Hydroxides of Aluminium, Zinc, Chromium and few other metals are soluble both in acids and alkali. They are known as amphoteric hydroxides.
An amphoteric hydroxides form complexes with hydroxides. The complexes formed under both acidic and basic conditions are charged ions and soluble in water. Those formed under conditions between these are soluble in water.
e.g. Aluminium hydroxide.
Al3+ +OH- = Al (OH) +2
Al (OH) 2+ +OH- =Al (OH) 2+
Al (OH)+ 2 + OH- =Al (OH) 3 Precipitate will absorb OH-
Al (OH) 3 + OH- =Al (OH)-4 Again soluble.
Under strongly acidic conditions, the OH- concentration is low and the species present will be Al3+ and Al (OH) +2. If a strong base is gradually added, the OH- concentration will increase and these ions will begin to add to the Aluminium complexes, reducing the charges until the neutral and insoluble species Al (OH) 3 is formed, which will then precipitate from solution. As base addition continues the concentration of OH- in the solution increases, the negatively charged and soluble Al (OH)-4 is formed.and the precipitate dissolves.
The Amphoteric property of Aluminium hydroxide is a factor limiting its use as a coagulant in water purification and industrial waste treatment.