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Amphoteric Nature of Aluminum Hydroxide

Summary

Aluminum hydroxide is prepared by mixing aluminum chloride and ammonium hydroxide in two hydrometer cylinders. Sodium hydroxide is used to dissolve the precipitate in one cylinder, hydrochloric acid in the other.

Hazards

Use care when handling 6M HCl and 6M NaOH. Both are corrosive and toxic.

Chemicals and Solutions

  • 0.1M AlCl
  • NHOH, conc. (use less) or 2% dilute (use about equal volume)
  • 6M HCl
  • 6M NaOH

Materials

  • 3 small hydrometer cylinders
  • 2 stirring rods

Procedure

  1. In each cylinder prepare Al(OH) by mixing approximately equal volumes of AlCl₃ and 2% NHOH (or by adding a smaller volume of conc NHOH). A gelatinous precipitation is formed.
  2. To the first cylinder add NaOH to the precipitate. To the second cylinder add HCl. In both cases the gelatinous suspension of aluminum hydroxide dissolves. (The third cylinder is the reference precipitate.)

Discussion

Substances that can act as either acids or bases are described as amphoteric. The reactions:

\( \ce{ Al(OH)3_{(s)} + 3H+_{(aq)} -> Al3+_{(aq)} + 3H2O } \)

\( \ce{ Al(OH)3_{(s)} + OH-_{(aq)} -> [Al(OH)4]- } \)

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