Aluminium hydroxide

Aluminium hydroxide (CAS 21645-51-2) has the formula Al(OH)3 and a molar mass of about 78.00 g/mol. It is a white powder or gel that is practically insoluble in water. It is amphoteric, so it dissolves in strong acids to form aluminium salts and in strong alkalis to form aluminates. In nature it occurs as the polymorphs gibbsite, bayerite, nordstrandite and doyleite. Gibbsite is a major constituent of bauxite.

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How is aluminium hydroxide used?

• Pharmaceuticals: It is used as an antacid and phosphate binder. As an adjuvant, it adsorbs antigens in many vaccines, often called "alum" adjuvants.
• Flame retardancy: Sold as aluminium trihydrate (ATH), it decomposes endothermically at around 180–200 °C and releases water. This makes it a common halogen-free flame retardant and smoke suppressant in polymers, cables and coatings.
• Industrial chemistry: It is calcined to produce alumina and serves as a starting material for other aluminium compounds. It also forms in situ during water treatment with aluminium-based coagulants.

Which grades and specifications should buyers check?

Pharmacopoeial monographs exist in the Ph. Eur. (EP), including hydrated aluminium hydroxide for adsorption, and in the USP (Aluminum Hydroxide Gel). Technical and ATH grades are usually specified by particle size distribution, purity, residual sodium content, whiteness and surface treatment. Request a CoA and SDS for each lot. Confirm the regulatory status under REACH or TSCA and the GHS classification with your supplier.

How should it be stored and handled?

Store the material in tightly closed containers in a dry place, away from strong acids and bases. Dust should be minimised in line with occupational exposure guidance given in the SDS.

Related concepts

Related entries include aluminium oxide (alumina), the Bayer process, bauxite, aluminium sulfate, magnesium hydroxide (an alternative flame retardant), vaccine adjuvants and amphoterism.