Aluminum Hydroxide

Aluminum hydroxide and its closely related compounds have a number of practical uses. In one process of water purification, for example, aluminum sulfate, Al₂(SO₄)₃, or alum (usually potassium aluminum sulfate, KAl(SO₄)₂), is mixed with lime (calcium hydroxide, Ca(OH)₂) in a container of water to be purified. The reaction between these compounds results in the formation of a gelatinous precipitate aluminum hydroxide. As the precipitate settles out of solution, it adsorbs on its surface particles of dirt and bacteria that were suspended in the impure water, which can then be removed by filtering off the aluminum hydroxide precipitate.

The ability of aluminum hydroxide to adsorb substances on its surface explains a number of its other applications. It is used in a number of chemical operations, for example, as a filtering medium and in ion-exchange and chromatography devices.

Aluminum hydroxide is popular as an antacid. It behaves as a base, reacting with and neutralizing excess stomach acid (hydrochloric acid) to bring relief from "heartburn." A few of the more common commercial antacids containing aluminum hydroxide are Amphojel, Di-Gel, Gelusil, and Maalox. Ingredients can change without notice, however, so the labels should always be checked.

Aluminum hydroxide is also used as a mordant in dyeing. In most cases, the compound is precipitated out of a water solution onto the fibers to be dyed. The material is then immersed into the dye bath. The color of the final product depends on the combination of dye and mordant used in the process. A similar process is used in the manufacture of certain paint pigments. A given dye and aluminum hydroxide are precipitated together in a reaction vessel and the insoluble compound thus formed is then filtered off.

Additional uses of aluminum hydroxide include the manufacture of aluminosilicate glass, a high melting point glass used in cooking utensils, the waterproofing of fabrics, and the production of fire clay, paper, pottery, and printing inks.

A close chemical relative of aluminum hydroxide, aluminum hydroxychloride Al₂(OH)₅Cl, is an ingredient in many commercial antiperspirants. The compound acts as an astringent, a substance that closes pores and stops the flow of perspiration.