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 AlCl₃
- NH₄OH, conc. (use less) or 2% dilute (use about equal volume)
- 6M HCl
- 6M NaOH
Materials
- 3 small hydrometer cylinders
- 2 stirring rods
Procedure
- In each cylinder prepare Al(OH)₃ by mixing approximately equal volumes of AlCl₃ and 2% NH₄OH (or by adding a smaller volume of conc NH₄OH). A gelatinous precipitation is formed.
- 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:
\( \ce{ Al(OH)3_{(s)} + 3H+_{(aq)} -> Al3+_{(aq)} + 3H2O } \)
\( \ce{ Al(OH)3_{(s)} + OH-_{(aq)} -> [Al(OH)4]- } \)