Aluminum Hydroxide
Properties
| State | Solid at room temperature |
| Color | White amorphous powder |
| Solubility | Insoluble in water; soluble in acids and strong bases |
| Melting Point | 300°C (decomposes) |
About Aluminum Hydroxide
Aluminum hydroxide is the textbook example of an amphoteric hydroxide — it dissolves in acid and dissolves in strong base, and is barely soluble at any pH in between. The acid-side chemistry is straightforward (Al(OH)3 + 3 H⁺ → Al³⁺ + 3 H2O); the base-side chemistry is more interesting, with the aluminum coordinating a fourth hydroxide to give the tetrahedral aluminate ion [Al(OH)4]⁻. That second pathway is what makes the Bayer process work: aluminum hydroxide dissolves selectively in hot concentrated NaOH while iron and silica oxides in bauxite ore stay solid, allowing the aluminum to be separated, then re-precipitated by cooling and CO2 acidification, and finally calcined to Al2O3 for electrolysis to aluminum metal. The natural form, gibbsite, is one of the major minerals in tropical and subtropical bauxite ores. The compound also has a long medical history as an antacid — the Al(OH)3 + 3 HCl reaction releases water and gives slow, sustained neutralization without producing CO2 the way carbonate-based antacids do. The other significant biomedical use is as an aluminum-salt vaccine adjuvant: amorphous Al(OH)3 binds antigens and slowly releases them, prolonging the immune system's exposure and enhancing the response to subunit vaccines like hepatitis B and HPV.
Where you'll encounter it
If you've taken Maalox, Mylanta, or any of the classic combination antacids, the aluminum hydroxide in there is what gives the slow, drawn-out acid-neutralization response that contrasts with the fast burst from calcium carbonate. The combination with magnesium hydroxide is intentional — aluminum hydroxide tends to be constipating on its own, magnesium hydroxide tends to be laxative, and the mixture cancels out the GI side effects. In a chemistry teaching lab, the amphoteric demonstration with Al(OH)3 is a classic — add HCl and the white gel dissolves, add NaOH and the white gel dissolves, dilute the result back toward neutral pH and the gel reprecipitates. It's also the basis of alum-based water treatment: aluminum sulfate added to raw water hydrolyzes to give Al(OH)3 floc, which sweeps fine particles out of suspension as it settles.
Common Uses
- Antacid for acid indigestion (slow, sustained neutralization)
- Aluminum-salt adjuvant in subunit vaccines (HepB, HPV, DTP)
- Fire-retardant filler in polymers and composites
- Bayer-process intermediate in aluminum metal production
- Coagulant floc in drinking-water and wastewater treatment
Safety Information
Low acute toxicity at the doses used in medicine and water treatment. Long-term high-dose oral use can interfere with phosphate absorption in the gut and shouldn't be used by patients with chronic kidney disease, where aluminum can accumulate. Inhalation of fine alumina-precursor dust over years has been linked to a fibrotic lung condition (Shaver's disease) in industrial settings. Otherwise practically inert to handle.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.