Amphoteric Nature of Aluminum Hydroxide

 

Summary: Aluminum hydroxide is prepared by mixing aluminum chloride and ammonium hydroxide in two hydrometer cyinders.  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 descibed as amphoteric.

The reactions:

Al(OH)3(s) + 3H+(aq) ---> Al3+(aq)+ 3H2O

Al(OH)3(s) + OH-(aq) ---> [Al(OH)4]-

To schedule a demonstration, please send an email to the demonstration lab.

 

Contact:

Eric Camp

Lecture Demonstration Technician

Bagley Hall 171

(206) 543-1606

ericcamp@uw.edu

 

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