Aluminum Potassium Sulfate

OVERVIEW

Aluminum potassium sulfate (uh-LOO-min-um po-TASS-see-um sul-fate) is also known as aluminum potassium sulfate dodecahydrate, potash alum, potassium alum, and kalinite. It normally occurs in the form of the dodecahydrate, meaning that each molecule of the compound is associated with twelve molecules of water. The formula of the hydrate is KAl(SO₄)₂·12H₂O. In this form, it occurs as white odorless crystals. The compound gradually loses its water of hydration when heated, giving up the first nine molecules of water at 64.5°C (148°F), and the remaining three molecules of water at about 200°C (392°F).

KEY FACTS

OTHER NAMES:

See Overview.

FORMULA:

KAl(SO₄)₂

ELEMENTS:

Potassium, aluminum, sulfur, oxygen

COMPOUND TYPE:

Inorganic salt

STATE:

Solid

MOLECULAR WEIGHT:

258.20 g/mol

MELTING POINT:

92.5°C (198°F; loses water of hydration beginning at 64.5°C (148°F)

BOILING POINT:

200°C (392°F; loses all water of hydration)

SOLUBILITY:

Soluble in cold water; very soluble in hot water; insoluble in alcohol, ether, acetone, and other organic solvents

Aluminum potassium sulfate belongs to a family of compounds known collectively as the alums. The term alum refers to a double salt that consists of aluminum, the sulfate group (SO₄), and one other metal. The presence of two metals, aluminum plus one other metal, accounts for the name double salt. Other common alums are aluminum ammonium sulfate and aluminum sodium sulfate.

The alums were known as far back as ancient Egypt and China, where they were used as deodorants. Alums are effective for this purpose because they are astringents, substances that cause tissues to shrink or contract, thus reducing the tendency of sweat glands to produce perspiration. As astringents, alums were also used in the field of medicine to treat wounds and prevent bleeding. Until the nineteenth century, however, chemists did not recognize that the substance they knew as alum was actually a variety of different compounds.

HOW IT IS MADE

Aluminum potassium sulfate occurs naturally in the form of the minerals alunite and kalinite. Where these minerals are available, the compound can be mined and purified to obtain potash alum. Where the minerals are not available, the compound can be produced synthetically by combining aqueous aluminum sulfate with aqueous potassium sulfate. The two compounds react with each other in solution to form the double salt, aluminum potassium sulfate, which can then be extracted by allowing the solutions to evaporate, during which the desired compound crystallizes out.

Interesting Facts

• During the fifteenth century, the Vatican achieved control of the alum industry in Europe. When King Henry VIII of England quarreled with Pope Clemens VII a century later over his desire to marry a second time, the pope cut off supplies of alum to Great Britain. Since alum was an essential product in the dyeing of clothes, the English clothing industry rapidly fell into dire circumstances.

COMMON USES AND POTENTIAL HAZARDS

One of aluminum potassium sulfate's major uses is in the dyeing of fibers and fabrics, where it is employed as a mordant. A mordant is a substance that reacts with a dye, helping it attach more permanently to a fiber or fabric. Aluminum potassium sulfate has also been used in the paper-making industry for many centuries, where it has a variety of applications. For example, it can be used to give paper a tough, shiny surface or to increase the intensity of inks, paints, and dyes used on the paper. Some water treatment plants also use aluminum potassium sulfate in their purification systems. The compound is added to water, where it combines with colloidal particles suspended in water to form larger clumps, which then settle out of the water. Other uses of aluminum potassium sulfate include:

• As a food additive, used to control the acidity of the product;
• In the manufacture of matches;
• For the waterproofing of fabrics;
• In the tanning of leather;
• In the manufacture of deodorants;
• To add hardness and toughness to cement;
• In the production of fireworks;

• As an astringent in medical treatments; and
• In the preparation of other compounds of aluminum in the chemical industry.

Words to Know

AQUEOUS SOLUTION

A solution that consists of some material dissolved in water.

ASTRINGENT

A material triggers a loss of water from tissue, thereby causing the tissues to shrink and contract.

COLLOID

A mixture consisting of tiny particles suspended, but not dissolved, in water.

DOUBLE SALT

A salt that includes aluminum and one other metal, such as potassium.

MORDANT

A substance that reacts with a dye so that it attaches to a fiber or fabric more easily.

FOR FURTHER INFORMATION

"Aluminum Potassium Sulfate Resources." http://names.mongabay.com/drugs/ingredients/ALUMINUM_POTASSIUM_SULFATE.html (accessed on September 18, 2005).

Hart-Davis, Adam. "Thomas Challoner and His Astonishing Alum Industry." ExNet. http://www2.exnet.com/1995/12/18/science/science.html#LINKS-BACK (accessed on September 18, 2005).

"Material Safety Data Sheet: Aluminum Potassium Sulfate Dodecahydrate." Department of Chemistry, Iowa State University. http://avogadro.chem.iastate.edu/MSDS/AlKSO4.htm (accessed on September 18, 2005).

"Potassium Aluminum Sulfate." Gale Encyclopedia of Science. Edited by K. Lee Lerner and Brenda Wilmoth Lerner. 3rd ed. Vol. 5. Detroit, Mich.: Gale, 2004.

See AlsoPotassium Sulfate