Aluminum Hydroxide

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Aluminum Hydroxide

Aluminum hydroxide is a common compound of aluminum, hydrogen, and oxygen that can be considered either a base, with the formula Al(OH)₃, or an acid, with the formula H₃ AlO₃. The compound is frequently treated as a hydrate—a water-bonded compound—of aluminum oxide and designated variously as hydrated alumina, or aluminum hydrate or trihydrate, hydrated aluminum, or hydrated aluminum oxide, with the formula Al₂ O₃ (H₂ 0)_x.

Properties

Aluminum hydroxide is found in nature as the mineral bayerite or gibbsite (also called hydrargillite). A mixed aluminum oxide-hydroxide mineral is known as diaspore or boehmite.

In a purified form, aluminum hydroxide is either a bulky white powder or granules with a density of about 2.42 g/mL. It is insoluble in water, but soluble in strong acids and bases. In water, aluminum hydroxide behaves as an amphoteric substance. That is, it acts as an acid in the presence of a strong base and as a base in the presence of a strong acid. This behavior can be represented by the following somewhat oversimplified equation.

In the presence of a strong acid such as hydrochloric acid, the above equilibrium shifts to the right, and aluminum chloride is formed.

Al(OH)₃ + 3 HCl → 3H₂O + AlCl₃

In the presence of a strong base such as sodium hydroxide, the equilibrium is driven to the left and a salt of the aluminate ion (AlO_2–) is formed.

NaOH + H₃AlO₃ → NaAlO₂ + 2H₂O

Sodium aluminate, NaAlO₂, has a number of practical applications, such as in water softening, paper sizing, soap and milk glass manufacture, and fabric and textile printing.

Uses

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 forms the 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. It is also used as a mordant (fixative) in dyeing. In most cases, the compound is precipitated out of a water solution onto the fibers to be dyed. The

KEY TERMS

Adsorption— The process by which atoms, ions, or molecules of one substance adhere to the surface of a second substance.

Amphoterism— The property of being able to act as either an acid or a base.

Equilibrium— The conditions under which a system shows no tendency for a change in its state. At equilibrium the net rate of reaction becomes zero.

Mordant— A material that is capable of binding a dye to a fabric.

Pigment— Any substance that imparts color to another substance.

material is then immersed into the dye bath. The color of the final product depends on the combination of dye and mordant used. 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 include the manufacture of aluminosilicate glass, a high melting point glass used in cooking utensils, fabric waterproofing, and the production of fire clay, paper, pottery, and printing inks.

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