Potassium Carbonate

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Potassium Carbonate

OVERVIEW

Potassium carbonate (poe-TAS-ee-yum KAR-bun-ate) is also known as potash, pearl ash, salt of tartar, carbonate of potash, and salt of wormwood. It is a white, translucent, odorless, granular powder or crystalline material that tends to absorb water from the air. As it does, it is converted into the sesquihydrate ("sesqui" = one-and-a-half) with the formula K2CO3·1.5H2O. That formula means that three molecules of potassium carbonate share two molecules of water among them.

KEY FACTS

OTHER NAMES:

See Overview.

FORMULA:

K2CO3

ELEMENTS:

Potassium, carbon, oxygen

COMPOUND TYPE:

Salt (inorganic)

STATE:

Solid

MOLECULAR WEIGHT:

138.21 g/mol

MELTING POINT:

898°C (1650°F)

BOILING POINT:

Not applicable; decomposes above melting point

SOLUBILITY:

Soluble in water; insoluble in ethyl alcohol

Potash is easily produced by pouring water over the ashes of burned plants and then evaporating the solution formed in large pots (hence the name: "pot" "ash"). The process has been known since at least the sixth century ce and the resulting product used in the manufacture of soap. Potash was one of the first chemicals to be exported by American colonists, with shipments having left Jamestown as early as 1608. Potassium carbonate is also called pearl ash and salt of tartar, both of which are impure forms of the compound. The impurities present include sodium chloride and some heavy metals (such as iron and lead). The primary uses of potassium carbonate are in the production of fertilizers, soaps, and heat-resistant glass.

HOW IT IS MADE

Most of the potassium carbonate made in the United States is produced beginning with potassium chloride (KCl) obtained from seven mines in New Mexico, Michigan, and Utah. The potassium chloride is first converted to potassium hydroxide (KOH) by electrolysis. The potassium hydroxide is then treated with carbon dioxide (CO2) to obtain potassium bicarbonate (KHCO3). Finally, the potassium bicarbonate is decomposed by heating, yielding water, carbon dioxide, and potassium carbonate.

Another method of preparation, called the Engel-Precht process, is a modification of this procedure. A mixture of potassium chloride, magnesium carbonate or magnesium oxide, and carbon dioxide is treated under 30 atmospheres of pressure, with the formation of a double salt, KHCO3·Mg-CO3·4H2O. The double salt is then heated to obtain potassium carbonate. The traditional method of obtaining potash from wood and vegetable ash is now obsolete.

COMMON USES AND POTENTIAL HAZARDS

An estimated 7 million metric tons (6.5 million short tons) of potassium carbonate were produced in the United States in 2005. Of that amount, nearly 90 percent was used for the production of fertilizers. Potash provides plants with the potassium they need to stay healthy and grow. Potassium is one of the three major nutrients required by plants, the other two being nitrogen and phosphorus. The next largest application for potassium carbonate is in the chemical industry, where it is used as a raw material to make other chemical compounds, potassium silicate being the most common.

Interesting Facts

  • The first patent ever issued in the United States was awarded in 1790 to Samuel Hopkins for a new and better way of making pearl ash.
  • Pearl ash was used in the United States in the eighteenth century as a leavening agent in the baking of bread.
  • The demand for potash began to fall off in the late eighteenth century as improved methods for the synthesis of sodium carbonate were developed. Sodium carbonate can replace potassium carbonate in many applications.
  • The term potash has historically had many different meanings. It has been used to refer to potassium hydroxide (KOH), potassium chloride (KCl), potassium sulfate (K2SO4), potassium nitrate (KNO3), or to some combination of these compounds.

Smaller amounts of potassium carbonate are still used for what was once its major application: the manufacture of soap. Potassium soaps (made from potassium carbonate) have some characteristics different from more common sodium soaps (made from sodium carbonate). They tend to be softer or even liquid and better able to create suds in water that contains a high concentration of minerals. Potassium carbonate is also used to make specialty glasses, such as television screens, cathode ray tubes, and optical lenses. Some other uses of the compound include:

  • For glazes in the making of pottery;
  • In the manufacture of pigments and printing inks;
  • As an additive in certain food products, chocolate being one example;
  • For the tanning and finishing of leather and the dyeing, washing, and finishing of wool; and
  • As a flux in metal working.

Potassium carbonate in dry or dissolved form is an irritant to the eyes, skin, and respiratory system. It can cause inflammation of the skin, eyes, throat, and stomach. Potassium carbonate's action is caused by its caustic properties in water solution, which are produced when it is dissolved in water or when it is absorbed by moist tissues in the body.

Words to Know

CAUSTIC
Strongly basic or alkaline; can irritate or corrode living tissue.
ELECTROLYSIS
Process in which an electric current is used to bring about chemical changes.
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

"International Potash Institute." http://www.ipipotash.org/index.php (accessed on October 31, 2005).

"The Potash Trade." Townships Heritage WebMagazine. http://www.townshipsheritage.com/Eng/Hist/Life/potash.html (accessed on October 31, 2005).

"Potassium Carbonate." Chemical Land 21. http://www.chemicalland21.com/arokorhi/industrialchem/inorganic/POTASSIUM%20CARBONATE.htm (accessed on October 31, 2005).

"Potassium Carbonate." J. T. Baker. http://www.jtbaker.com/msds/englishhtml/p5609.htm (accessed on October 31, 2005).

Willett, Jason C. "Potash." U.S. Geological Survey Commodity Statistics and Information. Available online at http://minerals.usgs.gov/minerals/pubs/commodity/potash/potasmyb04.pdf (accessed on October 31, 2005).

See AlsoPotassium Chloride; Potassium Hydroxide