the processes of welding and soldering (joining) metals.

WATER OF HYDRATION

Water that has combined with a compound by some physical means.

FOR FURTHER INFORMATION

"Borax (Hydrated Sodium Borate)." Amethyst Galleries, Inc. http://mineral.galleries.com/minerals/carbonat/borax/borax.htm (accessed on November 12, 2005).

"Rio Tinto Borax." Borax. http://www.borax.com/index.html (accessed on November 12, 2005).

"Sodium Tetraborate." International Programme for Chemical Safety. http://www.inchem.org/documents/icsc/icsc/eics1229.htm (accessed on November 12, 2005).