Why aluminum hydroxide is amphoteric hydroxide

Mar 03,2018


The so-called aluminum hydroxide actually refers to the hydrate of aluminum oxide. For example, the white colloidal precipitation obtained by adding ammonia or alkali to the aluminum salt solution has variable water content and uneven composition, which is collectively referred to as hydrated alumina.

True aluminum hydroxide can only be obtained by passing CO2 in an aluminate solution (containing Al (OH) 4-ions). Crystalline aluminum hydroxide is different from hydrated aluminum oxide, insoluble in acid, and does not dehydrate when heated to 373K. Only when heated at 573K for 2h can it be converted into aluminum metahydroxide (AlO (OH)).

Aluminum hydroxide is an amphoteric hydroxide. Since it exists in two ionizing forms, it is a weak acid, which can have the chemical formula H3AlO3 and a weak base, and can have the basic chemical formula Al (OH) 3. Aluminum hydroxide has amphoteric properties, which can react with both acids and bases. The acidity of aluminum hydroxide is that it is a Lewis acid and can be added to OH-, thus reflecting the alkaline Al (OH) 3. Due to the existence of two ionization, two salts can be produced:

Aluminum salts: Al3 + AlCl3, KAl (SO4) 2 · 12H2O (alum). Their aqueous solutions are acidic due to the hydrolysis of Al3 +. AgNO3 and dilute nitric acid are added dropwise, and Cl- is the one that produces white precipitation; Br- is the one that produces light yellow precipitation; I- is the one that produces yellow precipitation.

2. Metaaluminate, AlO2- NaAlO2, KAlO2. Their aqueous solutions are alkaline: AlO2- + 2H2O → Al (OH) 3 + OH- When the two types of salts are mixed, a double hydrolysis reaction occurs to generate Al (OH) 3 Al3 ++ 3 AlO2- + 6H2O == 4Al (OH) 3 ↓.