tetrahydroxoaluminate

Tetrahydroxoaluminate, commonly encountered as the tetrahydroxoaluminate anion [Al(OH)4]−, is a hydroxo complex of aluminum in which four hydroxide ligands are bound to a central Al3+ ion. In aqueous solution at high pH it exists predominantly as this anion; salts such as sodium tetrahydroxoaluminate Na[Al(OH)4] are well characterized and used in industry.

The aluminum center in tetrahydroxoaluminate is tetrahedrally coordinated to four hydroxide groups. The anion carries a −1 charge and is stabilized by hydration in solution. In water, the dominant species depends on pH; at strongly basic conditions [Al(OH)4]− forms, whereas at lower pH other hydroxo complexes may predominate and eventually Al(OH)3 can precipitate.

Formation occurs when aluminum or aluminum oxide/hydroxide dissolves in excess alkali. A representative reaction with sodium hydroxide is: 2 Al + 2 NaOH + 6 H2O → 2 Na[Al(OH)4] + 3 H2. Similar pathways operate with potassium or ammonium bases, yielding corresponding aluminate salts.

Applications of tetrahydroxoaluminate species include their use as sources of aluminate in water treatment and paper production, where they function as coagulants or pH regulators. Sodium aluminate solutions are also employed as cement admixtures and as reagents in various chemical syntheses, exploiting the aluminate anion's basicity and hydroxo ligands.

Safety considerations are important, as solutions of tetrahydroxoaluminate are highly caustic. Handling requires appropriate protective equipment and adherence to standard chemical safety guidelines to prevent skin, eye, or respiratory irritation.