AMPHOTERIC SUBSTANCES 217
- Aluminum Hydroxide. Add NH4OH to an aluminum
sulphate solution until a strong odor of ammonia persists
after stirring. A gelatinous white precipitate forms. Col-
lect some of this precipitate on a filter and wash it repeatedly
with hot water until all excess of ammonia is removed. Then
dip red and blue litmus paper into the precipitate and ob-
serve that neither is affected.
This experiment shows the insolubility of aluminum hydroxide
and the extreme weakness of its acidic and basic properties.
- Aluminum Hydroxide Acting as an Acid. To a little
aluminum sulphate solution add NaOH solution a drop at
a time until a copious precipitate is observed; continue to
add NaOH, and observe that the precipitate soon redissolves.
The precipitate is aluminum hydroxide, A1 2 (SO4) 3 + 6NaOH —•
2A1(OH) 3 + 3Na 2 SO4. That the aluminum hydroxide dissolves
in the solution of the base indicates that it has reacted to form a
soluble salt and that it is itself an acid. There are two acids of
aluminum differing in degree of hydration in the same way as
the different boric acids.
A1(OH) 3 —• H3AIO3 (ortho aluminic acid)
A1(OH) 3 -»• H 2 O + HAIO2 (meta aluminic acid)
The soluble salt may be either sodium ortho aluminate, Na 3 A10 3 ,
or sodium meta aluminate, NaA102, depending on the amount of
NaOH used.
- Aluminum Hydroxide Acting as a Base. To the solu-
tion left at the end of Experiment 3 add an acid (say HNO 3 )
drop by drop until a copious precipitate is observed. Con-
tinue to add acid and observe that the precipitate redissolves.
The precipitate is aluminic acid, displaced from its salt by the
stronger acid, NasAlO;, + 3HNO 3 -»• H 3 A1O 3 + 3NaNO 3. That
the aluminic acid dissolves in nitric acid indicates that it reacts to
form a soluble salt and is itself reacting as a base.
- Comparative Basic and Acidic Strength of A1(OH) 3.
Add NH4OH in excess to an aluminum salt solution, and note
that the precipitate does not redissolve in the excess of
NH4OH. Add acetic acid drop by drop. For a time the
