Aluminum hydroxide is widely used as an amphoteric adjuvant and adsorbent in biochemical and immunological applications. Its gel-forming layered structure and pH-dependent surface charge make it particularly suitable for vaccine formulation, protein purification, and phosphate binding processes.
Chemical Properties
Aluminum hydroxide (Al(OH)3, molecular weight 78.00 g/mol) appears as a white amorphous powder or gelatinous precipitate with a density of approximately 2.42 g/cm³. It exhibits a gibbsite-type layered structure in which Al3+ ions occupy two-thirds of the octahedral sites between double hydroxyl layers stabilized by hydrogen bonding (Al–O distance ~1.90 Å).
This compound is poorly soluble in water (Ksp ≈ 10−32) and demonstrates amphoteric behavior. It dissolves in acidic conditions (pH < 5) to form Al3+ ions and in alkaline conditions (pH > 9) to form aluminate ions [Al(OH)4]−. Upon heating to approximately 300°C, it undergoes dehydration to form γ-Al2O3.
Biochemical Applications
In immunology, aluminum hydroxide gel (commonly referred to as alum) is used at concentrations of 2–10 mg/mL as a vaccine adjuvant. It enhances immune responses by adsorbing antigens through electrostatic interactions, particularly due to its high isoelectric point (pH 11.4), promoting Th2-type immunity in vaccines such as DTaP and hepatitis B.
In protein purification, it is applied in chromatography for the selective adsorption of acidic proteins, including phosphatases and albumins, typically at pH 7–8, followed by elution using phosphate gradients. In molecular biology, freshly prepared precipitates are used to remove nucleic acid contaminants from enzyme preparations. Additionally, in pharmaceutical research, aluminum hydroxide is studied for its antacid properties, where 1 g can neutralize approximately 300 mL of 0.1 M hydrochloric acid.
