Question

Aluminum oxide is amphoteric yet is of little practical use as a buffer component. Why?

Answer 1

The actual molecules that do the reacting are hydrated Al+3 ions, e.g.:

Al(H2O)2(OH)4-1(aq) + H3O+(aq) -> Al(H2O)3(OH)3(aq) + H2O(l)

Al(H2O)3(OH)3(aq) + OH-(aq) -> Al(H2O)2(OH)4-1(aq) + H2O(l)

But you can't independently adjust the concentrations of the various hydrated Al+3 ions, so that you maintain a particular pH. As long as there is Al2O3(s) available, the concentrations of all the hydrated Al+3 species are set by the various K_eq, and so the pH of your "buffer" is set, not adjustable. I'd say that's the problem.