Amphoteric Hydroxides

The oxides or hydroxides of metals are basic in character and react with acids to form salts. The insoluble metallic hydroxides, such as ferric hydroxide, dissolve readily in acids to form salts but are insoluble in solutions of bases. Likewise the oxides of nonmetals are acidic in character, and insoluble forms are soluble in bases but not in acids. These characteristics serve as one basis of differentiating between metals and nonmetals.

The hydroxides of aluminum, zinc, chromium, and a few other elements are soluble in both acids and bases. They are known as amphoteric hydroxides, and advantage is often taken of this fact to accomplish separations in qualitative analysis and in chemical processing.

An insoluble metallic hydroxide, of course, exists in equilibrium with its ions. For example, ferric hydroxide dissolves to a limited extent to produce Fe3* and OH" ions:

and at saturation

When a strong acid is added, the OH" ions combine with the H+ ions of the acid to form poorly ionized water, and the [OH"] decreases so that

Therefore, ferric hydroxide dissolves in an attempt to establish conditions represented in Eq. (2.69). If enough acid is added, eventually all the ferric hydroxide will dissolve. A similar situation holds for insoluble nonmetallic oxides that are soluble in bases, except that the insoluble oxide is in equilibrium with H+ and some acid radical.

An amphoteric hydroxide forms complexes with hydroxide. The complexes formed under both acidic and basic conditions are charged ions and soluble in water. Those formed under conditions between these extremes are neutral in charge and insoluble in water. Aluminum hydroxide will serve as an example. Stepwise reactions for Al3+ and OH" are as follows:

Under strongly acidic conditions, the OH" concentration is low and the species present will be the positively charged ions, 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 aluminum complexes, reducing the charges until the neutral and insoluble species A1(0H)3(s) is formed, which will then precipitate from solution. As base continues to be added and the OH" concentration in solution increases further, the negatively charged and soluble Al(OH)4 ion is formed, and the precipitate dissolves.

Actually, metal ions such as Alw and Fe3+ do not occur as such in solution, just as H+ does not. Because of their strong positive charges, they readily become hydrated to form species such as [Fe(H20)6]3+ and [A1(H20)6]3+. However, the symbols Fe3+ and Al3"1" will be used in this book to represent these species m order to reduce the complexity of the equations.

The amphoteric property of aluminum hydroxide is a factor limiting its use as a coagulant in water purification and industrial waste treatment. The amphoteric properties of zinc and chromium hydroxides are important considerations in treating industrial wastes containing Zn2+ and Cr3"1".

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